Estimate the compression-test psi you should see from the static compression ratio.
The engine static compression ratio, e.g. 10 for 10:1.
About 14.7 psi at sea level, lower at altitude.
A rough target for a healthy compression test - real readings vary with cam timing and engine condition.
A common approximation is cranking psi = atmospheric x (compression ratio) raised to about 1.2-1.3, reflecting the polytropic compression of air. A 10:1 engine typically cranks around 175-210 psi when healthy.
Cam timing (especially performance cams), altitude, a cold engine, ring or valve wear and gauge differences all lower the reading. The estimate is a ballpark, not an exact figure.
Consistency between cylinders. A spread of more than about 10% between the highest and lowest cylinder points to a sealing problem worth investigating.
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