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Cranking Pressure Calculator
Estimate cranking compression pressure from dynamic ratio
Cranking pressure is the compression-gauge reading an engine produces while cranking, estimated here from its dynamic compression ratio.
Formula: Cranking pressure ≈ 14.7 × (DCR^1.3 − 1) — polytropic estimate (γ = 1.3), sea level
What this number means
This estimates cranking pressure from dynamic compression ratio using the standard polytropic estimate at sea level. It links the build-sheet numbers to the gauge reading you can measure in the pits.
What to change next
Compare cylinders against the estimate and against each other: one low hole points to that cylinder's rings, valves or head gasket, not the combination. A whole engine reading low suggests cam timing or a gauge and procedure worth checking.
Worked example
A dynamic ratio of 7.5:1 at sea level estimates ≈ 187 psi — Wikipedia's worked example for this formula lands on the same figure. Real gauges commonly read lower; consistency between cylinders matters more than the absolute number.