The design of combustion chamber has an important influence upon the engine performance and its knock properties. The design of combustion chamber involves the shape of combustion chamber, the location of the spark plug and the disposition of inlet and exhaust valves. Because of the importance of combustion chamber design, it has been a subject of considerable amount of research and development in the last fifty years. It has resulted in raising the compression ratio from 4:1 before the First World War period to 8:1 to 11:1 in present times with special combustion chamber design. Increase in CR increases the p and T of the charge at ignition, reduces the mass fraction of the residual gases - more favorable conditions are developed for ignition which reduces the first stage of combustion, and increases flame propagation rate in the main stage.

Increasing CR, increases Area/Volume ratio of the cylinder, increasing the cooling effects and the quench layers. Final stage of combustion is increased. Therefore while designing SI engine combustion chamber surface to volume ratio should be kept maximum for end gas region