Compression Ratio to PSI Calculator

Physics

The Compression Ratio to PSI Calculator estimates an engine's cranking compression pressure from its compression ratio using a polytropic approximation. Enter the compression ratio.

Formula

Absolute PSI ≈ 14.7 × CR^1.3; Gauge PSI = Absolute − 14.7
  • This is a polytropic estimate; actual cranking pressure varies with cam timing, altitude and engine condition.
  • The exponent 1.3 approximates real compression behaviour.

10:1 compression ratio

Inputs
  • Compression Ratio (X:1): 10

14.7 × 10^1.3 ≈ 293 PSIA, about 279 PSI on a gauge (estimate).

Frequently asked questions

Is this cranking pressure exact?
No — it is an estimate. Real cranking PSI depends on camshaft timing, altitude, and engine wear.