Compression ratio compares the cylinder volume at the bottom of the stroke to the volume at the top — how much the charge is squeezed.
CR = (Swept Volume + Clearance Volume) ÷ Clearance Volume
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.
Higher is more efficient but demands higher-octane fuel to avoid knock; forced induction usually runs a lower static ratio.
Use the Compression Ratio calculator in the automotive calculators.