Compression ratio

[1] When the piston is at the bottom of its travel (BDC), the volume of cylinder is measured (suppose the volume is X). Then the piston is placed at the top of its travel (TDC) and the volume of the cylinder is measured (suppose this volume is Y). The compression ratio is a comparison of these two values expressed as X:Y. Then the values are mathematically changed so that the second number is always 1. Thus you hear of ratios like 10.5:1 or 9.5:1 or 8:1. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely.
[2] Ratio of the volume of the clearance space to the total volume of the cylinder. In refrigeration it is also used as the ratio of the absolute low-side pressure to the absolute high-side pressure. See also primary compression ratio