Compression ratio (CR) is the ratio of the difference in the cylinder volume when the piston is at the bottom of the stroke to the volume in the cylinder above the piston when the piston is at the top of the stroke. The compression ratio of an engine is an important consideration when rebuilding or repairing an engine. ● See Figure 1.
Fig.1 Compression ratio is the ratio of the total cylinder volume (when the piston is at the bottom of its stroke) to the clearance volume (when the piston is at the top of its stroke).
Calculating Compression Ratio:
The compression ratio (CR) calculation uses the formula:
● See Figure 2.
Fig.2 Combustion chamber volume is the volume above the piston with the piston is at top dead center.
For example: What is the compression ratio of an engine with 50.3 cu. in. displacement in one cylinder and a combustion chamber volume of 6.7 cu. in.?
Changing Compression Ratio:
Any time an engine is modified, the compression ratio should be checked to make sure it is either the same as it was originally or has been changed to match the diesel compression ratio. Factors that can affect compression ratio include:
■ Head gasket thickness. A thicker than stock gasket will decrease the compression ratio and a thinner than stock gasket will increase the compression ratio.
■ Increasing the cylinder size. If the bore or stroke is increased, a greater amount of air will be compressed into the combustion chamber, which will increase the compression ratio.