Compression ratio calculator

Static and dynamic compression

Static compression is pure geometry: how much the cylinder volume shrinks from bottom dead center to top dead center. It ignores the cam, which is why two engines with the same 10:1 static ratio can behave very differently.

Dynamic compression starts counting only once the intake valve has closed, because the cylinder can't build pressure while the valve is still open. A cam that closes the intake late shortens the effective stroke and lowers dynamic compression. That's why a big cam can live with more static compression than a small one.

This calculator finds the piston's position at the moment the intake closes from your stroke and rod length, then uses that shorter effective stroke in the same ratio.

Which intake-closing number should you use?

This is where most builders get stuck. Cam cards list valve timing at different lift points: some at "advertised" or seat timing (often 0.006 in or 0.008 in of lift), others only at 0.050 in of lift. A calculator that wants one while you type in the other will be off by a lot.

If your card gives advertised or seat timing for intake closing, use that directly. If it only gives duration at 0.050 in and the intake centerline, this calculator works out intake closing at 0.050 in and adds 15°, a widely used convention that approximates seat timing. It's an estimate: cam profiles differ in how quickly they close.

Cranking pressure

The cranking pressure figure is a rough estimate using a polytropic exponent of 1.2 at sea level. Real gauge readings depend on cranking speed, ring seal, altitude and your gauge, so compare it with a compression test rather than treating it as exact.

Tested against a published builder example: 4.030 in bore, 3.48 in stroke, 65 cc chamber, 5 cc reliefs, 0.015 in gasket and 0.025 in deck gives 10.27:1 static, matching the 10.2:1 the builder reported.