Hi Panic . . .

"True, but 115psi/14.5 is 8:1 compression isn't." It is in fact a measure of static compression.

You are correct that effective C/R can be a hard thing to get ones arms around. And my original response was perhaps an over simplification.

C/R is a measure of volumes V@BDC/V@TDC - as the article you mention points out this is really a drawing-board measure - which is impacted by a lot of factors. Static compression is measure of pressures P@TDC/P@BDC.

I zeroed in on static compression or the overall-pressure-ratio because it can be very revealing about what is going on in the engine: Wiki Atricle on OPR

There is a table in the Wiki article which shows C/R versus OPR.
Because compression adds heat the pressure ratio is considerably higher than the C/R and grows faster as C/R increase. At a C/R of 10:1 the rise in pressure is 22:1. At sea level that is 14.7 X 22 or ~323 PSI.

I had a 327 V-thing that measured 12:1 compression by volumes. The cylinder pressures where just shy of 270 PSI. But that is way short of the theoretical pressure ratio of 10:1 C/R.

So a reading of 115 PSI at 425 feet of above sea level is just not indicative of a 12:1 C/R engine.

These two articles discuss the practical relationships between PSI readings and underlying C/R:
Van Ness
Andrews

regards,
stock49


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