A calculated ratio that relates the uncompressed volume of the cylinder with the piston
at bottom dead-centre to the compressed volume of the cylinder with the piston
at top dead-centre. Hence a 9:1 compression ratio means that the air-fuel mixture is compressed to one-ninth of its original volume before being ignited. The compression ratio bears little relationship to the actual effective pressure created in the cylinder. Can also be described as the ratio between the volume of the cylinder above the piston when the piston is at BDC (including the volume of the combustion chamber) to the volume when it is at TDC (that is, the combustion chamber only). Expressed as a formula, compression ratio is: cv + sv / cv where CV equals clearance volume and SV equals swept volume.