Engine Compression Ratio Calculator
Swept volume plus clearance volume, over clearance volume.
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Engine Compression Ratio Calculator by TheCalcTools
How the Engine Compression Ratio Calculator works
Enter the bore and stroke to find the swept volume.
Add the combustion chamber, head gasket and piston dish volumes to get the clearance volume.
The compression ratio is (swept + clearance) ÷ clearance.
The formula
Worked example
Using the default values shown in the calculator:
| Bore (mm) | 86 |
|---|---|
| Stroke (mm) | 86 |
| Combustion chamber volume (cc) | 45 |
| Head gasket volume (cc) | 5 |
| Piston dish volume (cc, minus for dome) | 4 |
- Swept volume = π ÷ 4 × 8.6² × 8.6 = 499.6 cc; clearance = 54.0 cc.
- Compression ratio = 10.25 : 1.
Result:
| Swept volume per cylinder (cc) | 499.6 |
|---|---|
| Clearance volume (cc) | 54.0 |
| Compression ratio | 10.25 : 1 |
Frequently asked questions
What compression ratio is normal?
Petrol engines are typically 9:1 to 12:1; diesels 14:1 to 22:1. Higher ratios need higher-octane fuel to avoid knock.
Does a turbocharger change it?
Static compression ratio is unchanged, but boost raises the effective compression, so turbo engines generally use lower static ratios.
Sources and further reading
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