The compression ratio is the amount that the volume of the incoming charge is compressed by, prior to combustion taking place. It is designed to be as high as reasonably possible. As a rough guide, normally aspirated engines would have a compression ratio of around 12:1, while forced induction engines, where the air is already pumped in under pressure, would be more like 7:1. That is, the volume of the gas is compressed to 1/7 of its original volume prior to the spark igniting it.
There are two main limiting factors on how high an engine’s compression ratio can be. The first is that the loads on the piston, rod and crank get too high, as a result of the increased combustion pressure. The second is the detonation limit of the fuel used. The figures above are for the static compression ratio, i.e. the theoretical values based on the engine being full of air at atmospheric pressure when the inlet valve closes. However, the actual, or dynamic compression ratio, will be different, since an engine will not necessarily have a 100% efficiency figure.