> I raised this topic a year or so ago to little response, but
> the composition of the list has changed a bit so I'm giving
> it another try.
I'm not a Spitfire wizard by any stretch of the imagination, but
I can answer your questions about engine modifications.
The system approach that I always recommend in engine modifications
not only stresses the fact that engines must work in harmony among
all components, but also stresses looking for the hierarchy of weak
links in the system. Find out what the worst point about your
motor is before you randomly go about changing things. I'm going to
make some assumptions, based on my fairly limited expertise in the
Spitfire 1500 motor; you really need a Spitfire expert to help you
with parts selection. But this article will talk to you about what
it takes to make more power out of the same engine, with some
assumptions based on what I know about the changes between US and
UK Spitfires, as well as early and late Spitfires (and recall that
while my main area of expertise is in M.G.s, the '75 and up Midgets
got the Spitfire 1500).
> Questions on my mind include: Can I just shovel in the UK 9:1
> pistons and a UK cam to good effect, or will I need to change out
> all sorts of other things, like the distributor, carb (ZS), etc.
Yes. In fact, I'd suggest changing all those other things first,
for a lot of reasons:
- you can do them each in a weekend or so, while the car is still
operational
- you can do them one at a time as budget permits and still have a
running car between changes (Hint: this is a BIG DEAL)
- they will be synergistic to some extent, in that each one will let
the preceding work a little better rather than worse
- most telling, until you do these changes, the high-compression
pistons and camshaft won't do you any good because you won't be
able to take advantage of them if you can't get the air in and
out of the engine fast enough through the existing system.
The truth of British sports car development from 1970-19870 is that
for the most part, the engines are the same, it's the add-ons that kill
the performance. Some facts:
1. Every full point of compression gives approximately a 4% boost in
power. If memory serves, the US Spits came with 7.5:1 CR and 53 bhp.
This means your projected compression enhancement will net you all of 3
bhp on the Spitfire, and that's *if* the exhaust, carburetion and
ignition timing will let you realize any benefit from the change. Most
likely, all that'll happen is that you'll ping like mad on any gas
below 92 octane, and the car will otherwise run about the same.
2. If you change the cam's timing and duration (I don't know what
sort of differences there are between the US and UK cams), you will
need to change the carburetor to take advantage of the new cam.
Cams change the speed at which air goes in and exhaust goes out
(as well as changing the volume, but for your application, it's
the speed rather than absolute volume of intake-exhaust gases
that you want to maximize). If you're trying to pull more intake
through the same carburetor, especially if you're trying to do
this with the same metering device, and then trying to push it out
through the same restrictive manifold, you'll lose. Oh, and you'll
need to change the exhaust as well, or the backpressure in the
stock unit will be high enough that you'll lose all the theoretical
benefits of the hot cam.
3. The distributor's advance curve is useful through about 3000 RPM.
Above that, you've got full advance. However, because this car is
a street-driven vehicle and you say you're more concerned with
acceleration than with absolute speed, this means the distributor
is probably the first thing you want to change.
4. Does your car have the integrated intake-exhaust manifold, with
log-style design for intake and exhaust? I'll bet it does. If
so, that's the single worst feature of your car's performance.
Go to a dual-carb intake manifold, a header with long branches
and large-radius, smooth curves where it changes directions, and
a reasonably large internal diameter in the exhaust system (2" is
probably big enough for the main pipe on a 1500).
If you're thinking about a UK cam (and I won't even begin to discuss
smog liability, that's your lookout depending on your state laws), go
ahead and shop around for a sportier cam to begin with. Talk to a
supplier of high-performance stuff, preferably someone who knows Spit-
fires (can you think of someone on this list who matches these specs?
Fat Chance! :-) But you'll probably be limiting the benefit of the
extra pistons and cam unless you improve the breathing of the engine.
For the same money and less trouble, I think you'll enjoy your car a lot
more by changing the externals -- go to a free-flowing exhaust, change the
needle on the carb or go to a dual SU setup from the UK, and put in a
hot recurved distributor (note that the distributor curve has exactly
zero effect on overall BHP production at the car's power peak, but it
has a tremendous effect on the car's responsiveness below about 3500 RPM,
or wherever the distributor develops full advance; once it's maxed, that's
it, but below that point, the distributor curve can make a big difference
in how the car *feels* to drive when you pull away from a light or
merge onto a freeway.)
All these changes might net you 5 to 10 bhp at the wheels, if you have
everything optimized. This is roughly a 10% to 20% increase over the stock
engine's output, which you *will* feel. When that's not enough, then you
can talk about tearing the insides of the motor apart and making it
more interesting. But putting a good cam and high-compression pistons
into the restrictive, late-Seventies breathing setup will be a waste
of sweat (nevermind the money :-).
While you're at it, remember to improve handling first: make sure
that all bushings are sound, that your shocks are in good shape,
and that your tires and brakes are up to the improved performance.
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