Compression ratio compares the cylinder volume at the bottom of the stroke to the volume at the top — how much the charge is squeezed.
Where
| Swept volume | Displacement of one cylinder (bore and stroke) |
| Clearance volume | Combustion chamber volume at top dead center |
Worked example
A cylinder with 500 cc swept and 50 cc clearance gives (500 + 50) ÷ 50 = 11.0:1.
FAQ
Why do higher-performance engines use higher compression ratios? Higher compression ratios extract more energy from each combustion cycle, improving efficiency and power, but they also increase the risk of engine knock, which is why higher compression engines typically require higher-octane fuel.
Does compression ratio account for forced induction like turbocharging? No, the basic formula describes the engine’s mechanical (static) compression ratio, not the effective compression that results once a turbocharger or supercharger adds pressurized air, which is usually discussed as a separate, dynamic figure.
Use the Compression Ratio calculator in the automotive calculators.