Compression Ratio Calculator
Enter the swept volume and clearance volume, or a bore and stroke, to find an engine's static compression ratio, the basis for fuel choice and tuning.
How to calculate compression ratio
- Enter the swept volume in cc, or a bore and stroke in mm.
- Enter the clearance (combustion chamber) volume in cc.
- Read the static compression ratio as an X:1 figure.
Examples
From volumes
swept 500 cc, clearance 50 cc
11.0:1
From bore and stroke
bore 86 mm, stroke 86 mm, clearance 55.5 cc
about 10.0:1
Frequently asked questions
What is compression ratio?
Compression ratio compares the cylinder volume at the bottom of the stroke to the volume at the top. A 10:1 ratio means the charge is squeezed into one tenth of its starting space.
What is the compression ratio formula?
Static CR equals the swept volume plus the clearance volume, all divided by the clearance volume: (swept + clearance) / clearance.
What is clearance volume?
Clearance volume is the space left above the piston at top dead centre, mainly the combustion chamber plus gasket and any piston dish or dome. It is the small number in the ratio.
Does a higher compression ratio need higher octane?
Usually yes. Higher compression raises cylinder pressure and temperature, which can cause knock, so engines with high static CR often require premium fuel or boost control.
Is this static or dynamic compression ratio?
This tool reports static compression ratio from geometry alone. Dynamic compression also depends on cam timing and valve closing, so it is usually lower than the static figure.
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