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Compression ratio
The compression ratio determines how hard the mixture is squeezed and drives both power and fuel requirements. Enter the geometry and all the volumes above the piston, and we work out the static ratio.
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Compression ratio
9,98:1
Static compression ratio. Combustion chamber and piston dome are always entered in cc (dome +, dish -).
How the calculation works
The compression ratio is the whole volume above the piston at bottom dead centre divided by the volume left at top dead centre. The cylinder volume comes from the bore and stroke. The clearance volume is the sum of the chamber volume, the gasket volume and the deck volume, minus any dome on the piston (a dish is added instead).
The right compression depends on the fuel, boost and cam timing. Too high a ratio on the wrong fuel gives knock and damage. When boosting you lower the compression, while race fuel and E85 tolerate more. This is the static ratio, the dynamic compression is also affected by the cam.
Example
96 mm bore, 80 mm stroke, a 50 cc chamber, a 96 mm gasket at 1,5 mm, 0,5 mm deck and a flat piston give about 10,0:1.
Guide values for compression and fuel
| Fuel or setup | Guide value |
|---|---|
| 95 octane (E10) | 9,5-10,5:1 |
| 98 octane | 10,5-11,5:1 |
| E85 | up to 13:1 |
| Race fuel | 12:1 and up |
| Turbo or supercharger | lower 1-2 steps compared with naturally aspirated |
Guide values for a naturally aspirated engine on pump petrol. An aluminium head often tolerates half a step to a full step higher than cast iron.
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Common questions about compression ratio
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