Compression ratio (CR) is an engineering term used to describe internal-combustion engines. It describes the difference in volume from maximum to minimum as a piston engine operates.
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.
CR= (π/4 x b2 x s + Vc) / Vc
The compression ratio is a single number that can be used to predict the performance of any internal-combustion engine.
Due to pinging the compression ratio in a petrol-fueled engine will not exceed 1:10.
In a turbo charged engine the CR wil be around 1:8.5.
In a diesel engine the CR will be 1:20 and higher.