How to Convert Compression Ratio to PSI
An engine's compression ratio (CR) describes how much the air-fuel mixture is compressed in each cylinder. A 10:1 compression ratio means the mixture is squeezed into one tenth of its original volume before ignition. The resulting cylinder pressure depends on how much that compression raises the pressure of the gas.
The standard formula for estimating cranking cylinder pressure is:
Cylinder PSI = Atmospheric Pressure x CR^n
Where n is the polytropic exponent. A value of 1.3 is the most widely used approximation for real engines during a cranking compression test. A theoretical ideal (isentropic) compression uses n = 1.4. The simple linear estimate (n = 1.0) gives the minimum and is sometimes used as a quick rule of thumb.
Worked Example
For a petrol engine with a 10.5:1 compression ratio at sea level using the standard approximation (n = 1.3):
- P0 = 14.696 PSI (atmospheric pressure at sea level)
- CR = 10.5
- n = 1.3
- Cylinder PSI = 14.696 x 10.5^1.3 = 14.696 x 21.259 = 312.4 PSI
- In kPa: 312.4 x 6.89476 = 2,154 kPa
- In bar: 312.4 x 0.068948 = 21.54 bar
Polytropic Exponent Explained
The polytropic exponent n determines how pressure and volume relate during compression:
| n value | Process | Use case |
|---|
| 1.0 | Isothermal (constant temperature) | Simple rough estimate only |
| 1.3 | Polytropic (real engine approximation) | Standard for cranking compression tests |
| 1.4 | Isentropic (ideal adiabatic) | Thermodynamic theory maximum |
Real engines fall between isothermal and isentropic because compression happens quickly but not instantly, and some heat transfer occurs. The n = 1.3 approximation produces results closest to what a compression gauge actually measures on a healthy engine.
Typical Compression Ratios and Pressures
| Engine Type | Typical CR | Approx PSI (n=1.3) | Notes |
|---|
| Economy petrol | 8:1 to 9:1 | 219 to 256 PSI | Older or budget engines |
| Modern petrol | 10:1 to 12:1 | 293 to 372 PSI | Most current naturally aspirated engines |
| Performance petrol | 11:1 to 14:1 | 332 to 454 PSI | High-output naturally aspirated engines |
| Turbocharged petrol | 8:1 to 9.5:1 | 219 to 274 PSI | Lower ratio due to boost |
| Light diesel | 14:1 to 18:1 | 454 to 630 PSI | Car and light truck diesels |
| Heavy diesel | 16:1 to 23:1 | 540 to 866 PSI | Trucks and industrial engines |
Why Measured Compression May Differ
The calculated figure represents the theoretical cranking pressure assuming a sealed cylinder with perfect gas behaviour. Actual compression test readings are typically 10 to 20 percent lower due to:
- Valve overlap and late intake valve closing (some air exits before compression begins)
- Ring and valve seal wear (leakage past rings or seats)
- Low cranking speed (less air ingested per cycle)
- Altitude (lower atmospheric pressure)
- Engine temperature affecting gas density
A compression reading more than 20 percent below the theoretical value, or with more than 15 percent variation between cylinders, typically warrants further diagnosis.
Related Calculators
Method: Polytropic compression formula P = P0 x CR^n, where P0 is atmospheric pressure (14.696 PSI at sea level), CR is the compression ratio, and n is the polytropic exponent (default 1.3). PSI to kPa conversion: 1 PSI = 6.89476 kPa. PSI to bar: 1 PSI = 0.068948 bar. References: Heywood, J.B., Internal Combustion Engine Fundamentals (McGraw-Hill, 1988); Taylor, C.F., The Internal Combustion Engine in Theory and Practice (MIT Press, 1985).
This calculator gives a theoretical estimate of cranking cylinder pressure. Actual compression test readings depend on engine condition, cranking speed, temperature, and altitude. Always use a calibrated compression tester for diagnostic purposes.
