What is the compression ratio of spark ignition engine?

The compression ratio of a spark ignition engine typically ranges from about 8:1 to 12:1, although some high-performance engines may exceed this. The ratio is defined as the volume of the cylinder when the piston is at the bottom of its stroke (BDC) divided by the volume when the piston is at the top of its stroke (TDC). A higher compression ratio generally improves efficiency and power output but can also lead to engine knock if the fuel's octane rating is insufficient.