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Cylinder pressure variation is a fundamental and widespread combustion problem in spark-ignited engines. The basic factors causing this problem are variations both in the start and in the rate of combustion. These variations occur not only from cycle to cycle within each cylinder but may also show up as consistent differences between cylinders. Our test results indicate that the major cause of cyclic combustion rate variation is the mixture velocity differences that exist within the cylinder near the spark plug at the time of ignition. As yet we do not know how to reduce the cyclic mixture velocity variations and thus reduce the problem at its origin. However, it is possible to circumvent some effects of cyclic pressure variation by increasing the average combustion rate.