David,
The static compression ratio is determined geometrically - you measure
the displacement D of a cylinder, then the combustion chamber volume C
(include the volume of the head gasket hole), and then the static
compression ratio is (D+C)/C. If you have domed or dished pistons, or if
the piston doesn't travel right to the deck, you need to compensate the
combustion chamber volume accordingly. When compression ratios are
quoted you'll typically get this number.
However, this figure doesn't take into account what happens when the
valves open and close. On a typical engine, the intake valve doesn't
close until after bottom dead center, so the cylinder doesn't start
getting pressurized until the piston is partway up the bore. This
results in the observed cylinder pressure as you'd measure with a
compression tester being less than what you might get if you just took
14.7 psi and multiplied by the compression ratio (I know there are other
effects at play: the air is being compressed more or less adiabatically,
and so on... let's ignore that for right now). The more radical the
camshaft, the later the intake valve will close, and the lower the
cranking pressure will be for a given static compression ratio.
When the engine is running, the situation changes again from the
low-speed cranking scenario, and the actual cylinder pressure is going
to be affected by the velocity of the air/fuel mixture moving into the
engine, and the exhaust gases being expelled. Racing cams typically have
significant overlap where both the intake and exhaust valves are open at
the same time, and the exhaust gas velocity is used to suck the intake
mixture into the cylinder. Then (especially in tuned-length intake
runner manifolds) the intake mixture builds up enough momentum that you
can get more than the manifold pressure in the cylinder, just as the
intake valve is closing. In this case, the cylinder has effectively been
slightly supercharged, and the cylinder pressure at TDC will be
correspondingly increased.
Increasing the static compression ratio makes the engine more
thermodynamically efficient, provided it's not so high that you run into
detonation territory. Race cams have altered valve timing that allows
higher static compression ratios and at the same time improve the
air/fuel mixture pumping efficiency at higher RPM, which puts more
mixture in the cylinder more often, and that makes more power.
Theo
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