From Paul
Geithner's Triumph Spitfire
Links, References
& Technical Information
Triumph
Spitfire Performance Enhancements (January
2010 edition)
Why
bother improving Spitfire performance?
The Spitfire is not a bad-handling car in factory spec. It bested its contemporaries in handling (see Car and Driver's "Showroom Stock Sports Car Comparison Test" from April, 1973) and it handles respectably in modern times. However, just a few easy changes can significantly improve handling and enhance safety. Power, acceleration and speed are where the Spitfire was mediocre in its day, and by modern standards it's nothing to write home about. But here again, a few simple, relatively inexpensive and easy changes can lead to 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 making for 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 racecar preparation
guide,
although many of the principles and some of the implementations apply.
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 engine swaps or complete 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 going to extremes (and since
GT6’s and
Herald’s and a few other cars are closely-related and share
the same
guts and design features, some of this information can be applied to
them as well). 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 chased down in several 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 in rough logical order (although it makes
sense to make certain particular changes in concert). I hope you find
this information helpful.
Handling
The
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
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).
Lose
Weight
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.
Understand and Set Alignment
Before discussing the following changes to the front and rear suspension, it is important to review alignment. 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.
Caster:
Camber:
Toe:
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 and
lower suspension links; the upper link for each side is the transverse
leaf spring and
fore and aft motion is constrained by a pair of trailing radius arms.
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:
A Short Course on Wheel Alignment
Install
Better Tires and Wheels
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 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. The table
above is based on a combination of real trial and error fitment results
on Spitfires and calculations. It is only a guide because assembly
tolerances vary
widely among Spitfires! When there is rubbing, 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. 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 camber change that Spitfires experience--especially at
the rear--some modest compliance in the tires is good. 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.
The above combinations will work with stock Spitfire Girling 12
calipers (mk1 and mk2) and Girling 14 calipers (mk3, mkIV and
1500), but there can be inteference if upgrading to the larger Girling
16 calipers for the GT6 (e.g., TR7 steel wheels typically interfere
with the larger GT6 front calipers--see brake upgrades later
in
this article). 175/70-13 tires on 13x5.5 inch rims with 3/4 inch (28mm)
positive offset are a nice upgrade to the stock Spitfire.
185/60-13
tires on 13x6 rims with 7/8 inch (23mm) positive
offset are a good performance combination,
especially if the rest of the suspension has been upgraded (read more
to follow).
Install Thicker, Longer Wheel Studs
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 overtorqued 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.
Changing
wheel alignment and lowering the
Spitfire can do a lot for handling. Most
Spitfires have their front ends too high for optimum
suspension action. Lowering brings the c.g. closer to the
ground,
which
helps reduce 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.
Dive
during braking and squat during acceleration are also
reduced, and bump steer
will be changed yet limited. But it's not just a matter of
lower is
better, because too
low is bad (not enough ground clearance for everyday use, and bad
suspension
geometry that makes handling worse). The
right amount of lowering and stiffening optimizes the
geometry of the suspension links so that the car will roll less and
maintain proper tire geometry for good grip. Good
static front
suspension geometry for an all-around Spitfire is when the lower A-arms
are about
parallel with the ground. This
happens when the compressed length (Lrest)
of each front coil spring is about 7
inches
installed on a standard damper (shock absorber) with fixed lower spring
perches and with the car at rest on level ground. This also
corresponds to a total distance between the center of the bolt
attaching the damper to the lower A-arm and the interface where the
upper spring perch meets the frame of 10.25 inches (Lrest
+ Dperch where Lrest
= 7 inches and Dperch = 3.25 inches).
Having
the
lower A-arms about parallel with the ground lowers the c.g. while
keeping the roll center in a favorable location for good handling.
There
are a three basic ways to lower the front: shorten the
stock springs, replace them with shorter ones or install front dampers
(shock
absorbers) that have adjustable spring perches. Adjustable
spring perch dampers allow fine adjustment of
static ride height and provide the most flexibility, but they are
expensive and you can avoid having to
use
them if you pick the correct stiffness and free length of coil springs. 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. Alternatively, you can 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 approximately 10 percent, and this
additional
stiffness is good because it will limit suspension travel and help
offset the
loss of suspension travel distance that comes with lowering. Another way to go with the
Spitfire 1500 is to install
springs from a mk3 or mkIV Spitfire. These
springs are shorter yet softer than the stock 1500
ones, so the
front end will come down. This
works, resulting in an installed spring length of about 7¼
inches, but
this approach has the disadvantage of softening while lowering. This is
bad because it increases the need for suspension travel at the same
time
it reduces the amount of travel available, thus
increasing
the likelihood of bottoming-out the suspension--it's not recommended. Running out of suspension
travel and hitting the bump
stops
is harsh and potentially damaging to your car. A
good way to go is to replace the stock springs with
shorter and stiffer ones that have the right combination of stiffness
and free
length such that the target installed length of about 7 inches is
achieved. Some
Spitfire parts
vendors sell such springs, or you can use my
spring calculator to specify your own, which you can then buy
from a
multitude of vendors that sell 2½ inch inside diameter coil
springs to
racers and custom car builders. Be
advised that going too stiff will limit body roll but may make for an
unpleasant ride on the street, and not going stiff enough may allow too
much
body roll and will increase the chance that you will bottom-out your
suspension
on the bump stops. The
front coil
springs that I use on my
1978
Spitfire 1500 are 350 pound per inch stiffness and 9 inches
free
length. They have
an installed
length of 7 inches in my light-weighted
car and help result in very nice, much improved handling. Here's how this setup
turned out:
After lowering, make sure there is adequate suspension travel before hitting the elastic bump stops contained in the front dampers, even if using stiffer springs (the use of stiffer springs will reduce the amount of travel required for a given load, thereby helping to offset the loss of suspension travel due to lowering). Trimming the front bump stops to be no less than 1/2 inch thick but no more than 3/4 inch thick will add suspension travel while also protecting the dampers against internal damage.
One thing that lowering the front will also do that is favorable is to push the camber of the front wheels toward the negative. An otherwise stock 1500 will end up at around 0.5 to 1 degrees of negative camber just by lowering described above such that the lower A-arms are parallel to the ground at rest. Some negative camber at the front is desirable in that it aids entry into turns (progressively more negative camber is generated in a turn due to the effect of positive caster), and a little bit in the straight-ahead, static position such as 0.5 to 1 degrees negative, will help cornering 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 still acceptable with the aforementioned changes, shimming the steering rack to raise it (if possible without interfering with the crank pulley of the engine) to make the steering tie rods more horizontal at rest can help compensate.
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 offsets this and the net
result is a car
that
handles much better.
Achieve Better Suspension Geometry—Optimize the Rear
The
Spitfire rear suspension is a swing axle architecture
employing a transverse leaf spring. This
design has the advantages of simplicity, low cost and low unsprung
weight, but it has the disadvantage of poor camber control
and potentially dangerous handling behavior. The roll center
of this architecture is relatively high, the wheels
are always normal, i.e., plane perpendicular to the axles, and
the design is prone to "jacking" or progressive raising of the car with
increased forces generated during braking and turning. In jacking, if
the
outboard axle (which is taking the load during the turn) angles up from
wheel toward the differential (which is sprung weight attached to the
chassis), then the force of the turn has a component pointing up
that raises the differential and hence 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 and camber gets more and
more positive. Quickly, grip is reduced to the point
that the rear tires break loose and the car goes into violent
oversteer.
Ways
to compensate for this dicey behavior include lowering the rear end to
set
the initial/static camber to be fairly negative (which also
lowers the c.g.), reducing
roll stiffness at the swing axle end of the vehicle, and lengthening
the swing axles. An
initial amount of negative
camber "pronates" the swing axles, i.e., angles them so
the ends of the axles at the differential are lower than the ends at
the wheels. This is a good idea on the Spitfire. With some negative
camber to begin with, the rear of the car has to raise and roll
more during cornering
before the camber of the outside wheel becomes positive, meaning the
angle of the axle points up from the wheel toward the
differential to add jacking force. In
a swing axle suspension, the wheels are always normal (i.e., plane
perpendicular) to the axles, which means changing camber is
accomplished by adjusting rear ride height (lowering the differential
changes the angle of the swing axles to generate negative camber, and
since the differential is mounted to the chassis, lowering the
differential means lowering the rear end of the car). It
is that
few degrees of negative camber at the rear wheels that gives the
Spitfire that "broken" look when viewed from behind. Lowering
the rear to get the
desired amount of negative camber also lowers the c.g., and that helps
handling too. Reducing
rear roll stiffness reduces rear weight transfer, which inhibits
jacking by suppressing development of jacking geometry. It
also
helps prevent the inside rear tire from unloading enough to lose grip.
Keeping the drive wheels and tires planted on the pavement and
retaining grip is especially important on a car with an
open differential where if the inside tire becomes unloaded,
it
spins and little or no torque gets applied to the loaded
outside
tire to drive the car forward. Triumph reduced rear roll stiffness in
switching from the "fixed spring" in
the early
Spitfires and GT6's to the "swing
spring" in the later models. The
trick
of the swing spring design is
that only one of the leaves is actually fixed to the differential while
the rest are allowed to float and pivot about a center bushing. Roll
stiffness is significantly reduced yet bump and droop
stiffness
is preserved.
Although
stiffening the transverse leaf assembly in bump and droop will help
reduce the rate of camber change for a given load on the suspension, it's
pretty difficult to do without also increasing roll
stiffness (e.g.,
adding another leaf), and
besides, it's not necessary on a street Spitfire. Lastly, longer
swing axles mean longer swing arms and therefore
a reduced rate of
camber change for a given amount of suspension movement, as well as a
lower roll center. Thus, Triumph
fitted the late, swing spring
equipped vehicles with swing axles measuring 1 inch longer.
The simplest, most straightforward method to lower the rear to increase negative camber and lower the rear c.g. is to use a shim in between the differential and the transverse leaf spring. Since the differential is attached to the chassis and is located below the leaf spring, adding a shim (a.k.a. "lowering block") between the spring and the differential lowers the rear of the car. While some initial negative rear camber is good, too much reduces tracking stability, degrades braking and causes uneven tire wear (and in the extreme, can also actually prohibit optimum tire contact geometry while cornering--especially with wider and lower profile tires). Experience suggests that compromising at 3 to 4 degrees initial/static negative rear camber is good for the sporty street Spitfire.
On
my '78 1500, I left the rear end alone except for some mild lowering to
match what I had done at the front and achieve an overall level stance
of the vehicle and to set the static camber at the rear to be about 4
degrees negative. I also added one rear toe setting shim between each
rear radius arm and the tub to set the correct amount of rear toe
subsequent to all of the other suspension changes. Lowering
and stiffening the front of my Spitfire simultaneously had the effect
of raising the
rear a bit, which was fine in my case because the rear of my Spitfire
was too low when I bought it. My
factory-original rear leaf assembly, like many others I have run
across, seems to have sagged
a bit since manufacture (be it due to compressed thrust buttons and/or
stress relief in the steel leaves). However, it has not changed and
been in equilibrium
since I bought the car in 1994. After making the front-end
changes, the
car ended up with a slight amount of rake (the front lower than the
rear by about 0.5 inches) and the rear settled at between 2 and 3
degrees negative
camber. To lower the rear a tad more to optimize rear camber and
further reduce roll and jacking, I fabricated and installed a lowering
block between the diff and the transverse
leaf
spring assembly. This 0.45 inch thick block (machined from 5/8 inch
thick 6061 T6 aluminum plate) virtually eliminated the rake and
increased the rear negative camber to about 4 degrees.
These small increments of c.g. lowering and camber change
resulted in quite a noticeable handling improvement--delayed and
reduced jacking and reduced roll during cornering--and more driving
enjoyment.
Here's
a calculation for estimating the thickness of lowering block
needed to
go from an initial camber setting to a more negative one:
target block thickness "T" = effective axle length * sin(initial camber - target camber)
where initial and target camber are in degrees. If your calculator does not calculate the sine using degrees, multiply degrees by pi/180 to convert to radians. Effective axle length is the distance from the swing axle u-joint to the bolt that attaches the swing axle trunnion to the vertical link. In the case of the "long" later model swing axles (used on later mkIV Spitfires--FH25001 and up, after February 1973--and all 1500 Spitfires), this measurement is 16 inches, and for the shorter swing axles used on earlier cars it's 15 inches. Also, note that block thickness means net thickness--i.e., the thickness of the middle portion that goes between the bottom of the spring and the top of the differential. This simple formula is valid for the small angles of only a few degrees being dealt with here.
If your rear end is too low or 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. The buttons need only be thick enough to barely separate the leaves at the button locations. You can fabricate your own by cutting slices from rod stock. Alternatively, some vendors on the internet sell them. This 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. It's a good idea to clean, repaint or coat the leaves and lubricate them before reassembly. This will change the arch of the spring a bit, 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 static rear camber.
As
mentioned earlier, the
later (mkIV and 1500) Spitfires have a low roll stiffness "swing
spring" rear
leaf assembly that works well to suppress the jacking effect and
consequential severe camber change and loss of grip during
cornering. In
the case of the early
fixed spring
Spitfires,
it is a good idea to update the design by replacing the fixed spring
and short swing axles with the later swing spring and long swing axles.
Since the early Spitfires using the short swing axles have
differentials with smaller stub
axle flanges, this updating requires either replacing
the larger U-joint flanges on the longer swing axles with the smaller
flanges from
the shorter early swing axles
or replacing the early differential and rear drive assembly with a
later one (the early stub
axles are slightly smaller than the later ones and do not swap).
Note that with fitment of the swing
spring,
the front anti-sway bar should be replaced with a stiffer (thicker)
one--just as Triumph did--to restore the loss in total vehicle roll
stiffness (more on anti-sway bars shortly).
If for whatever reason you want or need to stick with the early fixed spring design, then installing a camber compensator is a good idea. A typical camber compensator looks like a single leaf spring, usually mounted underneath the differential opposite the transverse leaf assembly, pivoting in the middle at the differential and attached at the ends to the vertical links out by the wheels. It helps prevent the rear wheels from developing extreme positive camber. Joe Curry sells camber compensator kits that do the job. This design is modeled after camber compensators used on racing Spitfires in the 1960's and is similar to ones that have been used on other swing axle cars like the early Porche 356 and VW Beetle.
Install
a Stiffer Front Anti-Sway Bar
Now,
moving on to the topic of anti-sway bars. If
you want to add more roll stiffness without adding
straight-line stiffness, then you can add anti-sway bars. These 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
Spitfire 1500 has a 7/8 inch diameter front anti-sway bar, but an
aftermarket 1 inch
bar has
1.7x more roll stiffness and provides about 535 lbs per inch of total
end-of-bar
deflection; the Spitfire mk3 has a 11/16 inch bar, but a stock
7/8 inch
Spitfire 1500 bar has 2.6x more roll stiffness and provides about 314
lbs per
inch of
total end-of-bar deflection. All else being equal, increasing roll
stiffness by means of a thicker front anti-sway bar will bias roll
stiffness toward the front and tend to make a car understeer more. However, on a Spitfire, installing a
thicker front anti-sway bar does not result in bad understeer. The
added tendency to understeer is offset by the
improved geometry of the suspension and tire contact due to reduced
body roll during cornering.
Adding rear roll stiffness by fitting a rear anti-sway bar is typically not a wise idea on a swing axle car because it can exacerbate jacking (note that although it is not disastrous to use a rear anti-sway bar on a Spitfire equipped with a swing spring, it essentially negates the advantage of the swing spring over the fixed spring). This is why zero-roll stiffness devices that limit camber change, like Z-bars and leaf-style camber compensators, have been applied to various swing axle cars for decades. The only time it might be worth adding rear roll stiffness to a Spitfire is if the front roll stiffness is so great that the car understeers significantly or if a limited-slip or locked differential has been installed, but at this point the car is probably set up for something much more serious than street use. Controlling rear camber change and reducing or precluding jacking are paramount.
Install
Good Dampers (Shock Absorbers)
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 "Goldlilocks" 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. Some medium
priced dampers like
KYB Gas-A-Just are
better but may 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 Spitfire
dampers
(e.g., Spax) allow damping adjustments in-place, without disassembling
the suspension, and also include adjustable spring perches 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)
STEP
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
STEP 1: With rebound set on full soft and the bump damping already set from the previous testing, drive the car one or two laps, paying particular attention to how the car rolls when entering a turn.
STEP 2: Increase rebound damping three (3) increments on all four dampers and drive the car one or two laps. Repeat this step until the car enters the turns smoothly (no drastic attitude changes) and without leaning excessively. An increase in the rebound stiffness beyond this point is unnecessary and may result in a loss of cornering power. Note: As with the bump settings, this point will probably be reached at one end of the car before the other, so keep adjusting the soft end until it feels smooth and does not lean too much. Rebound damping is now set.
In
summary, you can get away
with using stock dampers, but
installing
better
units enables finer tuning and permits matching damping levels with
spring rates, resulting in improved dynamic behavior of the suspension
and better handling.
Ensure Good Brakes (a.k.a., before you go, make sure you can STOP!)
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.
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
Power
Understand
Some Engine and Powertrain Basics
Power = Torque * angular velocity [note: this is a scalar product]
= Torque * 2*pi * rotational speed
When using the Imperial units foot-pounds for torque and horsepower for power, and using RPM (revolutions per minute) for rotational speed, the equation becomes:
hp = ft-lbs * RPM/5252
In the SI units of Newton-meters for torque and kilowatts (kW) for power, this becomes:
kW = N-m * RPM/9549
Other handy relationships are:
hp = 1.341 * kW kW = 0.7457 * hp
ft-lbs = 0.7375 * N-m N-m = 1.354 * ft-lbs
Spitfires came with basically four different engines--all variations on the same Standard-Triumph model inline-four, three main-bearing, pushrod-actuated overhead valve engine. Spitfire 4, mk1 and mk2 models came with 1147cc displacement units, mk3 and mkIV models came with two versions of the 1296cc or "1300" engine (the "small journal" models in the mk3 and "large journal" in the mkIV) that are bascially bored-out versions of the 1147, and the 1500 came with the 1493cc or "1500" engine, which is essentially a stroked 1300. There is further differentiation through the use of different cylinder heads and camshafts. The 1147 and small journal 1296cc engines have the lightest crankshafts and can spin up to the highest RPMs, and the small journal 1296 is popular with racers. The large journal 1300 and especially the 1500 have the heaviest cranks and flywheels of the bunch and so don't lend themselves to running as high RPMs as the small journal engines. However, the 1500, being longer stroke, produces more low RPM torque and is a perfectly adequate engine for a street Spitfire.
Lose
Weight
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.
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).
Optimize
the Ignition Timing
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 and not have so
much advance that you encourage pre-ignition (a.k.a., pre-detonation,
pinging, pinking, knocking). 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.
Until
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,
or
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.
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.
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.
Install
a Better Exhaust Manifold or Header
The
exhaust piping on any car is sort of like a musical instrument in
that it is tuned (whether by design or by accident) to operate best at
a
certain
engine speed. In a nutshell, the
low-pressure
left behind in the wake of a high-pressure pulse of hot exhaust gases
leaving
one cylinder and traveling down a branch of the exhaust manifold can
help
scavenge or vacuum-out the exhaust gases of another cylinder. How and when this occurs
depends on
engine speed and the length and diameter of the manifold runners. The longer the runner, the
longer it takes for a pulse of
gas to get to the end and the lower the engine speed at which the
low-pressure
wave can provide this beneficial scavenging effect to another cylinder. The stock manifold on the
later U.S.
Spitfires has runners so short that this scavenging effect never really
occurs
over the operating range of RPMs of the 1500 engine. The
best answer is to fit a system tuned to operate in the
range of RPMs where the engine will actually be used. There
are 4 into 1 headers--all the pipes leaving the
engine
collect together at the same place (like the stock U.S. spec 1500
manifold,
only longer)--and 4 into 2 into 1, or Tri-Y manifolds or headers--pairs
of
pipes collecting together first (1st and 4th
cylinders
collect, and 2nd and 3rd
cylinders collect), then the
remaining pair gathering together into one.
For
street use, the 4-2-1 systems are best because they provide
improved performance over a broader range of RPMs, but don't produce as
much
peak power as a 4-1 system (racers use 4-1 systems because their
engines spend
basically their whole time operating at high RPMs at or around the peak
power
point). This is
because the two
different lengths of tubes in a 4-2-1 system (the first ones leaving
the engine
are called primaries, and the second ones are called secondaries) are
tuned to
two different frequencies, or engine speeds, and in combination the
result is
higher performance over a broader range of engine speeds. This is nearly a bolt-on
change, where
the only custom work lies in joining the exit of the new
manifold/downpipe
combo or header to the rest of your exhaust system. You'll
almost assuredly have to retune your carb(s) after
this, and at this point you can benefit from richer needle(s) (in
constant depression/variable venturi carbs like SUs and
Zenith-Strombergs) or richer jets and emulsion tubes (in constant
venturi/variable depression carbs like Webers).
Install
Freer-Flowing Air Filters
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 this.
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, and the longer the intake air horns are, the
lower the
RPM a which the ram effect occurs.
Stub stacks are too short to provide a ram effect at a
useful RPM, but
they do provide a radiused entry for air to go into the carb(s), which
smoothes
airflow and does help performance a little.
Install
Better Intake Manifolding and Carburetors
While
the single Zenith Stromberg carb that came on the 1500
engines for the U.S market isn't bad, the manifold between it and the
cylinder head has
two abrupt
right-angle turns in it, which doesn't lend itself to good airflow. Moreover, the ports of this manifold are not same
size as the ones on the cylinder head, and so they are not matched. But
changing just the
manifold isn't
really an option here, so a good thing to do is to install twin SU HS2
or
HS4 carburetors
and a matching intake manifold, like the rest of the world 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 U.S. market
Spitfires have
this twin
carb arrangement using HS2s, and this is a simple bolt-on upgrade from
the single
Zenith-Stromberg
configuration. 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.
"Port and Polish" and "Flow" the Head
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 the Spitfire engines have figured out the modifications to make to cylinder heads to significantly improve performance, and some of this knowledge has been captured in various publications. You can try some of these mods yourself, or you can pay a professional to make them. A professional job should include equalizing the combustion chamber volumes, plus before and after measurements of gas mass flow rates through the head with the valves installed but open. Also, having a professional shape your valves and valve seats (e.g., 3-angle grind) will aid airflow and help performance too. Although you may not have your own flowbench, consider performing minor head work yourself. Just some modest grinding and polishing to clean-up leftover casting and machining irregularities and eliminate sharp transition areas in the ports, as well as grinding and polishing to ease the short-side radius of the intake ports can be done easily and inexpensively yourself and will noticeably help performance. You can even attempt to ease the vertical walls of the chamber or round over the sharp edges of the beveled steps inside the combustion chamber where the spark plugs poke through to further "unshroud" the valves and improve flow. Just be sure to make mods the same from cylinder to cylinder and to measure the volume of the combustion chamber recesses in the head after doing any work, and perform any minor rework necessary to equalize all the volumes. Base any compression ratio calculations and planned adjustments (e.g., head shaving) on these measured values.
Improving the flow characteristics of the head is very worthwhile if you want to get the most from your engine. For more insight here, check out these references:
Four-Stroke Performance Tuning in Theory and Practice, by A. Graham Bell
How to Build, Modify & Power Tune Cylinder Heads, by Burgess and Gollan
Theory and Practice of Cylinder Head Modification, by David Vizard, et al.
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.
Raise
the Compression Ratio
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 temperatures and elevates the likelihood of pre-detonation, which can damage or destroy pistons. The practical limit on compression for a Spitfire engine running on contemporary pump gasoline is about 9.5 or 9.75:1 without having to deleteriously retard the ignition to avoid pre-detonation. More modern cars can and sometimes do have higher compression ratios, but that's because they employ knock-sensing systems that dynamically adjust (retard) the ignition to avoid prolonged pre-detonation. Racers use very high compression, but they also use camshafts with very long valve opening duration and overlap that lowers the dynamic compression ratio of the engine, which is its effective compression ratio while it is operating. Racers also spend lots of money on special, tougher materials (e.g., forged instead of cast pistons), frequent engine rebuild or swaps, and other costly 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.
Change
the Camshaft
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. 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. 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. As a rough
guide for 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. Moreover,
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. 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)
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. This is loosely termed "blueprinting" by some. 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, more closely match the weights of the connecting rods and pistons and more precisely balance the rotating parts such as the crank, flywheel and pulleys. 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 do things like magnafluxing to check for cracks in ferrous parts and assure their structural integrity. All of this will enable the engine to run smoother with less self-induced stress and enable it to be run at higher RPMs and generate more power. If you are going to the trouble of doing everything mentioned thus far, then such precision machining and balancing is an investment worthy of serious consideration.
Fix Some Bottom-End Weaknesses
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 ensure delivery of adequate amounts of oil here, particularly on a 1500 engine. 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, especially 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.
Stages
of Improvement
A
convention of sorts has been established by popular fiat to
identify different levels of engine preparation. This
"stage" convention for Spitfires varies a little bit
from one person to the next, but a common one is summarized below:
A
Stage 1 engine is relatively easy and inexpensive to set-up
and gets you the most bang for the buck, especially on a North American
market Spitfire. Stage
2 isn't too tough or expensive to implement, even
for the
automotive novice, and such an engine will make your Spitfire much more
enjoyable. Stage 3
is still perfectly
usable on the street, but things start to become expensive. Full Stage 4 and beyond is
reaching beyond the scope or intent of this
article. Don't be
wooed by high peak horsepower
numbers. Your
Spitfire will be
more fun if you have power where you need it and can use it, which for
street
use means in the middle RPM range and below 5500 RPM. Why
have an engine that makes more than 120 peak bhp at 7000 RPM but
can't idle smoothly or has anemic performance in the low and mid ranges
in a street
Spitfire?
Spend
More to Get Less
Now
is the point at which you can spend proportionally more and
more money for proportionally less and less improvement, which is
treading
outside the cheap and easy theme of this article.
But for completeness, I mention some things briefly here
in
case you want to go further.
Switch
to Fuel Injection
Fuel
injection is intrinsically more efficient than carburetion,
and modern technology fuel injection control systems like MegaSquirt
allow a great deal of closed-loop control over fuel
injection, ignition
and engine performance, and enable superior fuel efficiency. I
have driven a few
fuel-injected
Spitfires, and I can testify first-hand that fuel injection on a
Spitfire or GT6
can work very well! For
more information and an example of a well done fuel
injected Spitfire, check out Paul
Tegler's "FIS6" (FIS6 is an acronym for Fuel Injected Spit 6
and is pronounced "physics"). This car is
actually a Spitfire with a
fuel-injected 2.0 liter GT6 engine installed in a special way.
Use
Forced Induction
Naturally aspirated engines rely on ambient atmospheric pressure and the acoustic resonance "ram effect" to get air and fuel into them. Superchargers are compressors of one form or another that are driven by a belt or chain off of the engine, and a turbocharger is a compressor connected directly to a turbine spun by the flow of exhaust gases exiting the engine. The purpose of both is the same, and that is to force additional air and fuel into the engine at above ambient atmospheric pressure, increasing volumetric efficiency and enabling the engine to generate more torque and power. Since Spitfire engines have only three (instead of five) main crank bearings, and in the case of the 1500 engines the crankshafts are relatively heavy and flexible, there are limits to how much extra torque and power can be reliably generated. While some people have adapted superchargers for application to Triumph Spitfire engines (such as Bennett French of Standard Performance in North Carolina, U.S.A.), there is scant data so far on their long-term use in street-going Spitfires and their effect on reliability. Probably only a modest level of boost (e.g., 1/2 atmosphere or less) on a low static compression (e.g., 7.5:1) configuration should be considered, if at all, and ignition timing should be chosen carefully. Properly done however, a forced-induction small displacement engine can be a lot of fun. I installed a supercharger on my first car, a Volkswagen Scirocco with a 1.8 liter four cylinder engine. Running modest boost (6 psi--a little less than 1/2 atmosphere) added torque throughout the operating range, made the engine feel like it had more displacement and significantly improved performance. The primary concern with forced induction on a Spitfire, particularly with the 1500 engine, is about exceeding the capabilities of the bottom-end to handle it over the long-term.
Above and Beyond
Beyond the aforementioned things, getting more power out of a Spitfire engine is a matter of diminishing returns, and is going 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, by themselves add durability, and will make the engine capable of higher performance, but they aren't 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. Besides, at this point, your Spitfire may not be very well mannered or suited for street use and would be beyond the "enhanced street performance for reasonable cost" theme of this article.