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Compression ratio
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Compression ratio

The compression ratio is a single number that can be used to predict the performance of any internal-combustion engine.

To calculate the compression ratio;
b = cylinder bore (diameter)
s = piston stroke length
Vc = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated.

CR= (π/4 × b2 × s + Vc) / Vc

Due to pinging the CR in a petrol or gas (LPG or CNG)-fueled engine will not exceed 10:1
In a turbo charged engine the CR wil be around 8.5:1
In a diesel engine the CR will be 20:1 and higher.