Bob Kramer wrote:
>
> FOT,
> I know that some of the more seasoned and/or engineering types out there
> know the answer to this question. My machinist claims that an increase in
> compression from 10.5 to 1 to 12.5 to 1 will produce serious HP gains,
> something like 40 to 50%. Is that true?
It's about an order of magnitude off. Standard seat-of-pants estimate
is around 4% per point, so you may be looking at an 8% increase from the
two-point CR bump alone.
On the plus side, CR increases throttle response all the way up the RPM
range, which is a good thing. And there may also be related effects,
such as making a hot cam more (or less) effective due to increased
cylinder pressures changing the way exhaust scavenging works. System,
system system.
It's been my experience, however, that if you plan to go over 10.5 or 11
to 1, you really need race gas and forged pistons. I have a lovely
"custom" piston in my garage that has big pieces missing from the top,
to remind me not to run an 11.5:1 head on 92 octane gas. (See "system"
comment, above.) You'll be running higher cylinder pressures (meaning
more stress on head gasket, rings, valves, esp. if this is on one of the
wet-liner engines) and higher cylinder temperatures (meaning more stress
on valves, piston face, and cooling), so if your car is or has been
marginal in those areas to begin with (what? a British sports car
marginal in cooling, cylinder head sealing, or valve life? I'm
shocked), you may want to consider enhancing those while the machinist
is skimming your car's head.
Also, remember that a head sliced down to provide a 12.5:1 CR may well
be a use-once, then discard item. You may not end up with enough metal
left in it to clean it up again after it warps because of overheating,
or burns through when the gasket goes, or whatever causes its demise.
Hmmm, let's visualize this: taking the nominal 2138cc displacement of
certain TR engines because it's what I happened to type into the
calculator :-), your 10.5:1 CR gives you a combustion chamber of 50.9
cc. Going to a 12.5:1 CR means you have to reduce the combustion
chamber to 42.8 cc, a reduction of 8cc. You might be better off
(depending on valve interference issues) going with a piston that has an
extra 8cc in the dome, but then your machinist probably isn't selling
pistons...
Bottom line, though, is that even the 8% increase will make the car more
fun to drive, crisper and more responsive; there's a special feel to
high-compression engines. You just have to know you're making the
tradeoffs against higher probability of failures elsewhere in the car as
a result of the extra pressure and heat.
--Scott Fisher
typing with Triumph grease spots on his hands today to qualify as an
FOT
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