What Is Compression Ratio?

Remember what happens during the compression stroke of the four-stroke cycle: Both the intake and exhaust valves are closed so no air can escape, and the piston moves upward from bottom dead center (BDC) to top dead center (TDC) so that the air/fuel mixture in the cylinder is compressed into the combustion chamber. Compression ratio is the relationship of cylinder volume (or displacement) with the piston at BDC to cylinder volume with the piston at TDC. If the volume of the cylinder with the piston at BDC is 10 times greater than the volume of the combustion area with the piston at TDC, then 10 units of volume get squeezed into 1 unit of space, and the compression ratio is 10.0:1. There are five factors that affect compression ratio: cylinder swept volume, clearance volume, piston dome or dish, head-gasket volume, and chamber volume.