Compression Ratio Calculator
Calculate your engine’s compression ratio from swept volume and combustion chamber volume — essential for engine building.
Compression Ratio Calculator
Swept volume + chamber volume → CR
How It Works
Compression ratio compares the total cylinder volume at BDC (bottom of stroke) to the clearance volume at TDC. Higher CR increases thermal efficiency but demands higher-octane fuel.
How to Use This Calculator
- Divide total engine displacement by cylinder count to get swept volume per cylinder.
- Measure or look up the combustion chamber volume (cc) — from head casting data or measured with a burette.
- Include the compressed head gasket thickness volume (bore × gasket thickness × π/4) for accuracy.
- Enter both values and click Calculate.
- Compare the result against the fuel octane requirement for your application.
Worked Example
Reference Table
| Application | Typical CR | Min Octane (RON) | Notes |
|---|---|---|---|
| Standard diesel | 16:1–22:1 | N/A (CI) | Compression ignition, no spark |
| Standard petrol NA | 9.5:1–11:1 | 91–95 | Most production cars |
| Performance petrol NA | 11:1–13:1 | 95–98 | Sports and GT engines |
| Turbocharged petrol | 8:1–9.5:1 | 95–98 | Lower CR to avoid knock |
| Naturally-aspirated race | 13:1–15:1 | 98–102 | Full race engines |
| Rotary (Wankel) | 9:1–10:1 | 91–95 | Different geometry |
Frequently Asked Questions
Why do turbocharged engines have lower compression ratios?
The turbo compresses the intake charge before it enters the cylinder, raising the effective CR at boost. Using a lower static CR prevents detonation (knock) under boost while maintaining high effective pressure.
How does head gasket thickness affect compression ratio?
The compressed gasket adds to the combustion chamber volume. A thicker MLS gasket (0.9 mm vs 0.5 mm standard) can reduce CR by 0.2–0.5 points on a typical engine — important for boost applications.
Can I increase compression ratio without machining?
Yes — removing material from the head (decking) reduces combustion chamber volume and raises CR. Fitting higher-compression pistons or changing to a dish/flat/domed piston also alters CR effectively.
What happens if compression ratio is too high for the fuel octane?
Detonation (knock) occurs — the air-fuel mixture auto-ignites before the spark, creating a shockwave. Modern ECUs retard ignition timing to suppress knock, reducing power and efficiency. Prolonged knock causes piston damage.
