Compression Ratio to PSI Calculator — Engine Cylinder Pressure
Convert a compression ratio to estimated cylinder pressure in PSI. The calculator uses the adiabatic formula P₂ = P_atm × CR^γ (γ = 1.4 for air) — the same rapid-compression process that happens inside an engine during a compression stroke.
PSI
Estimated peak cranking compression at top dead centre (upper bound)
- 1
Adiabatic growth factor (CR^γ)
10^1.4 = 25.1189γ = 1.4 for air; the factor by which pressure multiplies during adiabatic compression. - 2
Isothermal pressure (lower bound)
14.7 × 10 = 147 PSI - 3
Adiabatic cylinder pressure
14.7 × 25.1189 = 369.2
How does this calculator work?
Cylinder pressure ≈ P_atm × CR^1.4 (adiabatic compression, γ = 1.4 for air). At a standard 14.7 PSI atmosphere and 10:1 CR, this gives ≈ 185 PSI. Real engine test readings run 10–25% lower. Also shows the isothermal lower bound (P_atm × CR) and conversions to kPa and bar.
Formula
How this is calculated
When a piston compresses the air–fuel charge in a cylinder, the process is approximately adiabatic — it happens too quickly for significant heat exchange with the cylinder walls. For a diatomic ideal gas (air, γ = 1.4), the pressure ratio equals the volume ratio raised to the power γ: P₂/P₁ = (V₁/V₂)^γ = CR^γ. Multiplying atmospheric pressure P_atm by CR^1.4 gives the theoretical peak cranking pressure.
For comparison, the isothermal pressure (CR × P_atm, which assumes the temperature stays constant throughout compression) is also shown as a lower bound. The true result lies somewhere between isothermal and adiabatic, depending on compression speed, ring seal quality, and intake charge temperature. Real measured cranking pressures are typically 10–25% below the adiabatic value due to leakage past rings and valves, late intake valve closing (which reduces effective CR), heat losses, and slow cranking speed.
Typical compression readings: naturally aspirated petrol 130–180 PSI; turbocharged petrol 120–220 PSI; diesel 250–450 PSI. Readings significantly outside these ranges when testing a running engine indicate worn rings, leaking valves, or a failing head gasket.
Frequently asked questions
Air is mostly diatomic nitrogen and oxygen. Diatomic gases have 5 degrees of freedom (3 translational + 2 rotational), giving a heat-capacity ratio γ = Cp/Cv = 7/5 = 1.4 at typical engine operating temperatures. This is the standard value used for air in adiabatic compression calculations.
Most manufacturers specify 130–180 PSI (≈9–12 bar) at cranking speed, with no cylinder deviating more than 10–15% from the highest reading. Low readings indicate worn piston rings, burned valves, or a blown head gasket.
The formula assumes perfect sealing, ideal gas behaviour, and adiabatic compression. In practice: slow cranking speed allows more heat loss; rings and valves never seal perfectly; late intake valve closing reduces the effective CR; and cold oil increases blow-by. Actual readings are typically 10–25% below the adiabatic theoretical value.
Also known as
TG we-Calculate Editorial Team. (2026). Compression Ratio to PSI Calculator — Engine Cylinder Pressure [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/compression-ratio-to-psi-calculator
TG we-Calculate Editorial Team. "Compression Ratio to PSI Calculator — Engine Cylinder Pressure." TG we-Calculate. 2026. https://we-calculate.com/calculator/compression-ratio-to-psi-calculator.
TG we-Calculate Editorial Team, "Compression Ratio to PSI Calculator — Engine Cylinder Pressure," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/compression-ratio-to-psi-calculator
@misc{wecalculate_compression_ratio_to_psi_calculator, title = {Compression Ratio to PSI Calculator — Engine Cylinder Pressure}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/compression-ratio-to-psi-calculator}}, year = {2026}, note = {TG we-Calculate} }
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