Anyone know how to figure compression psi?????

I can give you the equation for the "ideal" pressure however it will give you a calculated value that is way too high. One reason it doesn't work exactly is because the equation is based on a constant mass of air being compressed when in fact, the exhaust valve timing is such that it doesn't close til after BDC so the mass isn't constant. Typically the actual pressure is about 60-70% of the ideal. The ideal equation is as follows: p2 = p1 X (compression ratio)^1.4. (1,4 is not the "exact value, but for what we are doing it's OK)
So if you are sea level and the pressure is 14.7 psia and your compression ratio is 16, then p2=14.7 x (16)^1.4 = 713 psia. If we take 60% of that, p2act = 428psia. Since your gauge doesn't read absolute pressure, it reads gauge pressure, we subtract the ambiet pressure of 14.7 psia. That gives 413 psi. If we use 70 %, p2act = 484 psi.

Hope this helps you.

Yall take care now

John T

Atmospheric pressure drops by about 1 psi for every 2300 feet in elevation above sea level. So if you live much above sea level, you need to calculate the local atmospheric pressure and use that number instead of 14.7.

Of course no engine ever reads the maximum, but this will give you an idea of whether yours is in the ballpark.

Keith

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