Compression Ratio to PSI Calculator

Convert your engine's compression ratio to estimated cylinder pressure (PSI), or vice versa.

:
psi
Standard at sea level is 14.7 psi.
PSI
Note: This is a direct pressure ratio conversion. Real cranking pressure varies with cam timing (IVC) and thermal efficiency.

How to Convert Compression Ratio to PSI

The conversion between Compression Ratio (CR) and PSI (Pounds per Square Inch) estimates the pressure inside the cylinder assuming a perfect seal and linear compression relationship relative to atmospheric pressure.

Formula

The formula to calculate the theoretical pressure is:

Pressure = (X / Y) × Atmospheric Pressure

Where:

  • X is the numerator of the compression ratio (e.g., 10 in 10:1).
  • Y is the denominator of the compression ratio (usually 1).
  • Atmospheric Pressure is typically 14.7 psi at sea level.

Reverse Calculation

If you know your cylinder pressure (e.g., from a compression test) and want to estimate the compression ratio:

CR = Pressure / Atmospheric Pressure

(Assuming Y = 1)

Note: Real-world cranking compression (psi) is affected by the camshaft's intake valve closing point (Dynamic Compression), heat transfer, and leakage, so this theoretical value is a baseline maximum.

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