From Paul
Geithner's Triumph Spitfire
Links, References
& Technical Information
Triumph
Spitfire Performance Enhancements
Why
bother?
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. 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.
Handling
Lose
Weight
Handling
is the Spitfire's strength, and it seems a shame not to
improve something with such potential.
All else being the same, a lighter car will outhandle 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 lightweighting will not only make the car lighter,
but
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 "bumper backdating" improves the car's
appearance. Best of
all, it's free.
Install
Better Tires and Wheels
Get
better tires, and also better wheels if you can afford
it. The factory
155/80-13 tires
are rather tall and skinny, even considering the 4.5 inch wide rims
that most
Spitfires had on the showroom floor.
Try 175/70-13s instead.
This size has the same overall diameter as the 155/80-13s,
so the ride
height and speedometer readings won't change and will be accurate. 175/70-13s will fit
acceptably on 4.5
inch wide rims, and won't lead to rubbing or any other interference
issues on a
stock Spitfire. However,
switching
to wider rims will help a great deal.
Wider rims enable the tires to form a better contact patch
with the road
and thus perform better, especially in turns.
175/70-13s on 13x5.5 inch rims with 1 inch of positive
offset (i.e., about 4.3 inches of backspacing) will fit the Spitfire
nicely. If you
don't mind your
speedometer reading about 4 percent faster than you are really going,
then
shodding 13x5.5 rims with 185/60-13 tires will lower the car by
½ inch
and help improve handling some more by lowering the center of gravity
(c.g.) of
the car. A set of
good quality
alloy rims, like Superlites,
is a good way to go. They
are
truer and lighter than steel wheels, and while any weight savings is
good, in
this case the reduction is in unsprung weight, which helps handling
even
more. If the cost
of alloy rims is
prohibitive, you can adapt
steel TR7 wheels to the Spitfire.
Stock steel TR7 wheels are 13x5.5 with about 1 inch of
positive
offset. Even wider
rims are
possible, but choice of offset is critical to avoid interference
between the
tires and the car's bodywork or suspension elements.
13x6 rims with 1 inch positive offset with either
175/70-13
or 185/60-13 tires will work, as will 14x5.5 or 14x6 wheels with 1 inch
of
positive offset and 175/65-14 or 185/60-14 tires.
However, clearances will be tight, and given the normal
assembly
tolerances of a Spitfire, you could have some rubbing between the tires
and the
inner edges of the fender lips—particularly at the front. Other wheel and tire
combos are
possible, but there is potential for interference.
Moreover, ever-larger wheels with lower aspect tires
aren't
as compliant and can produce a harsher ride and lead to higher shock
stresses
on the suspension elements.
Achieve
Better Suspension Geometry—Lower and Stiffen
the Front
The
purpose of good suspension geometry is to ensure proper
positioning of the tires relative to the road for good grip, which
results in
safer and faster handling.
Changing wheel alignment and lowering the
Spitfire—particularly
the front end—can do a lot for handling.
Most Spitfires have their front ends too high for optimum
suspension action. Lowering brings the c.g. is 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.
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, especially the front,
optimizes the
geometry of the suspension links so that the car will roll less for a
given
c.g. location. Good
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 of each front coil spring is about 7
inches
with the car at rest. Having
the
lower A-arms about parallel with the ground favorably locates the car's
roll
center while also lowering the c.g.
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 shocks allow fine adjustment of
static ride height, 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 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 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 but 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, thus
increasing
the likelihood of bottoming-out the suspension.
Running out of suspension travel and hitting the bump
stops
is harsh and potentially damaging to your car.
The best 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 business 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 250 pound per inch stiffness and 10 inches
free
length. They have
an installed
length of 7 1/8 inches in my lightweighted car and they are neither too
stiff
nor too soft for my tastes, and the car handles very nicely. Here's how this setup
turned out:
Something that you should do when lowering the front (even if you use stiffer springs, which will reduce the amount of travel for a given load) is to make sure you have adequate suspension travel before hitting the rubber or rubber-like bump stops contained in the front shocks. Trimming the front bump stops to be no less than 1/2 inch thick but no more than 3/4 inch thick should ensure adequate travel while also protecting the shocks against internal damage (at least when using coil springs of 250 lbs/inch and up).
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 such that
the lower
A-arms are parallel with the ground and the compressed length of the
coils is 7
inches with the car at rest. Some
negative camber at the front is desirable in that it aids entry into
turns, and
a little bit such as 0.5 to 1 degree negative will not cause uneven
tire
wear. Another thing
that lowering
the front end will do is change the at-rest toe setting, so be sure to
realign
the front wheels. 1/16 inch toe-in as prescribed in
the
repair manuals is a good setting for all-around use.
I've developed a simple, inexpensive and effective way to
do
make your own front
toe adjustments.
Note
that stiffening the front of a vehicle typically increases
understeer (everything else being equal), but the improved geometry
gained from
the lowering described above offsets this and the net result is a car
that
handles much better and close to neutral.
Correct
the Rear, and Install a Camber Compensator on Early
Spitfires
With
so much attention to the front, it's time to turn attention
to the rear. Luckily,
making the
aforementioned changes to the front will not adversely affect the rear,
and you
probably will not have to do anything at the rear.
The Spitfire rear suspension is a swing axle architecture
employing a transverse leaf spring.
As such, it is a good idea to have about 3 degrees of
negative camber at
each rear wheel as prescribed in the repair manuals.
Moreover, it is also a good idea not to soften it so that
favorable geometry is maintained.
While stiffening the transverse leaf will help maintain good
geometery, it is also good not to stiffen it much for a street vehicle
with an open differential or else the extra
weight transfer
during a turn may allow the inside wheel to spin and lead to oversteer. In the case of my 1500 as
with many
others, my factory-original rear leaf assembly seems to have sagged after manufacture,
but it
has stabilized. With
the
adjustments to the front end discussed above, the rear end actually
rose up a
little bit and the resulting geometry is good with 3 degrees negative
camber.
The
mkIV and 1500 Spitfires have a "swing spring" design rear
leaf assembly that works to compensate for unfavorable camber change
and the
jacking effect intrinsic to the swing axle architecture. In the case of the early
fixed spring
Spitfires that are prone to dangerous jacking under hard braking and
cornering,
it is a good idea to install a camber compensator.
It looks like a single leaf spring, pivoting in the middle
at the differential and attached at the ends to the vertical links out
by the
wheels. Joe Curry
sells camber
compensator kits that do the job.
Add
a Stiffer Front Anti-Sway Bar
Now
on to the topic of anti-sway bars.
If you want to add more roll stiffness without adding
straight-line stiffness, than you can add anti-sway bars. These are simply torsion
springs that
act only when the car rolls. The
stock 1500 has a 7/8 inch diameter front anti-sway bar, but a 1 inch
bar has
1.7x more roll stiffness and provides 535 lbs per inch of total
end-of-bar
deflection; the stock mk3 Spitfire has a 11/16 inch bar, but a stock
7/8 inch
Spitfire 1500 bar has 2.6x more roll stiffness and provides 314 lbs per
inch of
total end-of-bar deflection (stiffness goes with the 4th power of
thickness, so even a small change in bar diameter makes a big
difference). As
already mentioned, all else being the same, adding stiffness at the
front
without adding it at the rear will make a car understeer more. But improved suspension
geometry during
turns due to reduced body roll helps offset this and so the Spitfire
doesn't
seem to suffer from the added total front roll stiffness that a thicker
front anti-sway bar provides.
Adding
roll stiffness at the rear by means of a small anti-sway bar is
not a wise idea on a swing axle car because it exacerbates jacking
(although it is
acceptable to use a rear anti-sway bar on Spitfires equipped with the later swing spring). Moreover, adding roll
stiffness at the
rear will cause the inside wheel to unload more in a turn, and with an
open
differential, the power will go to the unloaded inside wheel, perhaps allowing
it to
spin and thus slowing the car down.
The only time it's probably good to add roll stiffness at
the rear of a Spitfire is if the front stiffness has been increased so much that the
car
understeers significantly, but at this point the car is probably so
stiff as to
be used for serious autocrossing or racing and not the street. For street use, leaving
the rear roll
stiffness alone is fine.
Install
Good Dampers (Shock Absorbers)
Last
is the subject of dampers (shock absorbers).
Some are really expensive!
For street use, you can use the stock
variety. The fronts
won't be ideal
if you stiffen and lower the front, but it's the least expensive way to
go. Some medium
priced shocks like
KYB Gas-A-Just
are better. If you don't mind spending some money, then
adjustable shocks by Koni, Spax, Gaz or Avo will do nicely and will
allow you
to match the bump and rebound characteristics of the dampening to the
stiffness
of the springs. As
mentioned
earlier, you can buy front shocks with adjustable spring perches that
allow
fine adjustment of static ride height, but these are the most expensive
variety
and you can avoid having to use them if you pick the right stiffness
and free
length of coil springs.
Stock
spec rear shocks work at the rear, or you can go with
air-adjustable '63-'78 Corvette rear shocks (like Monroe MaxAir
MA785), which are
very competitively priced.
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 shocks.
Because you can add air to these, you can fine-tune the
height of the
rear (which also changes rear suspension stiffness a small amount), and
if you
make separate air feeds for each of the two shocks, you can adjust each
side
independently. As
for the front,
you can also buy premium shocks for the rear.
In summary, you can use stock shocks, but installing
better
units will improve handling and your driving enjoyment.
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
Lose
Weight
Most
motorcycles can out-accelerate even 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 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 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).
Advance
the Ignition Timing
First thing, advance the ignition timing to around 10 or 12 degrees before top dead center (BTDC) at idle (with no vacuum advance connected while you set it) or as close to this as possible without inducing pre-detonation (a.k.a. pre-ignition, pinging, pinking, or knocking). The ignition timing of many Spitfires was really retarded from the factory (at least in the U.S.), which may help pass emissions tests but it hurts performance. Just this simple change can make your engine feel like it has come alive. Like lightweighting, it's free. Also, if you have a vacuum retard setup, switch it to vacuum advance. You might have to retune your carb(s) after this.
Convert from Points to 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., electronic ignition. The
Pertronix series of products are easy, reliable drop-in replacements
that take the place of points in your distributor. Another way to
go is a distributorless electronic ignition 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, use open-source software and have a large user community, so
support is readily available. In any case, switching from points
to electronic ignition can enhance performance and will reduce
maintenance and improve reliability.
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 like a musical instrument in
that it is tuned (either by design or by accident) to operate best at a
certain
engine speed. 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 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. 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, 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. You can also try Weber
side-draught
carbs (e.g., DCOEs) instead of SUs.
These are even more expensive, but they look good and 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 can help.
"Flow" or "Port and Polish" 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 round-over the sharp edge of the beveled step inside the combustion chamber where the spark plug pokes through can be done easily and inexpensively yourself and will noticeably help performance. You can even attempt to ease the vertical walls of the chamber to "unshroud" the valves, which will improve flow. Just be sure 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
Triumph Competition Prep 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 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. 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 or in the little gap
between the piston and the cylinder wall and above the
top
ring. On a Spitfire
1500 with a
7.5:1 static compression ratio, as all were in the U.S. except 1976
models,
changing
the pistons from the stock dished ones to flat-top ones will reduce Vcc
by about
6.7 cubic centimeters and raise compression to 8.3:1.
This helps performance some.
You can raise compression further by shaving or milling
material off of the head surface, which shrinks the volume in the head
(part of
Vcc) and thus increases the ratio.
However, increasing compression increases the chance 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, which lowers the effective or
dynamic
compression ratio of the engine when it is running.
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 low-compression Spitfire by
switching from
dished-top pistons to flat-top pistons (if you don't already have
them) and shaving material off your cylinder head will help performance.
Changing pistons isn't
too difficult but new pistons are moderately expensive.
Shaving your cylinder head
is easier and 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 to reach
your
desired static compression ratio.
But
if
you start out with a stock, unmolested 7.5:1 1500 engine, replace the
dished
pistons with flat-top ones, and shave about 0.080" off the head, then
you'll have an engine with a static compression
ratio
of 9.5 to
9.75:1.
Change
the Camshaft
If
you have an engine with raised 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 vacuum at low engine speeds and lowers the effective
compression ratio, reducing torque and power at low RPM.
So, choosing
a camshaft 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
new ones,
but be aware of material and surface finish choices and compatibility.
You'll need new tappets and maybe different valve springs
and pushrods 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 270 (particularly
in a 1500) or
280 degrees and
valve overlap under 60 degrees or thereabouts so that adequate
performance at
low RPMs is preserved.
Moreover, don't bother with valve lifts greater than about 0.380"
unless some flow improvements have been made to the head. These
guidelines are not hard and fast rules because many factors determine
camshaft performance, and
these guidelines
assume that you have raised the static compression ratio and
have
installed good-flowing exhaust and intake parts, the specifics
of which affect
camshaft selection.
For more on camshaft
terminology, theory and practice, check out these references:
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 manufacturing and assembly tolerances.
If you are asking more from your engine, it's a good idea
to tighten many of these tolerances with more precise machining.
Have a machine shop align bore your block (i.e., precision
machine 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,
and 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. In
the
process, the shop will do things like magnafluxing to check for cracks
in
ferrous parts and assure the structural integrity of parts. All of this will enable
the engine to
run smoother with less self-induced stress and enable it to be
run at
higher RPMs. 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.
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:
Stage
1:
-
properly
advanced ignition and upgrade to electronic ignition (e.g., Pertronix)
-
4-2-1
exhaust
-
better
flowing carbs and intake manifold (e.g., twin SU
carburetors or single Weber DCOE carburetor)
-
K&N
filtration
Stage
2:
-
minor
head work (matching of head and intake manifold ports, and removal of
casting and machining
irregularities in the head)
-
increased
compression (e.g., flat-top pistons in a 7.5:1 1500
engine for 8.3:1 compression)
Stage
3:
-
align
bored block and fully-balanced rotating and reciprocating
parts
-
9.5
to 9.75:1 compression
-
fast
road cam
-
more
extensive head work (e.g., unshrouding of valves in head, 3 or 5-angle
grind on valve seats, shaped backsides of valves, narrowed
valve stems, recessed valve guides)
-
"big
valve" head (from early 1500; intake valve diameter=1.438", as
compared to 1.380" for other 1493cc, 1.303" for 1296cc and 1.246"
for 1147cc engines)
Stage
4:
-
twin
Weber DCOEs or fuel injection
-
>9.75:1
compression (and use of advanced fuels, ignition)
-
ultimate
road or race cam
-
4-1
exhaust
-
advanced
materials (lightened connecting rods, push rods and
tappets, forged pistons, steel crankshaft, roller rockers, etc.)
A
Stage 1 engine is relatively easy and inexpensive to set-up
and gets you the most bang for the buck.
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 (the difference will be most dramatic with a late U.S. market
Spitfire). Stage 3
is still perfectly
useable on the street, but things start to become expensive. Full Stage 4 and beyond is
really the
realm of racing and the like and 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 115 peak bhp at 6500 RPM but
can't idle smoothly or has anemic performance at 2000 RPM 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. For
more detail, see the article "Building
a
Reliable Spitfire Engine for High Performance" by Calum
Douglas, ""A Guide
to Racing your Triumph Spitfire or GT6" by Jon Wolfe or one of the
writings of Kas
Kastner.
Switch
to Fuel Injection
Fuel
injection is intrinsically more efficient than carburetion,
and modern technology fuel injection control systems like MegaSquirt
and electronic
ignition controllers like MegaJolt
enable a great deal of control over fuel injection and ignition. I have driven a few
fuel-injected
Spitfires, and I can say it 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 Spitifire with a
fuel-injected 2.0 liter GT6 engine installed in a special way.
Go
Crazy
Beyond
the aforementioned things, getting more power 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. Other performance
enhancements like
forced-induction (supercharging or turbocharging), higher performance
fuels,
oxidizers (like Nitrous Oxide) and other exotic things can produce more
power
but often at the expense of durability and reliability and can require
significant modifications to strengthen the block as well as the
aforementioned
special ultra-high performance parts.
Such things are typically not suitable for a street
vehicle, and are
outside the theme of this article.