>
> The TR4 stock compression ratio is 9:1, I think. Doesn't that
> mean that the cylinder compression reading should be about 130
> psi (given 14.7 psi ambient pressure). Your numbers look more
> like from an engine with 10:1 or 11:1 compression.
>
> Ever curious,
> Charlie Farwell
> 1962 TR4 #CT3852L
Cylinder pressures are not a simple linear function. The reason being
heat gain. As you squish the stuff in the cylinder, the temperature
goes up. This causes more pressure, which causes more heat, which causes
more.... with quickly diminishing returns on each iteration.
One thing that affects cylinder pressures at crank speed (compression
test scenario) is valve timing. Compression ratios are a calculation of
change in volume from absolute bottom (BDC) to absolute top (TDC) of the
stroke. However, the actual compressing won't start until the intake valve
closes. A typical street engine will pull test pressures of around 150 psi.
My street car did... until I put in some "hot" (stage 1) cams, and left
the compression ratio alone. It now pulls about 120 psi, and I'm shopping
for 10:1 pistons. :>
This is why valve adjustment has such a major effect on compression
pressures. The very tail end of the valve movement is a drawn out
deceleration so that theh valve is not slammed into it's seat. The movement,
normally termed in thousandths (of an inch) per degree, here is better
termed in degrees per thousandth. A few thousandths off on the valve
adjustment can move the actual valve close point many degrees.
Randy
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