Compression Ratio to PSI Calculator
Estimate the peak cylinder pressure created by a given static compression ratio, ambient pressure, and boost level.
How to Use This Calculator
Enter your static compression ratio
Use the ratio from your engine build or calculate it with our compression ratio calculator.
Set the ambient pressure
Sea level is 14.7 psi. Reduce for high elevations or use actual barometric pressure.
Add boost pressure
Enter manifold gauge pressure supplied by turbochargers or superchargers. Use zero for naturally aspirated engines.
Adjust efficiency
Account for real-world heat losses and leakage. Naturally aspirated engines are often 90–98% efficient.
Formula
Cylinder PSI = (Ambient + Boost) × Compression Ratio × Efficiency
Efficiency: Entered as a decimal (e.g. 95% → 0.95) to model heat losses and valve leakage.
Example with CR 10.5:1, ambient 14.7 psi, 8 psi boost, 95% efficiency:
Intake absolute = 14.7 + 8 = 22.7 psi
Ideal cylinder = 22.7 × 10.5 = 238.4 psi → Effective = 238.4 × 0.95 = 226.5 psi
Gauge pressure = 226.5 − 14.7 = 211.8 psi
About the Compression Ratio to PSI Calculator
Converting compression ratio to pressure helps tuners estimate the stress on internal components, choose head gaskets, and anticipate octane requirements. This tool combines your compression ratio with intake pressure and real-world efficiency to produce realistic cylinder pressure estimates.
When to Use This Calculator
- Boost planning: Evaluate cylinder pressures before raising boost levels.
- Fuel selection: Determine whether your setup can run pump gas or needs race fuel / E85.
- Engine builds: Check if connecting rods, pistons, and head studs can handle projected pressures.
- Altitude tuning: Adjust expectations for engines running in thin air.
Why Use Our Calculator?
- ✅ Boost-aware: Automatically includes turbo or supercharger pressure.
- ✅ Multi-unit outputs: View results in PSI, kPa, and bar simultaneously.
- ✅ Efficiency factor: Avoids overestimating pressure by incorporating real-world losses.
- ✅ Instant adjustments: Perfect for comparing new heads, cams, or boost targets.
Common Applications
Dyno prep: Estimate pressures before scheduling tuning sessions.
Engine simulation: Combine with detonation modeling tools for safe tuning.
Maintenance planning: Assess head gasket and fastener requirements for raised boost levels.
Tips for Best Results
- Use actual barometric pressure for your location when racing at altitude.
- Lower the efficiency percentage for high-compression engines with significant heat soak.
- Compare calculated pressures to manufacturer limits for rods, pistons, and head gaskets.
- Pair this tool with a dynamic compression calculator to factor in cam timing.
Frequently Asked Questions
Is this the same as a compression test reading?
Compression testers measure cranking pressure with the starter turning the engine. This calculator estimates theoretical peak pressure at top dead center during full compression, so numbers will be higher than a cranking test.
How should I pick the efficiency percentage?
Healthy naturally aspirated engines often operate between 90–98% efficiency. Forced-induction engines may experience more heat and leakage—start around 90–95% and adjust based on experience or data logs.
Can I use this for diesel engines?
Yes, but remember that diesels have much higher compression ratios and intake pressures. Ensure your efficiency factor reflects diesel combustion characteristics and high boost levels.
Does cam timing affect the result?
Cam timing changes dynamic compression, which alters real cylinder pressure. Use this calculator for static estimates and combine it with a dynamic compression tool to account for valve timing.