Triumph Spitfire Performance Enhancements (March 2013 edition)
Why bother improving Spitfire performance?
Triumph Spitfires are basically fun, economical street-legal go-carts. Many are still around (more than 300,000 were manufactured from 1962 to 1980), they are simple and easy to work on, and parts are relatively inexpensive and readily available. The later model Spitfire is not a bad-handling car in factory spec. It bested its contemporaries in handling and it holds its own even now. However, some simple changes and upgrades, especially to the early Spitfires, can significantly improve handling and enhance safety. Power, acceleration and speed are where the Spitfire was mediocre in its day, and by today's standards it's underwhelming. But here again, a few simple, relatively inexpensive and easy changes can yield significant improvement, and by putting it all together you will have a much more fun car. The following article is a brief summary and represents my own observations and findings for putting together a fun, reliable and safe Spitfire that can be used with confidence on the street for years and years.
This article is aimed at improving Spitfire performance for sporty street use at reasonable cost. It’s not a race car preparation guide, although the physics is universal and many of the specific enhancements apply to both street and track use. There are several good race preparation guides for Spitfires written by people with experience building and racing Spitfires. These include the Triumph Race Prep Manuals, Kas Kastner's race prep guidelines and Jon Wolfe's "A Guide To Racing Your Triumph Spitfire or GT6." Furthermore, I’ve limited the scope of enhancements herein to things that keep a Spitfire a Spitfire—or at least a Triumph for the most part—and do not go into things like radical engine swaps or drivetrain transformations (notwithstanding how fascinating, fun and exciting they can be). This article is meant to benefit Spitfire enthusiasts who want to reliably use their Spitfires often, improve their performance and derive more enjoyment from them without having to go to extremes. I attempt throughout to explain "why"—the principles behind the improvements—followed by "what"—the specific improvements themselves—to promote understanding and foster creativity. More in-depth information can be found in references cited throughout the article. Exact details of “how” to implement the improvements are more the realm of shop manuals and so "how-to" information is not covered in great detail here. This article is divided into two main sections—Handling and Power—and in each section, the enhancements are listed serially in rough logical order. However, like any engineered assembly, the Spitfire is a system. Changes involve trade-offs and often these changes need to be considered in the context of the whole vehicle and executed in concert. I hope you find this information helpful.
CONTENTS (shortcuts to sections of this article):
Understand Some Suspension Basics
main purpose of the suspension is to ensure proper positioning of the
tires relative to the road for good grip, despite bumps, dips and
cornering forces, to enable safer and faster handling. The entire
suspension--tires, wheels, linkage, springs and dampers (shock
absorbers)--and the overall rigidity and mass properties of the
vehicle are a system that should be considered as a whole when making
changes to any of the individual elements. The mass of the vehicle and
how it is distributed, the geometry of the suspension elements and the
stiffness of the springs and the damping coefficients of the damping
elements determine how much and how quickly the vehicle will roll,
pitch and yaw in reaction to gravity and handling forces.
At any given point in time with any given set of forces, weight is being transferred through the suspension to the ground, and the key principle behind suspension tuning is that the load path with the most stiffness will transfer the most weight. If more force is transferred through a given tire than it can handle, exceeding it's ability to adhere to the road, then it will lose grip and slide, and control will be relinquished. If the front of the vehicle slides first, it's called understeer (or pushing, or plowing), and if the rear of the vehicle slides first, it's called oversteer (or getting loose or fishtailing).
The objective of performance suspension tuning is to manage weight transfer, enabling higher cornering and acceleration/braking forces while staying within the limits of tire adhesion. This is done primarily by managing mass, the height of the center of mass (a.k.a. center of gravity, or c.g.), and the length of the moment arms that forces on the c.g. react through to reach the ground by managing the geometry of the suspension linkage, and by setting the compliance of the suspension by managing the geometry of the suspension linkage and the stiffness of springs and dampers. Generally, the following are good things for handling: less mass, more concentrated mass (i.e., lower polar moment of inertia), lower c.g., shorter distances between mass centers and roll centers (i.e., shorter moment arms—good for reducing lateral weight transfer due to body roll of sprung mass), lower roll centers, wider track, and longer wheelbase. These may sound simple but their implementations sometimes work against each other and other objectives of vehicle design, and entail various costs and disadvantages, so suspension solutions are many and varied.
ultimately matters here are the forces where the rubber literally meets
the road, i.e., the forces of the accelerating mass of the car versus
the reaction forces of the ground. Wheel rates, which are the product
of spring rates and the leverage and motion of the suspension, are the
actual rates of vehicle stiffness. Simply adding wheel rate
contributions from individual suspension elements yields total wheel
rates and vehicle stiffness. Spring rates alone mean nothing as a
measure of vehicle stiffness and they can be very misleading--it is
wheel rates that matter. I've
tabulated various factory and aftermarket front coil, front anti-roll
bar (ARB) and rear leaf spring
and resultant wheel rates and roll stiffnesses for
nominal Spitfires and GT6s.
All else being the same, a lighter car will out-handle a heavy one. Trimming weight is easy to do on the later U.S. market Spitfires by removing all the extra bumper and bumper reinforcing hardware they were burdened with. This light weighting will not only make the car less massive, it will eliminate weight at the ends of the vehicle, thus reducing its polar moment of inertia, meaning it will be easier to start and stop it turning. Moreover, many people think this conversion to the European spec look or pre-1974 U.S spec look improves the car's appearance. Best of all, it's free. There are other ways to “add lightness” such as replacing various bits and pieces with lighter ones. Weight reduction boosts performance, and every little bit counts.
Before discussing the following changes to the front and rear suspension, it is important to review alignment. There are six spatial degrees of freedom: translation along and rotation about each of three spatial axes. The three most basic alignment measures are Caster, Camber and Toe, and they are critical to handling, tire wear and safety. Changes to the suspension will result in changes in alignment that need to be measured and adjusted.
On the Spitfire, caster, camber and toe can be adjusted at the front reasonably easily. The Spitfire front suspension is a classic unequal A-arm design. Caster and camber can be adjusted using shims between the lower A-arms and the frame rails. Addition of shims adds negative camber. Adding or subtracting unequal numbers of shims between the front and rear A-arm interfaces changes caster (more shims at the aft mounting adds positive caster). Adjusting the tie rods sets toe. Note that because of the tie rod geometry, Spitfires exhibit a significant amount of toe change with up and down (bump and droop) suspension motion, called bump steer. Therefore, when changing camber and caster, toe must be checked and reset, and toe should be adjusted last.
At the rear of the Spitfire is a swing axle architecture with a single transverse leaf spring. The swing axles themselves are the primary suspension links; the transverse leaf spring and trailing radius arms guide the swing axles up and down and locate them fore and aft. Rear caster is for all practical purposes not adjustable, camber is determined by rear ride height (an intrinsic feature of the swing axle design), but toe is adjustable using shims where the radius arms interface with the vehicle tub. After achieving final ride height and hence rear camber, toe should be measured and then adjusted if necessary (rear toe changes much less than front toe).
Stock alignment specs vary across models and can be found in shop manuals. Specific non-stock settings are mentioned during the subsequent discussion of suspension mods. Overview information on generic alignment metrology and the effects of alignment can be found in the following primer:
The use of even wider wheels and tires is possible, but selection of the proper wheel offset is critical for avoiding interference between the tires and the car's bodywork or suspension elements. Moreover, offset combined with the steering axis, rim width, camber and overall wheel/tire diameter determines scrub radius, which is the distance between the steering axis and the centerline of the tire at the ground (on the Spitfire, the steering axis is the line that goes through the center of the vertical link ball joint and trunnion; stock Spitfires have positive scrub radius). Minimizing scrub radius will distribute loads better on the wheel bearings, help enable good tracking and straight-line braking (i.e., help minimize pull to one side or the other), make steering easy and help minimize tire wear.
There are wheel and tire fitment procedures you can follow to check and see by direct measurement what wheel and tire combinations ought to fit your car. When there is rubbing on a Spitfire, it typically occurs at the inner edges of the front fender lips during up and down suspension motion and at the firewall just aft of the front tires during turns. Too much width and/or not enough offset is usually the root of the problem. Other rub areas can be at the rear radius arms and rear fenders. Generally, using tires wider than 185mm and bigger than 22 inches in overall diameter requires rolling or folding-up of the inner edges of the front fenders, especially if the vehicle has been lowered. Of course, if you do body work to flare the fenders, you can build-to-suit and avoid rubbing the fenders this way and expand the range of wheels and tires even more.
Note that ever-larger wheels and lower profile tires are not necessarily better. Bigger diameter rims and tires are typically heavier, so they add unsprung weight. Moreover, their mass is distributed at a larger radius from the hub so they have more rotational inertia and so they are more difficult to start and stop rotating. Lower aspect ratio tires are less compliant, so they may not be matched to the rest of the suspension. Given the large amount of rear camber change that Spitfires exhibit, a moderate profile tire with some compliance is best for grip. Besides, very low aspect tires leave the wheels more susceptible to damage from objects and potholes in the road. Lastly, be sure that the wheels won't interfere with the front brake calipers. Virtually all 13 inch wheel options will work with stock Spitfire Girling 12 calipers (mk1 and mk2) and Girling 14 calipers (mk3, mkIV and 1500), but TR7 wheels interfere with the larger Girling 16 calipers for the GT6 due to their offset (see brake upgrades later in this article).
Something to seriously consider, especially when using alloy rims, is to upgrade from the stock 3/8 inch, 24 threads per inch wheel studs to ones that are thicker, longer and stronger. Being as thin as the stock studs are, they are susceptible to over-torquing, and being as old as many are, they likely have been over torqued by someone in the past. Fitting wider wheels with wider, stickier tires allows the car to generate more cornering force, which puts more stress on the studs. Furthermore, alloy rims have thicker center sections than steel ones, and so the lug nuts engage fewer threads, making a somewhat marginal situation even more so. Upgrading to 7/16 inch or 12mm studs that are longer than the stock Spitfire/GT6 ones is good insurance, and a really good idea if you are considering driving your Spitfire sportingly. One very good solution is to upgrade to Land Rover Freelander studs (part number CLP9037L). These are 12mm thick, 1.5mm per thread (i.e., M12x1.5) and are 2 inches long, 1 1/4 inches of which is threaded, versus the stock studs that are 1.5 inches long, 3/4 inches of which is threaded. The comparison photo below clearly illustrates the dramatic difference.
For more details, see Upgrading Triumph Spitfire Wheel Studs.
Achieve Better Suspension Geometry—Lower and Stiffen
Changing wheel alignment and lowering and stiffening the Spitfire can improve handling. Lowering brings the c.g. and roll centers closer to the ground, which helps reduce jacking and roll in turns, which helps maintain proper suspension geometry during turns, which keeps the tires in proper contact with the road so they can grip, which enables faster speeds through turns. Similarly, some suspension geometry changes and stiffening can reduce pitch (i.e., dive during braking and squat during acceleration) and limit bump steer. Stiffening helps by limiting the amount of roll and suspension geometry change for a given amount of acceleration. Moreover, stiffer springs reduce the amount of travel for a given load to offset the loss of bump travel due to lowering.
lowering and stiffening is good, but too much makes things worse. Too
much lowering generates poor suspension geometry that lowers the roll
center too much, thereby lengthening the lever arm between the roll
center and the c.g. to increase the roll moment that increases lateral
weight transfer due to body roll and increases changes in suspension
geometry to actually make
handling worse (note however that simply reducing overall wheel/tire
diameter beneficially lowers the c.g. and roll centers without changing
the linkage geometry). Moreover, too much lowering leaves inadequate
clearance for everyday use. Too much stiffness actually reduces grip
and makes for an uncomfortable ride. Too much change in stiffness at
one end of the car relative to the other, or stiffness not matched to
the static weight distribution of the car, will lead to excessive
understeer or oversteer. The right amount of lowering and stiffening
can optimize the geometry of the suspension links so that the car
transfers less weight for a given amount of roll, as well as rolls less
in response to a given set of forces to help maintain better tire
geometry for good grip.
A good compromise static front suspension geometry for a stiffened Spitfire at rest on level ground is when the lower A-arms are parallel to the ground. This is a reasonable compromise that lowers both the c.g. and the roll center without increasing the roll moment too much. This geometry occurs when the total distance between the center of the bolt attaching the damper to the lower A-arm and the upper spring perch is 10.25 inches. Because the distance from the lower A-arm to damper (shock absorber) connection and the bottom spring perch is nominally 3.25 inches, this also corresponds to a compressed length of the front coil springs installed of 7 inches.
Ways to lower the front include 1) shortening the stock springs, 2) replacing the stock springs with shorter ones and 3) installing front dampers (shock absorbers) that have adjustable height spring perches. Dampers with adjustable spring perches allow fine adjustment of static ride height and provide the most flexibility, but they are relatively expensive, and fixed perch dampers will work with springs that have the right combination of stiffness and free length. Stiffer springs reduce the amount of travel for a given load, thereby helping to offset the loss of bump travel due to lowering. The least expensive way to lower the front is to simply cut one free coil off one end of each of the stock springs. This shortens and slightly stiffens the springs, and the pigtail left by this operation will compress and not be an issue once installed in the car. On the Spitfire 1500, this will result in the desired installed static coil length of about 7 inches. Another option is to cut about half a free coil off one end of each spring and then heat and carefully reform and flatten the cut ends. However, this is not easy to do correctly. The application of too much heat can ruin the temper of the springs, and getting two springs to come out the same is difficult and not guaranteed. In either case, this shortening of the stock springs will stiffen them only marginally (about 10 percent). While additional stiffness is good because it limits suspension travel and helps offset the loss of bump travel that comes with lowering, such a marginal increase is not enough. Another approach for the Spitfire 1500 is to install springs from a Spitfire mk3 or mkIV. These springs are shorter in free length but also softer than stock 1500 ones, and using them on a 1500 will lower the front end and result in an installed spring length of about 7¼ inches. Softening while lowering is bad though because the need for suspension travel is increased at the same time the amount of bump travel is reduced, thus greatly increasing the likelihood of bottoming-out the suspension. Therefore, using softer springs like those from an earlier model Spitfire on a 1500 is not recommended. Running out of suspension travel and hitting the bump stops is harsh, leads to sudden understeer and can cause loss of control and is potentially damaging. Replacement springs need to be stiff enough to be compatible with the reduction in bump travel but not so stiff as to make handling worse, so the right approach is to replace the stock springs with shorter and stiffer ones that have the right combination of stiffness and free length. I've developed a spring calculator that determines which combinations of spring stiffnesses and free lengths produce the targeted installed spring length of 7 inches on Spitfires and GT6's. Solutions from this calculator and good choices for sporty road going Spitfires are 350 pound per inch, 9 inch free length coil springs for the 1500, 325 pound per inch, 9 inch free length ones for the mkIV and 300 pound per inch, 9 inch free length springs for the lighter mk3 and earlier Spitfires. Compare these with the popular aftermarket "lowering" springs for Spitfires that are 330 pounds per inch, 9.2 inches free length. A good choice for GT6's with their added mass and additional front weight bias are 400 pound per inch, 9 inch free length springs. Such 2.5 inch inside diameter coils springs are available from a multitude of vendors. Below are profile photos of my 1978 Spitfire 1500 with 350 pound per inch, 9 inch free length springs and my 1968 Spitfire mk3 with 300 pound per inch, 9 inch free length springs. These front springs help result in much improved handling, and form follows function with an attractive, level and purposeful stance.
When lowering, ensure that enough suspension travel remains in bump. Lowering the car as described above will reduce bump travel by about half, leaving approximately 0.75 inches between the top of dampers and the elastic bump stops installed on them. However, due to the geometry of the front suspension, this translates into about twice as much travel at the tire, or about 1.5 inches. The spring rates called out above are approximately twice the stock factory spring rates and therefore are compatible with, and effectively compensate for, the factor of two reduction in bump travel. If it is necessary to gain more bump travel, then some minor trimming (up to 3/8 inch, or 1 cm) of the bump stops is typically possible.
One thing that lowering the front will also do is to push the camber of the front wheels toward the negative. An otherwise stock Spitfire will end up at around 1 degrees of initial negative front camber due to the lowering described above (i.e., with the lower A-arms are parallel to the ground at rest). Some amount of initial negative camber at the front is desirable in that it aids entry into turns. Motion toward additional negative camber is generated during cornering by the motion of the unequal length and non-parallel A-arms and rotation of the steering axis with positive caster. This helps offset the motion toward positive camber due to roll during cornering such that the net result is to square the tire with the ground for optimal contact and grip. A minor amount of initial negative camber like 1 degree at the front of a lowered and stiffened Spitfire can benefit handling without causing excessive or uneven tire wear or severely reducing straight-line braking distance.
As mentioned previously, front toe will be changed significantly by lowering, so toe must be measured and reset. 1/16 inch front toe-in as prescribed in the repair manuals is a good setting for all-around street use. I've developed a simple, quick and inexpensive way to make your own toe adjustments. Bump steer, which is the change in toe and steering behavior due to up and down motion of the suspension, is an unfortunate feature of the Spitfire, and lowering the front suspension exacerbates it by increasing the angle of the steering tie rods. While bump steer is acceptable with the aforementioned changes, shimming the steering rack to raise it (if possible without interfering with the crank pulley of the engine) can compensate some by making the steering tie rods more horizontal at rest.
Note that stiffening the front of a vehicle means more weight transfer occurs at the front, which typically increases understeer (everything else being equal), but the improved geometry gained from the lowering and stiffening described above largely offsets this and the net result is a car that handles much better.
The Spitfire rear suspension is a swing axle architecture. Much maligned and archaic today, some celebrated high-performance cars of the past like the "Gullwing" Mercedes-Benz 300SL of the 1950's and the Auto Union race cars of the 1930's employed swing axles. On the Spitfire, the transverse leaf spring and trailing radius arms connect to the swing axles via pivoting vertical links to locate the swing axles fore and aft. The swing axle suspension has the advantages of independence, simplicity, low cost and low unsprung weight, but it has the disadvantage of poor camber control and potentially dangerous handling behavior. The defining characteristic of swing axles is that the axles shafts are coaxial with the hubs and so the shafts are normal (plane perpendicular) to the wheels. There is only one articulating joint, which is the u-joint at the differential. Swing axles serve as both driveshafts and primary suspension links and because swing axles themselves are the suspension swing arms, which are short, the design has a high rate of camber change. The roll center is particularly high and thus is prone to “jacking," which is progressive raising of the car due to braking and turning forces. During a turn, if the outboard swing axle (which is taking most of the load during the turn) angles up toward the differential (which is sprung weight attached to the chassis), then turning force transmitted to the sprung mass has a component pointing up that raises the rear end of the car, i.e., the axle helps to push the rear of the car up even higher. As the effect progresses, it gets worse and worse, camber gets more and more positive and grip becomes less and less. Quickly, grip is reduced to the point that the rear tires break loose and the car goes into violent oversteer.
Reducing or precluding jacking and controlling rear camber change are paramount objectives of swing axle control. Methods include lowering the rear/adding negative camber, reducing roll stiffness while increasing droop stiffness, and lengthening the swing axles.
Swing Axle Ride Height and Camber
Camber of swing axles is solely determined by the height of the differential relative to the hubs and the length of the swing axles. Because the differential is sprung mass attached to the chassis, this means rear camber of a given set of swing axles is determined by rear ride height. Lowering the rear generates negative camber; raising the rear generates positive camber. Initial negative camber on swing axles helps two ways. First, it lowers the rear c.g. and roll center, which reduces jacking during braking and turning. Second, it "pronates" the swing axles, angling them so the ends at the differential are lower than the ends at the wheels, which means that during a turn, the rear of the car has to rise and roll more before the angle of the outboard axle points up toward the differential (i.e., assumes positive camber) to add more jacking force.
If your rear end is actually too low or if the spring feels like it has lost its spring, one remedy is to renew the thrust buttons. Thrust buttons are little disks captured in dimples at the ends of the leaves and serve as bearing surfaces between adjacent leaves. The stock buttons are made of a rubber or rubber-like material, and by today many are crushed or missing so as to be ineffective. Popular substitute materials are Teflon and Delrin (Teflon creeps; something with better dimensional stability like Delrin is more deterministic).
The buttons need only be thick enough to barely separate the leaves at the button locations, and I mean barely (e.g., 0.040"/1.0mm net separation, or less) because spring arc and hence rear camber after renewal are very sensitive to leaf separation and hence button thickness. You can fabricate your own buttons pretty easily from rod stock. Alternatively, some vendors sell versions of replacement buttons, as does The Dutch Triumph Spitfire Club. Button replacement is a good and much less expensive alternative to buying a whole new spring. Unless the leaves have lost their temper or have otherwise been damaged, there's no reason to replace all that spring steel. To install new thrust buttons, simply remove the spring assembly, disassemble, replace the thrust buttons and reassemble. To reduce the chance of buttons falling out, a dab of flexible adhesive (e.g., RTV silicone) between button and leaf is a good idea, as long as the adhesive sticks to the buttons (good luck with Teflon). Another method that works with slippery buttons is to drill a hole through each button, countersinking the bearing-surface end to create a key, and inject the adhesive through the button hole and to the leaf. A more serious alternative is to drill a hole through each button and leaf (it's spring steel, so have many sharp bits on hand) and rivet them together, countersinking the holes in the buttons so that the heads of the rivets are recessed and don't contact the neighboring leaves. It's also a good idea to clean, repaint or coat the leaves and lubricate them before reassembly. Lubrication will ensure that the leaves slide freely and reduces wear on the buttons. Button renewal will change the arc of the spring and hence raise the rear end, so reinstall the spring and settle the car, remeasure rear camber and then fabricate and install a lowering block if necessary to set the desired ride height and static rear camber.
OK, after all that about swing axles, there is another small chassis Triumph rear suspension scheme that offers superior camber control and more tuning flexibility. Recall that swing axles are, by definition, both primary suspension links and drivetrain. A way to radically reduce a Spitfire's rate of rear camber change and lower the rear roll center is to eliminate the swing axles altogether and upgrade to a fully-independent multi-link rear suspension architecture in which the drive elements are not suspension links. Triumph achieved this with the "rotoflex" suspension it used on the GT6 mk2, a.k.a. GT6+, and early GT6 mk3 (rotoflex was used on the 2 liter Vitesse mk2 too but the GT6 parts are compatible with the Spitfire). Instead of swing axles, a lower wishbone plus a radius arm on each side serves as the primary lower suspension link. The transverse spring is the upper suspension element and a different vertical link houses the hub bearing. The drivetrain element is a two-piece shaft connected by an elastic "donut" that is stiff enough in rotation to transmit drive torque yet flexible enough orthogonal to rotation (i.e., along the direction of the shaft) to allow the assembly to vary in length (plunge and extension) and thus permit inboard and outboard motion of the wheels and proper suspension articulation.
This last point is absolutely key, because one cannot simply add a lower wishbone and keep the swing axles. In a fully-independent multi-link suspension, the vertical links holding the hub bearings rotate on virtual swing arms that are much longer than the distance to the differential, so they move on different arcs than swing axles do and thus move inboard and outboard relative to the differential with vertical motion. When such a suspension is used at the drive wheels, the driveshafts must be able to vary in length as well as tip and tilt at both ends or the whole linkage will be over-constrained and cease to be a suspension.
The rotoflex suspension has the distinct advantage of reduced camber change, a lower rear roll center and greater tunability, but it is much heavier than the swing axle setup (about twice the weight), has a higher parts count and is difficult to maintain. The cast iron vertical link is rather massive and unsprung (but some of the extra weight is due to the brakes being larger than a Spitfire's). The lower wishbone is cast iron and is partially unsprung, and the elastic rotoflex donuts are heavy, mostly unsprung, prone to wearing-out and difficult and expensive to renew. One other note on this setup is that it has a 49 inch track (1 inch wider than the "short swing axle" Spifires and GT6 mk1, but 1 inch narrower than the "long swing axle" cars). So, the GT6 mk2 rotoflex design is a compromise. Given the swing axle's advantages in simplicity and reduced parts count, low cost, low maintenance and reduced mass, plus things like the swing spring to compensate for its primary disadvantage, it's not a mystery why swing axles reappeared in the GT6 mk3 and persisted in the Spitfire.
There is one very good upgrade to the rotoflex design that addresses most of its shortcomings, and that is to replace the rotoflex driveshafts themselves with constant velocity (CV) joint driveshafts. Canley Classics has developed such a driveshaft, deriving it from one that Triumph used in its front wheel drive 1500. The Canley driveshafts weigh less, are easy to install, have a smaller envelope and require essentially no maintenance.
Canleys also offers lighter weight alloy vertical links and aluminum wishbones to significantly reduce mass (the wishbones are curved to clear the rotoflex donuts, but CV driveshafts permit straight ones).
The combined mass savings of the Canley driveshafts, alloy vertical links and aluminum wishbones is roughly 5 kg per side, reducing total mass of the setup by about 25 percent and splitting the difference in mass between the rotoflex and swing axle setups:
GT6 mk2 rotoflex setup = ~20 kg one side (includes brakes and radius arm)
Spitfire swing axle setup = ~11 kg one side
Canley setup with Canley driveshafts, alloy links and aluminum wishbones = ~15 kg one side
Switching from swing axle to rotoflex-type rear is not a bolt-on but sort of close to it. First, brackets for linking the wishbones to the frame must be welded to the frame as on the GT6 mk2. Canleys sells brackets with three different pairs of holes for tuning the suspension geometry (specifically, this changes the angles of the wishbones and varies the lengths of the virtual swing arms and therefore changes the rates of camber change and relocates the roll center). Second, the rotoflex donuts are so large that they interfere with regular Spitfire dampers running from the vertical links to the chassis. A different, longer rear damper specific to the GT6 mk2 must be employed and an upper attachment for them must be welded to the tub in the wheel well. However, if the Canley CV axles are used then regular Spitfire length dampers will clear and can be used and attached as usual. Third, the ends of the brake lines run differently on the smaller Spitfire rear brakes versus the larger GT6 rear brakes, so some plumbing modifications must be made. Fourth, rear roll stiffness is not the handicap that it is with swing axles, so it's not so detrimental to use a fixed transverse spring (rotoflex GT6's and Vitesse's had fixed rather than swing springs). Lastly, different radius arms and mountings need to be used. GT6 mk2 radius arms are adjustable in length and are attached further inboard on the GT6 mk2 tub heelboard than on the Spitfire. The inboard location puts the trailing arm inner pivot in line with the inboard, frame-mounted pivot point of the wishbone so that the combo of wishbone and trailing arm becomes one large triangulated articulating A-arm assembly. So, new radius arm mounts need to be welded to the tub heelboard as in the GT6 mk2 configuration. An alternative is to substitute an entire GT6 mk2 frame for a Spitfire one, which is more practical when doing a complete rebuild, but at the very least new inner radius arm attachments still have to be installed on the Spitfire tub. I'm doing precisely this to produce a modified "roundtail" early Spitfire-bodied version of a GT6 mk2. Paul Tegler has documented a discussion of this conversion applied to his highly-modified '75 Spitfire.
Now, moving on to the topic of anti-roll bars (ARBs), also called anti-sway bars. If you want to add more roll stiffness without adding vertical stiffness, then you can add anti-roll bars. ARBs are simply torsion springs that act only when the car rolls. A thicker bar is a stiffer bar, and a little goes a long way because stiffness goes with the 4th power of bar thickness. The swing spring Spitfires have a 7/8 inch diameter front ARB, but an aftermarket 1 inch bar is 1.7 times stiffer; the fixed spring Spitfires have a 11/16 inch bar, but a stock 7/8 inch Spitfire 1500 bar is 2.6 times stiffer. All else being equal, increasing roll stiffness by means of a thicker front ARB will bias roll stiffness toward the front and tend to make a car understeer more. However, on a Spitfire, installing a thicker front ARB may not result in too much understeer. The added front roll stiffness and consequent tendency to understeer is offset somewhat by the improved geometry of the suspension and tire contact due to reduced body roll during cornering. See my table of Spitfire and GT6 spring and wheel rates to see how different front ARBs contribute to vehicle roll stiffness.
Adding rear roll stiffness by fitting a rear ARB is typically not a wise idea on a swing axle car because it exacerbates jacking (note that although it is not disastrous to use a rear bar on a Spitfire equipped with a swing spring, it negates one of the advantages of the swing spring over the fixed spring). This is why zero-roll stiffness devices that add vertical stiffness to limit camber change, like Z-bars and leaf-style camber compensators, have been applied to various swing axle cars for decades. In fact, an ARB, which adds roll stiffness without affecting vertical stiffness, is the mechanical converse of a camber compensator, which adds vertical stiffness without affecting roll stiffness. Reducing or precluding jacking and controlling rear camber change are paramount objectives of swing axle control.
If switching to a multi-link independent rear architecture like the rotoflex, then adding a rear ARB is an option, and keeping a thinner bar up front is OK. The point here is that the multi-link independent rear offers more flexibility of tuning with ARBs in that both front and rear bars may be employed and tuned separately to achieve a wide range of roll stiffnesses and front to rear roll stiffness biases.
Sometimes ARBs are unintentionally preloaded on one end or the other because they are not flat and/or the mounts on the frame rails are not plane parallel with the A-arm attachments. The result can be a Spitfire that leans to one side or the other due to the jacking effect of the asymmetric preload. A way to eliminate this and set the ARB to zero preload at rest is to replace the factory ARB links with adjustable length ones and adjust them. Jon Wolfe sells adjustable ARB link kits, or you can kit your own from rod ends, jam nuts, spacers and bolts.
Dampers, also called shock absorbers, are critical to the dynamic behavior of the car, i.e., the nature of the car while it is moving and in transition from one suspension "set" to another, like when the car is initiating (entering) or completing (exiting) a turn or going over bumps and dips. Damper settings have a significant effect on ride and handling. Whereas force from linear springs is proportional to distance of motion, i.e., the amount of spring compression (spring force = spring rate * compression distance), viscous dampers exert force proportional to the speed of motion (damper force = damper piston velocity * damping coefficient).
The right amount of damping makes a big difference in handling. Too little damping causes the car to oscillate after hitting bumps or changing directions. This makes it difficult for the driver to provide the right steering input at the right times and therefore makes the car difficult to control. Moreover, the car can oscillate so much as to lose contact with the road, resulting in loss of control. Too much damping and the car becomes too rigid in transition and the slightest bump or steering input causes one or more tires to lose contact with the road, also resulting in loss of control. A good damping ratio for a sporty car is around 0.3 (the ratio of the actual damping coefficient to the critical damping coefficient of the spring-and-mass system). If you change to stiffer springs and have adjustable dampers, don't crank up the damping too much. The right amount of damping varies as the square root of spring stiffness because critical damping is proportional to the square root of spring stiffness, so to maintain a given or optimum damping ratio, the amount of damping should be changed in proportion to the square root of stiffness. Premium dampers can be adjusted in both the bump (compression) and rebound (droop or extension) directions--sometimes independently, and bump and rebound damping affect handling in different ways.
Bump damping affects the corners of the vehicle being loaded and is used primarily to control the motion of the unsprung weight of the vehicle (wheels and tires, hubs, and portions of the suspension links, springs and drive axles). Bump damping adjustments should not be used to control the downward movement of the vehicle when it encounters dips, nor should it be used to control roll or bottoming. The ideal bump setting for a vehicle can occur at any point within the adjustment range of the dampers and depends on many variables. This setting is when "side-hop" or "walking" in a bumpy turn is minimal and the ride is firm and responsive but not uncomfortably harsh. Any additional bump damping beyond this ideal setting and the "side-hopping" condition will be problematic and the ride may be too harsh. If there is too much bump damping at the front, then upon hitting a bump the whole front of the vehicle--both the sprung and unsprung weight--will move upward, reduce or lose contact with the road and "walk" off the road first, resulting in understeer. Similarly, if there is too much bump damping at the rear, then upon hitting a bump the whole rear of the vehicle--both the sprung and unsprung weight--will move upward, reduce or lose contact with the road and "walk" off first, resulting in oversteer. However, sufficient front bump damping is useful for reducing corner entry understeer, and likewise sufficient rear bump damping is useful for reducing corner exit oversteer. Clearly, an optimum "Goldilocks" setting is desired.
Rebound damping affects the corners of the vehicle being unloaded and is used primarily to control transitional roll behavior, i.e., how the vehicle leans when entering and exiting a turn. Remember that dampers do not limit the total amount of roll but rather control the rate of roll, or much time it takes the vehicle to achieve a given amount of roll. Total, steady-state roll is determined by other things like the stiffness of the springs and anti-roll bars and the locations of the roll centers and center of gravity (c.g.), etc. Too much rebound damping on either end of a vehicle can keep the suspension from extending fast enough to keep the tires in good contact with the road and so cause an initial loss of grip, which will make the vehicle oversteer or understeer excessively when entering or exiting a turn. Specifically, rebound damping at the rear affects corner entry because the vehicle is decelerating and weight is being transferred from rear to front during corner entry, so too much rear rebound damping can lead to corner entry oversteer. Rebound damping at the front affects corner exit because the vehicle is accelerating and weight is being transferred from front to rear, so too much front rebound damping can lead to corner exit understeer. Thus rebound damping adjustment can also be used to help tune the way the vehicle behaves entering and exiting corners. Moreover, too much rebound damping in relation to spring rate will cause a condition known as "jacking down," wherein after the vehicle hits a bump and the spring is compressed, the damper prevents the spring from returning to a neutral position fast enough before the next bump is encountered, which compresses the spring some more and so the whole vehicle falls or jacks downward. This can repeat with each subsequent bump until the car is lowered onto the bump stops. Contact with the bump stops causes a sudden and drastic increase in suspension stiffness, upsetting the car and resulting in a loss of contact and grip. If this condition occurs at the front, the car will push/understeer; if it occurs at the rear, the car will get loose/oversteer.
For street use, you can use the stock variety of dampers, even if you modestly increase the stiffness of the springs. They won't be ideal if you stiffen and lower the car, but they are the least expensive way to go. However, beware the dimensions of some Spitfire dampers sold as stock replacements. Some such front dampers have spring perches that are too high, i.e., the distance between the center of the lower A-arm mounting and the lower spring perch is 3.875 rather than 3.25 inches. Using such dampers will cause the front end to sit noticeably too high, resulting in a higher c.g., suboptimal suspension geometry and poor handling:
Also, beware the compressed length of some rear dampers sold as stock replacements. The fully compressed length of rear dampers grounded on the bump stops should be no more than 9.5 inches (eye center to eye center), not 10.5 inches like some "stock" replacements. A Spitfire with 3 to 4 degrees negative rear camber will have rear dampers compressed to about 10.5 inches, so fitting such incorrect dampers leaves no rear bump travel, resulting in poor handling and noticeable oversteer. The remaining 1 inch of damper bump travel provided by the correct dampers, which translates into about 1.6 inches of tire travel due to the rear suspension geometry, is necessary and adequate:
Some medium priced dampers like KYB Gas-A-Just are good medium-quality dampers, but they do not have adjustability. If you don't mind spending some money, then good quality adjustable dampers (e.g., Koni) will do nicely and will allow you to more finely tune the car's handling characteristics. Some dampers available for the Spitfire and GT6 (e.g., Spax) allow damping adjustments in-place, without disassembling the suspension, and also include adjustable spring perches on the front units for adjusting ride height without changing the springs.
To fit them to the Spitfire, just press-out the bushings they come equipped with (including the metal tubes in the lower bushings) and substitute ones from Spitfire dampers. If you make separate air feeds for each of the two dampers, you can adjust each side independently, which is one way to corner jack the car and compensate for a minor side-to-side ride height difference.
COMPRESSION (BUMP): Adjust bump damping first, before adjusting extension (rebound)
1: Set all four dampers on minimum bump and minimum rebound settings.
STEP 2: Drive one or two laps to get the feel of the car. Note: When driving the car during the bump adjustment phase, disregard body lean or roll and concentrate solely on how the car feels over bumps. Also, try to notice if the car "walks" or "side-hops" in a rough turn.
STEP 3: Increase the bump setting three (3) increments on all four dampers. Drive the car one or two laps. Repeat this step until a point is reached at which the car starts to feel hard over bumpy surfaces.
STEP 4: Back off the bump adjustment two (2) increments. Note: The back off point will probably be reached sooner on one end of the vehicle than the other. If this occurs, keep increasing the bump on the soft end until it, too, feels hard, and then back it off two (2) increments. Bump damping is now set.
EXTENSION (REBOUND):Adjust rebound damping only after setting bump damping
The final segment here under handling is braking. Brakes are of course important for safety, but good braking is key to good handling and crucial to making a car faster if you want to do more than just go in a straight line. Besides, fast effective braking is fun! Braking is simply the conversion of the kinetic energy of motion into waste heat energy. Braking force is the product of the coefficient of friction of the braking material and caliper piston force, and piston force is proportional to total caliper piston area, so larger calipers with more total piston area and pads with higher coefficient of friction material will improve braking. Brake effectiveness is also dependent on the ability to eliminate or dump waste heat, so bigger rotors, vented rotors and larger pads will dissipate heat better and yield better braking with less fade. Spitfire brake components in good working order make for adequate braking, but there is a range of enhancement options to consider. The simplest and most significant change you can make for the better with the stock setup is to use premium brake pads. Mintex, Hawk and Porterfield brand pads are good upgrades. Another easy and relatively inexpensive upgrade is to replace the stock rubber flex lines with less complaint ones, typically covered in braided stainless steel. The stock flex lines connecting the hard lines on the frame to the brakes on the suspension swell a little bit under pressure. Replacing them with less compliant ones will transfer more braking force faster to the brakes, improving performance and feel. There are many alternative hardware options for further enhancing braking. One relatively easy and essentially bolt-on upgrade is to install GT6 brakes. GT6 caliper piston area, pad area and rotor area are all greater than that of Spitfire brakes. Swapping front brakes involves substituting GT6 vertical links, spindles, hubs, wheel bearings, rotors and calipers for the Spitfire parts (everything outboard of the A-arms); you can't substitute just rotors or calipers due to component compatibility. The rear drum brakes of the GT6 are larger too, and these parts can be substituted as well, but this is not as important and not absolutely necessary. Because the process of braking transfers weight to the front, the front brakes do most of the braking and are most important. Another, nearly bolt-on upgrade that is readily available and relatively inexpensive is to install GT6 hardware but use '79-83 Toyota pick-up truck front brake calipers (you can modify the following adaptation of Toyota brake calipers to TR Triumphs for fitment to the Spitfire). Make sure your wheels will accommodate the larger GT6 or Toyota calipers without interference. Lastly, there are several, albeit relatively expensive, aftermarket brake kit options using parts from manufacturers such as Alcon, AP and Wilwood that can significantly improve brake performance.
Handy metrics of braking capability are total swept area of braking surface and swept area per ton of vehicle weight. Typically the greater these values, the better the braking. The greater the swept area, the more fade resistant the brakes are likely to be, and the greater the swept area per ton, the shorter the braking distance is likely to be. Early Spitfires (Spitfire 4/mk1 and mk 2, which have 12P front calipers) have 199 sq. inches of total swept area (144 sq. inches front discs and 55 sq. inches rear drums) yielding around 249 sq. inches/ton. Later Spitfires (those with 14P calipers, i.e., mk3, mkIV and 1500) have 205 sq. inches of total swept area (150 sq. inches front discs and 55 sq. inches rear drums), yielding 248 sq. inches/ton (mk3) to 219 sq. inches/ton (late U.S. market 1500). For comparison, upgrading to GT6 brakes leads to 260 sq. inches swept area (197 sq. inches front discs and 63 sq. inches rear drums) and makes possible 325 sq. inches/ton on the earliest Spitfires (31 percent increase) to 277 sq. inches/ton on the late U.S. market Spitfire 1500 (27 percent increase). The results are summarized in the following table:
For more insight and information on vehicle handling and suspension design, check out:
How to Make Your Car Handle, by Fred Puhn
Tune to Win, By Carroll Smith
Many motorcycles can out-accelerate the hottest performance cars, even though they have much less power because proportionally they weigh even less such that the ratio of their power to their weight is much higher than that of most cars. The easiest and cheapest way to make your Spitfire feel more powerful is to shed mass and improve the power-to-weight ratio. As mentioned earlier under handling, this is easy to do on the later Spitfires in the U.S. by removing all the extra bumper and bumper reinforcing hardware they were burdened with.
A good metric of performance is power per unit mass (or weight). Horsepower at the wheels divided by total test weight of the car is the ideal metric, but since wheel horsepower is typically not published information and has to be obtained on something like a rolling road that introduces additional variables, peak brake horsepower (bhp) is a good proxy for relative comparison purposes. Below is a plot of actual test data of Spitfires (from "Triumph Spitfire Gold Portfolio") and some assorted vehicles circa 2011 (from "Road & Track" test reports) spanning the range, from the 70 bhp Smart fortwo coupe to the 1001 bhp Bugatti Veyron Grand Sport supercar, plus the 119 bhp Ducati 848 EVO motorcycle for comparison. The trendline is a best-fit of the actual data (weights are as-tested and include driver weight), and given the mathematical relationship it is a straight line on a log-log plot. The point of the plot is to illustrate the relationship between acceleration and the power-to-weight ratio. The greater the power-to-weight ratio, the quicker the vehicle; each doubling of bhp/ton cuts acceleration time approximately in half:
"Adding lightness" is very high return for lower bhp/ton cars like factory-spec Spitfires. Note that driver weight is a larger proportion of total test weight for lighter cars. In the case of a Spitfire, the weight of the typical driver is around 10 percent of the car's weight so the driver's weight has a bigger effect on a Spitfire's performance than it does on a heavier car like a Ford Mustang. Having a passenger or some cargo aboard your Spitfire will noticeably reduce acceleration. Besides shedding vehicle weight, shedding driver weight is something to consider! This underscores why horse jockeys are small, and why minimum weight constraints exist for horse racing and auto racing. For a typical Spitfire, a reduction of 25 to 30 pounds is equivalent to adding one horsepower (not counting other benefits). Roughly speaking, increasing Spitfire engine peak power to 90 bhp and beyond (i.e., 1 bhp or more per cubic inch displacement) enables crossing the 100 bhp/ton, 10 second 0-60 threshold. Clearly, you can take your Spitfire's quickness from pathetic to acceptable by increasing bhp/ton by losing weight and adding power.
The following is basically about better breathing to get the fuel and air into and the exhaust gases out of the engine more effectively (carbs, exhaust, head work, cam), and increasing the engine's thermodynamic efficiency so as to better convert the potential chemical energy of the fuel+air charge into kinetic energy (ignition timing, higher compression).
Ignition timing is critical to Otto cycle engine performance. Why? For best performance, maximum pressure developed by the burning mixture in a cylinder should occur a little after the piston has passed top dead center (TDC) and is on its way back down the cylinder to take advantage of the mechanical leverage of the position of the piston connecting rods and the crankshaft and optimally develop torque. Developing pressure too early or too late wastes performance potential, and in the extreme can lead to damage. But it takes a finite amount of time for the combustion, initiated by the spark, to propagate throughout the mixture in the cylinder. Pressure, temperature, motion of the mixture and fuel grade all affect combustion speed--pressure being the most important factor. To account for the finite amount of time it takes combustion to occur and for max pressure of combustion to be reached at the optimum position of the piston, the spark needs to be "advanced," typically to occur some number of degrees before the piston reaches TDC. As engine speed increases, the speed of combustion doesn't increase as fast as the engine, so progressive spark advance is needed to keep the max pressure of combustion occurring at the right time. This is true to first order up to a point, because as engine speed (RPM) increases, pre-combustion cylinder pressure (up to the limits of the engine's induction system to flow mixture), mixture motion and fuel atomization increase too, and these things speed-up combustion. Beyond around 3000 to 4000 RPM in most engines, these effects keep pace and so no further advance is needed. Finally, the amount of load on the engine affects optimum spark timing. Less advance is needed when accelerating, with the throttle wide-open and lots of air is cramming into the engine and raising absolute pressures inside it, as opposed to when cruising or decelerating and pressures are reduced. Therefore, at a given RPM, more advance is needed at low engine loads (i.e., low absolute manifold pressures).
As you progress with engine modifications, the best amount of total ignition advance for a given engine speed and load changes, so to fully reap the benefits of engine performance modifications, it pays to revisit your ignition timing when making performance changes. For example, as you raise an engine's compression, less advance is needed at a given engine speed, and you want to be careful not to have so much advance that you encourage detonation (a.k.a., knock, pinging, pinking). If you go with a "bigger" cam, volumetric efficiency and dynamic compression are reduced at low RPM but increased at high RPM, thereby reducing combustion speed at low RPM but increasing it at high RPM and requiring an ignition map with a different shape and a steeper "slope" (camshafts are discussed later in this article). So to get the most out of a given camshaft and overall intake setup, ignition timing is crucial to preserving low RPM behavior and maximizing high RPM performance. "More" is not "better" when it comes to advance. Don't be fooled by an ignition map with big advance numbers everywhere. Proper spark timing is about optimizing performance for a given engine configuration.
recently and before the advent of electronics, spark timing on most
Otto cycle engines has historically been determined and controlled by
the distributor. Static advance is set by the position of the
distributor such that the rotor and plug wire electrodes in the
distributor cap are aligned relative to piston position at rest as
desired (e.g., to set timing at idle). Dynamic advance with a
distributor is accomplished via two mechanisms--centrifugal and vacuum.
Centrifugal advance adjusts spark timing as a function of engine speed
and works this way: little weights constrained by little springs inside
the distributor fling outwards more and more as the distributor spins
faster and faster with increasing engine speed, up to a limit imposed
by pins or some other hard stop. The stiffness of the springs and the
mass of the weights controls how far the weights move for a given
speed. The outward motion of the little weights rotates a plate that
holds the spark triggering mechanism (e.g., a cam actuating contact
points, or a multi-pole magnet interacting with a Hall effect sensor,
a chopper wheel interrupting the light path between a light source and
an optical sensor, all of which function as switches to interrupt
current flowing in the primary winding of the coil and resulting in
magnetic field collapse and induction of high voltage in the secondary
winding of the coil). Vacuum advance (or sometimes retard) adjusts
spark timing as a function of engine load, where vacuum in the intake
system is used as a measure of engine load (e.g., cruising,
accelerating or decelerating). Vacuum can be sensed via a tap or
multiple taps on the intake manifold, or a port near the throttle
plate. A vacuum line goes from the intake vacuum tap to a vacuum
canister on the distributor containing a diaphragm, and the diaphragm
is attached to a link that is attached to the plate holding the spark
trigger mechanism, so motion of the diaphragm, in response to intake
vacuum, can advance (or on some distributors, retard) the spark.
Convert from Points to Electronically-Triggered Ignition, or to Fully-Electronic Ignition
Something to consider is making the switch from a mechanical spark trigger (points) to an electronic spark trigger (Hall effect (magnetic) or optical trigger), i.e., electronically-triggered ignition. The Pertronix series of products are easy, reliable drop-in replacements that take the place of points in your distributor. Switching from points to electronic ignition can enhance performance and will reduce maintenance and improve reliability.
A worthwhile modification to make, especially if you make other engine mods, is to eliminate the distributor and replace its function with a fully-electronic ignition control system like MegaJolt or the ignition portion of the MegaSquirt fuel management system. These systems, which sense engine position using a toothed "trigger" wheel attached to the crankshaft pulley rather than the shaft of the distributor, enable precise and fully-programmable ignition advance settings vs. engine speed and load. These are a bit more involved than simply replacing the points assembly in the distributor with a little electronics module, but they work well and have a large user community, so support is readily available. I installed MegaJolt on my modified '78 Spitfire 1500 and I'm thrilled with it.
So, why not stick with a distributor instead of going with a fully-electronic, programmable system for controlling ignition timing? Two main reasons. First, changing the advance map on a distributor is very difficult, time consuming and not really deterministic. It involves adding (welding on) or subtracting (grinding off) mass from the centrifugal weights, changing the centrifugal weight springs, and changing the range of motion that the vacuum/retard mechanism makes or imparts to the distributor. Being able to simply change spark timing incrementally at the click of a mouse, as with MegaJolt, is completely deterministic, much faster and allows results to be assessed immediately before environmental conditions change that can affect interpretation of results (like air temperature, humidity, etc.). Moreover, ignition timing maps can be implemented that simply are not physically possible to create with a mechanical system (if such a thing is warranted). Second, fully-electronic systems like MegaJolt eliminate several mechanical interfaces involved in communicating piston position and delivering the spark that add uncertainty and reduce precision in spark timing. In a traditional distributor-equipped engine, the crankshaft turns the camshaft through a chain or belt, and the camshaft incorporates a gear that engages and turns the distributor shaft, which actuates the centrifugal advance weights and springs that actuates a mechanism for interrupting primary current to the coil as well as turns a rotor that spins past contacts in the distributor cap to distribute spark to each spark plug. In the case of many fully-electronic ignition schemes, piston position is sensed directly off the crankshaft by the variable reluctance sensor "looking at" the toothed wheel and all spark triggering, switching and distributing is done purely electronically, and so the timing "slop" contributed by the rest of the mechanical interfaces in a conventional distributor-based ignition system are eliminated, thereby resulting in much more precise timing and virtually eliminating jitter. Lastly, all-electronic ignition systems sense engine load, via a measure of manifold pressure using a pressure transducer or throttle position via a potentiometer, more accurately than a vacuum diaphragm attached to a distributor plate carrying spark triggering components.
you want to retain a distributor and still have the advantage of
electronically-selectable alternative advance curves, a good
aftermarket option is 123Ignition. Standard 123Ignition
distributors have 16 different preset curves to choose from, and there
is a line of programmable distributors that afford further
customization. 123Ignition is a nice, simple drop-in alternative to
systems like MegaJolt and MegaSquirt.
Better breathing is about making it easier for gases to get in and get out of the engine and it can cost some money, but the expense is not unreasonable for the corresponding improvement in performance and many of the following enhancements are relatively easy to make, so they fit the theme of this article.
purpose of the exhaust system is to enable the evacuation and
scavenging of waste exhaust gases and even aid the intake of the next
cycle's fresh air+fuel mixture. When an exhaust valve opens, a
compression wave of acoustic energy rushes from the valve, out of the
head and down the manifold or header tube at the speed of sound, which
is about 1700 feet per second (roughly 20 inches or 50 cm per
millisecond) at such temperature. The actual exhaust gas follows behind
as a chunk of mass moving at a subsonic speed of about 300 feet per
second (roughly 3.6 inches or 9 cm per millisecond)--about five times
slower than the acoustic pressure wave ahead of it. The moving mass of
exhaust gas leaves behind a wake of low pressure as it moves along, and
this low pressure helps suck gas from the cylinder. This is inertial
pumping. When the faster-moving acoustic pressure wave reaches a
transition where it experiences a sudden expansion, like at a collector
where exhaust tubes from other cylinders join together, part of the
acoustic compression wave continues down the exhaust system but part of
it is reflected back up the same manifold tube as an expansion or low
pressure wave. If this reflected low pressure wave reaches the exhaust
valve late in its cycle while it is still open and when the intake
valve is also open early in its cycle, i.e., during valve overlap, then
it will reduce the pressure inside the cylinder, which will not only
extract remaining exhaust gas from the cylinder but also suck fresh
air+fuel mixture into the cylinder. This is acoustic pumping. A
performance exhaust system is designed to make use of both inertial and
acoustic pumping to improve the engine's volumetric efficiency and
An exhaust manifold or header is sort of like a tuned musical instrument. Pipe lengths and diameters determine its 'tune'. Longer pipes mean longer travel times, resulting in more torque at low engine speeds below the torque peak but less torque at high engine speeds above the torque peak. Larger diameter pipes mean slower gas flow (longer travel times) but less backpressure and reduced pressure loss with pipe length, shifting where peak torque occurs to higher engine speeds. Thermally-insulated pipes keep heat in the exhaust for higher exhaust gas temperatures, which means faster gas flow (but also a benefit in reduced intake temperatures for improved volumetric efficiency). Collector sizes and shapes are also important to header performance in that they influence the strength of the reflected low pressure acoustic wave and affect the way the pulses of exhaust gas blend and flow together.
The stock manifold on the later U.S. Spitfires has tubes so short that beneficial acoustic scavenging never really happens within the operating RPM range of the 1500 engine, although they are do allow gas flows to combine so as to at least not interfere with each other and enable some degree of inertial pumping. Almost any header can help a later Spitfire's performance simply by enabling smoother flow and delaying the timing of acoustic pumping, but a properly tuned header of good design and workmanship can significantly improve performance by optimally timing acoustic pumping to really harness it.
For four-cylinder engines like the Spitfire's, there are 4 into 1 headers in which all the pipes collect together at once (like the stock U.S. spec 1500 manifold, only much longer; see image on left below), and 4 into 2 into 1, or Tri-Y systems in which pairs of pipes for cylinders with exhaust strokes 360 crank degrees (180 distributor degrees) apart collect together first to form a single pair of pipes that later collect into one. Collecting pairs of pipes that aren't separated by 180 distributor degrees would degrade performance because their exhaust valve openings would overlap and gas flows would collide instead of meshing and one primary would pressurize the other, thus impeding flow. For the 1-3-4-2 firing order of the Spitfire and most other inline four-cylinder engines, #1 and #4 cylinders collect and #2 and #3 cylinders collect together. These first pipes leaving the head are called primaries, and the second ones after the merging of primaries are called secondaries.
For street use, the 4-2-1 systems are best because they provide improved performance in the middle range and over a broader range of RPMs, even though they typically don't produce as much peak power as 4-1 systems. 4-2-1 systems 'play a chord' and have broader torque curves with more mid-range performance than 4-1 systems, which 'play one note' and produce more high-end torque and peak power. Four-cylinder race cars often use 4-1 systems because their engines spend basically their whole time operating at high RPMs at or around the peak power point, and maximum power is more important than broad power or power in the low and middle range.
Note that for inline six-cylinder engines, like the GT6 2-liter, there are two ways to have a 'compound' header like the 4-2-1: the 6-2-1 (two sets of three primary pipes gathering into two secondary pipes that gather into one) and the 6-3-1 (three pairs of two primary pipes gathering into three secondary pipes gathering into one). By far the better performing configuration is the 6-3-1. It combines pairs of pipes from cylinders with exhaust strokes that are 360 crank degrees (180 distributor degrees) apart and thus one exhaust valve is closed while the other is open and the operation is bascially the same as that described above for the 4-2-1 configuration. The Triumph inline six firing order is 1-5-3-6-2-4, so the correct way to gather primaries is 1+6, 5+2 and 3+4. Contrast this with the usual 6-2-1 configuration where cylinders 1, 2 and 3 are gathered and 4, 5 and 6 are gathered, all cylinders that are only 120 distributor degrees apart so exhaust valve openings overlap and backpressure is an issue. In V8s, cylinders are usually combined like two separate four cylinder systems. This is easy to implement for single-plane crank V8s because each side of the engine is configured like a single inline 4 cylinder so all cylinders on the same side of the engine easily combine as 4-1 or even 4-2-1. However, correct pairing of cylinders on cross-plane crank V8s is not so easy because ideally two cylinders from each side need to combine, so a practical alternative is to configure like two 4-1 sets of pipes connected by a balance pipe.
For more information about exhaust tuning theory and practice, see:
Exhaust Science Demystified, by David Vizard, from Popular Hot Rodding (February 2009)
How Headers Work, from Super Chevy (February 2009)
Four-Stroke Performance Tuning in Theory and Practice, by A. Graham Bell
K&N air filters are proven to flow more air than standard paper elements, and are sometimes even better than no filter. Some folks think they look nice too. You'll probably have to retune your carbs after installing K&Ns.
Install Short Air Horns on the Carbs
This will provide a smoother path for air to get into your carb(s). Short ones, called stub-stacks, will help performance a little yet still fit inside your air filters. Like the principle behind exhaust runner lengths above, air horns (sometimes called ram pipes) can help air enter the engine through tuning of inertial flow, and the longer the intake air horns are, the lower the RPM at which the ram effect occurs. Stub stacks are too short to tune the ram effect very much, but they do provide a radiused entry for air to go into the carb(s), which smooths airflow and does help performance a little.
While the single Zenith-Stromberg (Z-S) carb that came on the 1500 engines for the US market isn't bad, the manifold between it and the cylinder head has two abrupt right-angle turns in it. This 'log' manifold works, but a more direct path from carb to head can improve flow. Moreover, the ports of the US market Z-S log manifold are smaller that the ports on the cylinder head, so they are not matched. But changing just the manifold isn't really an option for the Zenith-Stromberg setup, so a good alternative is to install twin Skinners Union (SU) HS2 or HS4 model carburetors and a matching ram-style intake manifold like what the 1500s for other markets got from the factory. The twin SU setup in which one carb feeds two cylinders allows for a nice straight-through flow of the air and fuel mixture into the cylinders. The earlier US market Spitfire 1300s have this twin carb arrangement using HS2s, and this is a simple bolt-on upgrade from the single Zenith-Stromberg configuration. HS2s will work fine on a 1500, while HS4s will perform better than HS2s at high RPM on modified engines. Used SU carb and manifold assemblies are available from many suppliers for not too much money. New SU carbs are available too (manufactured by Burlen Fuel Systems) but they are more expensive than used ones. You'll need to figure out what needles and dashpot springs to use too, which will depend on the engine's state of preparation and your altitude and general climate conditions. For suggested SU needles for various stages of tune, see the "stages of improvement" section near the end of this article.
You can also try other carbs, like Weber or Dellorto side-draught carbs instead of SUs. These are more expensive, but they are capable of terrific performance.
At this point, the way to improve breathing is to focus on modifying the cylinder head to get it to flow better. This can get expensive, but some minor mods involving simply matching the ports of the cylinder head and the manifold and removing any irregularities in the ports that might disturb airflow are easy and inexpensive to do and will help performance if done correctly.
The capability of the cylinder head to flow gases and promote good combustion has enormous bearing on the engine's ability to generate power. People with much experience at wringing power out of engines have empirically determined the modifications that improve flow in cylinder heads to significantly improve performance, and some of this knowledge has been captured in various publications (e.g., Bell, Vizard). You can pay a professional to make head mods or you can try some of the more pedestrian ones yourself.
On intakes, it's good to match the head ports and the manifold ports. You don't want to enlarge them, just match them so that there's no step and the transition from manifold to head is virtually seamless. On exhaust ports, almost the opposite is best for performance. Empirical evidence indicates that a step from the head exhaust port to a slightly larger manifold port/header pipe is good. It reduces exhaust backflow and helps acoustic scavenging. Therefore, don't try to match exhaust head and manifold ports. However, do make sure there are no locations where the manifold ports are inside of/smaller than the head ports so as to protrude into the flow leaving the head.
Care must be taken when modifying the port runners in the head between the mainfold ports and the valves. Don't significantly alter the shape or size of the port runners, but rather remove sharp edges and transitions within the port runners that can disrupt flow. Just some modest grinding and polishing to clean-up leftover casting and machining irregularities and eliminate sharp transition areas is good. Often there is a little sharp edge just before the valve seats left over from casting and/or installation of the valve seats, particularly on the short radius side. Grind away the sharp edge and make a nice smooth transition while keeping a nice radius. It's also good to smooth the areas around the valve guides, but you don't need to remove a bunch of material here. You want to have a little bit of a venturi shape behind the valve seats; ideally a valve port throat that is about 90% of the valve diameter is good. Be careful not to get carried away and grind too much. Do not simply enlarge the port runners everywhere or you'll make flow worse. Tapered valve guides help reduce obstruction to flow too.
Also, it's beneficial to have flow that enters each chamber with some curve or swirl to it so that as the compression stroke occurs and the mixture gets 'squished' it mixes rapidly and well, which is conducive to good propagation of the combustion flame front from the spark plug to all reaches of the mixture in the combustion chamber. There is often a noticeable step between the machining left over from port drilling and the cast portion of the intake port runners in Spitfire 1300 and 1500 heads. Grind this to seamlessly blend them together. Rounding-off the little sharp edges of the bevel near the spark plug hole can help gas enter and exit the cylinder but it can let the curl of the swirl open-up, so it's a balancing act there. Don't degrade swirl by grinding away on the inboard side wall of the port runners except to blend discontinuities, and beware the little bump in the outer wall of the exhaust port runner near the exit--it's close to a stud hole.
What matters is flow through the head as it will be operating--with valves installed and open to varying degrees. You can achieve big flow through a head with wantonly enlarged runners and no valves installed, but that's not the operating configuration of the head and such flow numbers are misleading and will not be representative of the head in actual operation. Besides, flow quantity is only part of the story--high flow alone doesn't directly translate into more torque and power. Proper, efficient flow into and out of the cylinders with the valves present is what counts. Flow around valves in heads that have rather asymmetrical port flow like the Spitfire's will be improved by shaping the backsides of the valves to be less 'tulip' shaped and more 'nail head' shaped. A backside radius of about 0.2 times the valve diameter is about optimum for heads with asymmetrical port runners and 'bathtub' shaped combustion chambers like the Spitfire's (stock radius ratio is 0.25 to 0.3). Getting rid of that first ridge where the single stock valve grind meets the backside with a multi-angle grind helps flow around the valve, even if you choose not to tighten the radius on the backside. A multi-angle grind on the valve seats helps too, producing a nicely radiused venturi while ensuring adequate seat contact for good sealing and heat transfer (crucial for conducting heat from the exhaust valves to the head and coolant).
There are limits to what can be done to 'unshroud' the valves in Spitfire heads to further aid flow because the valves are very close to the walls of the combustion chamber recesses, and these recesses already reach out to and line up with the edges of the cylinders. But, if there is any difference (perhaps from boring the cylinders), then a little relief to the walls of the combustion chamber to unshroud the valves can help. As mentioned above, some rounding-over of the sharp edges of the beveled step near the spark plugs will serve to unshroud the valves some and improve flow into and out of the cylinders. Intake valves appear to benefit more by unshrouding than do exhaust valves.
Be sure to make the same mods from cylinder to cylinder and measure the volume of each of the combustion chamber recesses in the head. Perform any minor rework necessary to equalize all the volumes to within 0.2cc or less. Be sure to base any compression ratio calculations and planned adjustments (e.g., head shaving) on these measured values.
For more insight here, check out these references:
Four-Stroke Performance Tuning in Theory and Practice, by A. Graham Bell
Tuning Standard Triumphs Over 1300 cc, by David Vizard
Theory and Practice of Cylinder Head Modification, by David Vizard, et al.
How to Build, Modify & Power Tune Cylinder Heads, by Burgess and Gollan
Triumph Competition Preparation Manual
One last thing that fits under improved breathing is the camshaft. Changing the camshaft in most Spitfires can help, but only after other things have been done, like improving the exhaust and intake and head flow, and especially raising the static compression ratio of the engine, which is described next.
Increasing compression increases the efficiency of the thermodynamic cycle of the engine, which yields more torque and more power. Static compression ratio is simply the ratio of the volume above a piston when the piston is all the way down at bottom of its stroke (bottom dead center, or BDC) to the volume above a piston when the piston is all the way at the top of its stroke (top dead center, or TDC). If you think of the volume left when the piston is at TDC as Vcc (combustion chamber volume) and the volume swept by the piston as Vd (cylinder displacement volume), then the volume above the piston at BDC is Vd+Vcc and the compression ratio CR = (Vd+Vcc)/Vcc.
Vcc includes not only the volume of the recess in the cylinder head, but also the volume of the gap determined by the head gasket's thickness, the volume of the cavity in the top of a dished piston (or the negative volume of the space taken by a crowned piston) and what little other volume that might remain between the piston and the top surface of the block at TDC ("deck volume") or in the little gap between the piston and the cylinder wall and above the top ring (negligible for practical purposes). For Spitfire 1300 and 1500 engines, gasket volumes and deck volumes are all the same, so compression ratio is determined by the head volume and the piston type (dished or flat). On a Spitfire 1500 with 7.5:1 static compression ratio, as all North American models were (except 1976 models, which had 9:1 static compression), changing the pistons from the stock dished ones to flat-top ones will reduce Vcc by the volume of the dished space, which is about 6.7 cubic centimeters per cylinder, and thus raise compression to nearly 8.4:1. This helps performance some. You can raise compression further by substituting a compatible head with smaller combustion chambers and/or shaving/milling material off of the head surface to shrink combustion chambers. However, increasing compression increases combustion pressures and temperatures and elevates the likelihood of detonation and detonation-induced pre-ignition, which can damage or destroy pistons. A static compression ratio of 9.5:1 or 9.75:1 is the practical limit for a Spitfire street engine with a reasonable cam profile running on contemporary pump gasoline. More modern cars sometimes have higher compression ratios, but that's because they employ knock-sensing systems that dynamically adjust (retard) the ignition to avoid prolonged detonation. Race engines typically have very high static compression ratios, but they also have more aggressive camshafts with very long valve opening duration and overlap, which reduces the effective or real compression inside an engine while it is operating. Racing competition and the overarching objective of winning a race typically favors peak power over long-term reliability and low or moderate-speed driveability and justifies spending more for special, tougher materials (e.g., forged instead of cast pistons), frequent engine rebuild or swaps, and other expensive and time-consuming things.
Raising compression on a Spitfire, by swapping and/or shaving heads and switching pistons, will help performance. Changing pistons isn't too difficult but new pistons are moderately expensive. Swapping or shaving a cylinder head is easier and can be less expensive. Removing the cylinder head isn't too tough a task, and once you have it out of the car, a good machine shop can quickly remove the amount you desire for a very modest fee. I've constructed a compression ratio calculator spreadsheet that you can use to figure out how much you need to shave off a given head to reach a desired static compression ratio. I've also compiled information about interchangeable cylinder heads for Triumph 1300 and 1500 four-cylinder engines (see table below--still a work in progress) that can be used in conjunction with the compression calculator to generate various engine configurations and compression ratios.
Triumph 1300 and 1500 heads are interchangeable, and evidently were made in three castings (identified by different cast-in numbers) that in turn were machined different amounts to yield different combustion chamber sizes (stamped with different part numbers) for various car models for different markets around the world. All three castings accommodated valve seats for the same size exhaust valves, but different size inlet valves. Note that the hot ticket for the North American market "low compression" 7.5:1 1500 with dished pistons is to fit one of the "big inlet valve" heads, use either dished or flat pistons, and perhaps do some shaving of the head. The bigger inlet valves will support greater airflow and permit higher performance to be extracted from more extensive mods (like a bigger cam). For example, nearly 9:1 compression can be achieved on a "low compression" dished piston short block by simply installing a stock 218142 head from a world market (non-North American) late Spitfire mkIV (FH25001 and later). A little shaving of this head (about 0.040 inches or 1mm) before you install it and you'll have a 9.5:1 engine with big inlet valves and without taking anything apart in the bottom end. Or, take a head from a Toledo 1300 (part number 218141) and put it on a 1500 with flat-top pistons and get about 9.6:1. Some "big inlet valve" heads came in North American market cars, and these can be shaved and used with flat-top pistons to get the same result. Another option for the "low compression" 1500 is to replace the dished pistons with flat-top ones and shave about 0.080" off the "low compression" head it came with to produce an engine with a static compression ratio of about 9.5:1. Note that merely changing from dished to flat-top pistons in this instance will not result in 9:1 but rather 8.4:1 because the 9:1 1500 configuration also uses different heads (e.g., TKC1155 and TKC2748) that have smaller combustion chambers. 1500's that started out as 9:1 engines can benefit from "big inlet valve" heads and/or shaving to raise compression too. 1300 engines, particularly the more robust "small journal" ones popular with racers, can be similarly upgraded by substituting shaved versions of one of the "medium" or "big inlet valve" heads. All of the above heads are interchangeable on 1300 and 1500 blocks, so beware of certain "bad" head, block and piston combinations that either raise compression too much for typical street use or result in a downsizing of inlet valves.
If you have an engine with raised static compression and improved flow characteristics, then it's possible to take advantage of a more aggressive camshaft (i.e., more valve lift and longer valve opening duration and overlap). The air+fuel mixture has inertia of course, so opening valves earlier and closing them later (i.e., increasing duration) enables more mixture to get into the cylinders at higher engine speeds, thus improving volumetric efficiency and increasing power at high RPM. The trade-off is that longer duration and overlap (principally later intake valve closing) dilutes pressure at low engine speeds and lowers the effective compression ratio, reducing torque and power at low RPM. So, choosing a camshaft for a fixed valve timing engine is a compromise between increasing performance at high RPM and decreasing performance at low RPM.
Reground cams, which are ordinary/stock cams machined to have a new profile, are less expensive than virgin ones, but be aware of material and surface finish choices and compatibility. You'll need resurfaced or new tappets and maybe shorter valve guides and different valve springs too, depending on cam profile particulars. There are many aftermarket Spitfire camshafts to choose from, and manufacturers/vendors only reveal certain data about their cams (exact profiles are usually closely-guarded), so choosing among them can be difficult. To first order, don't get too aggressive with duration or overlap or your engine will not perform well at low RPMs and you may be disappointed in the way it behaves driving it around in normal use. Furthermore, increased valve lift beyond a certain amount will not add to performance because of the limitations of the head to flow gases. According to Dmitri Elgin, valve lift greater than about 0.380" in a Triumph engine will yield diminishing returns unless some flow improvements have been made to the head and valves. However, this is not to say that a cam that generates peak lift greater than 0.380" is wasteful. This is because valve velocity and acceleration are important too. A cam that has a fast 'ramp' will open the valves more quickly and allow the valves to be more open more often and thus capable of more flow. But, if the change in cam ramp is too abrupt, then valve acceleration will be too high, generating excessive forces on the valve train. A way to get sooner and quicker valve openings and later and quicker valve closings without excessive acceleration is to have a cam with a higher peak lift than the optimal max opening. Obviously, cams and valve operation are more complicated than just duration, overlap and peak lift.
If you are not sure what to pick, be conservative and don't over-cam. The objective is to achieve a compatible match between compression ratio, cam duration, overlap and lift, and the flow characteristics of the head, intake and exhaust, while keeping drivetrain forces within design limits. As a rough guide for sporty street use, keep duration under 280 degrees or so and valve overlap under 60 degrees or thereabouts so that adequate performance at low RPMs is preserved and good performance in the mid-range is achievable. These guidelines are not hard and fast rules because many factors determine camshaft performance, and it is assumed that you have raised the static compression ratio and have installed good-flowing exhaust and intake parts, the specifics of which affect camshaft selection.
Actually, the Triumph factory 270 degree duration (25-65-65-25) cam that was stock in many of the 1300 engines is a really nice street cam. Be advised that this cam and the other early "small journal" cams ran in bearings installed in the block, while the later 1300 and all 1500 engines had "large journal" cams that ran right in the block without camshaft bearings. You can use a small journal cam in a late block as long as you install cam bearings.
For more on camshaft terminology, theory and practice, check out these references:
Four-Stroke Performance Tuning in Theory and Practice, by A. Graham Bell
Camshaft Glossary (Elgin Cams)
Camshaft Theory (Second Chance Garage)
Performance Camshafts (Dimitri Elgin of Elgin Cams)
Camshaft Selection (Newman Cams)
Cam and Valve Train Questions (Crane Cams)
Camshafts and Valve Train Basics (Street Racers Online)
"Blueprint" the Engine
The engine, like any manufactured thing, is imperfect and has certain machining specifications and assembly tolerances. If you are asking more from your engine, it's good to reduce variance and tighten the dispersion about the specs. There is no such thing as perfection in machining and assembly, but with extremely precise machining and meticulous parts screening guided by excellent metrology, it is possible to minimize variance about an ideal spec to be within the resolution of the measuring equipment itself and be so tiny as to be inconsequential, and thereby achieve "practical" perfection. The process of rebuilding an engine to exact specifications is "blueprinting." Get all the moving and rotating parts matched and balanced. Have a machine shop align bore your block (i.e., precision machine the main bearings to be in alignment and the cylinder bores to be more parallel to each other and more perpendicular to the crank), resurface the top of the block to ensure flatness and orthogonality to the cylinders, grind and/or polish the crank to be more straight. Some moving parts, like connecting rods, can be lightened before they are matched to reduce acceleration forces, but lightening of such critical parts must be done carefully and should be performed by someone who knows what they are doing. Note that lightening of the flywheel, particularly on the 1500, will let the engine rev-up quicker, but it won't change the output torque or power and can make for rougher idling, so be forewarned. A good machine shop will check for cracks (magnafluxing, die penetrant) to assure integrity of parts. Blueprinting will enable the engine to run smoother with less self-induced stress and enable it to be run at higher RPMs and thus generate more peak power. If you are doing more than just bolting on some better intake and exhaust parts, then have the engine blueprinted.
There are a few specific enhancements that ought to be made to Spitfire engines to fix some intrinsic weaknesses. One is to enlarge the passageway that feeds oil to the center main bearing and subsequently the big end bearings and connecting rods for pistons 2 and 3. Enlarging it to 5/16 inches will aid in the delivery of adequate amounts of oil here. This task should be performed by a competent machinist. Baffling the oil pan to prevent oil surge and starvation of the oil pickup, particularly during left-hand turns, is a really good idea. You'll appreciate this if you've ever watched an oil pressure gauge on a Spitfire with an unbaffled sump during a long left-hand turn at speed. Lastly, pinning the thrust washers to the mains to keep them from falling-out after they wear-down is a good idea that can provide peace-of-mind and prevent costly damage to the block.
For more details on Triumph engine rebuilding, see the following:
Building a Reliable Spitfire Engine for High Performance, by Calum Douglas
A Guide to Racing your Triumph Spitfire or GT6, by Jon Wolfe
Triumph Competition Preparation Manual
the writings of Kas Kastner
More Work but Bigger Results
One engine swap that I'll mention in this article since it is still "keeping it Triumph" is to put a Triumph in-line 6 cylinder lump into a Spitfire. The resulting creation is commonly referred to as a "Spit-6." Many people have tried this mod and there are some important nuances to note. First, adding the extra mass of the 6 pot lump and distributing it differently will significantly change the handling characteristics of the Spitfire. To safely deal with it, the brakes should be upgraded to GT6 type all around and the front springs should be uprated. One way to do this is to take a GT6 chassis and put a Spitfire body on it, which is more precisely referred to as a "convertible GT6" or CGT6. But there's more. Mounting the engine the way it is in the GT6 puts the two additional cylinders forward and tilts and raises the engine, necessitating the substitution of a bulged bonnet, like the GT6 bonnet, and a forward-mounted GT6 radiator. Also, the front suspension towers on the GT6 are slightly different than the Spitfire's, so GT6 towers should be used when mounting the 6 cylinder the way it's mounted in the Spitfire's small chassis cousin. Also, Spitfire and GT6 transmissions and bellhousings are different and not interchangeable. If a 2.5 liter engine from a TR5/TR250 or TR6 is used instead of the 2 liter from a GT6, be advised that the oil pan is deeper (to accommodate the longer stroke) and it will interfere with the steering rack and frame. A remedy is to swap the 2.5 liter pan out for a 2 liter oil pan that has been locally 'modified' to provide clearance for the 2.5 liter crank. Also, the TR6 transmission sits higher and requires modifications to the transmission tunnel cover and the tub's propshaft tunnel to accommodate it. You get more torque and power with the 6 cylinder swap, and the same kind of enhancements already described can be applied to enhance performance (download David Vizard's "Tuning Standard Triumphs over 1300cc" for more detail and specifics), but the car will handle quite differently than a Spitfire due to the extra few hundred pounds and its distribution so far forward and a little bit up.
An innovation that significantly enhances the Spit-6 is to substitute a Spitfire front engine plate for the normal 6 cylinder one and mount the engine like a Spitfire engine is mounted. This puts the front of the 6 cylinder in the same location as the front of the 4 cylinder and shoves the entire engine and transmission aft by 6 inches from the normal GT6 configuration. This absolutely transforms the way the car feels and handles for the better. However, this is more work as it requires fairly involved trimming and modification of portions of the frame (some localized notching and reshaping to accommodate the bellhousing and exhaust, and a new mount for the transmission 6 inches farther aft), the tub (cutting and refabricating parts of the firewall) the gear shifter (removing 6 inches from the GT6 or TR6 cantilevered gear shift extension, and rewelding) and the propshaft (shortening), among other things. It also requires some relocation of items depending on whether the car is right or left hand drive. For example, left hand drive (e.g., North America, Continental Europe) requires implementing a take-off plate at the stock 6 cylinder oil filter location and mounting the filter remotely to accommodate the accelerator pedal, and right hand drive may require the brake master cylinder and pedal to be relocated slightly to accommodate the intake (depending on what it is). Having driven GT6s and "ordinary" Spit-6's myself, as well a Spit-6 with such an "aft mounted" configuration, it's my humble opinion that aft mounting is well worth the extra work. However, it's somewhat involved and more difficult than anything mentioned so far in this article. I am in the process myself of putting together an early-bodied Spit-6 (more properly, a CGT6) with an aft-mounted engine and fully independent rear suspension. Having seen Stephen Attenborough's aft-mount modified GT6 in person and having driven Paul Tegler's aft-mount, fuel-injected Spit-6 called "FIS6" (pronounced "physics"), I'm excited to have my own version.
Comparison of engine positions--normal GT6 mounting and innovative aft-mounting (photos c/o Paul Tegler):
Stephen Attenborough's right hand drive GT6 and Paul Tegler's left hand drive Spit-6 "FIS6" (both are fuel injected):
injection is intrinsically more efficient than carburetion,
and modern technology fuel injection control systems like MegaSquirt
feedback control over fuel
injection and ignition
and enable superior fuel efficiency and performance. I
have driven a few
Spitfires, and I can testify first-hand that fuel injection on a
Spitfire or GT6
can work very well! Paul
Tegler's Spit-6 "FIS6"
a modified Spit-6 with a
fuel-injected 2.0 liter 6 cylinder GT6 engine installed in the
configuration described above. Paul's car is a fine piece of
engineering--a "21st century" Triumph and a real
blast to drive! Paul lists some other folks
who have fuel-injected a Triumph.
Stages of Improvement
Beyond the aforementioned things, getting more power out of a Spitfire engine or other Triumph lump can be a matter of diminishing returns, and is likely to require large sums of money and sometimes will lead to reduced reliability and increased hassles and operating costs. Special, ultra-high performance internal engine parts (e.g., forged pistons, special steel cranks, special connecting rods) are very expensive, add durability and will make the engine capable of higher performance, but are they warranted for a street Spitfire? Extreme levels of power (e.g., in the realm of 100hp per liter of displacement and up) often come at the expense of durability and reliability and can require significant modifications to strengthen the block as well as special ultra-high performance parts. Moreover, at some point the drivetrain cannot handle the added stress, so even if you have a terrific, reliable, high-powered engine, you won't be able to apply that power without breaking the transmission, differential or other drivetrain parts, or replacing them with more robust units from unrelated cars. Besides, at this point, your Spitfire may not be very well mannered or suited for street use, or would not really be a Triumph anymore and would be beyond the theme of this article. The important thing is to enjoy your Spitfire, and what constitutes "enjoyment" is a matter of personal taste and choice. Enjoy!