The cycle-by-cycle variations of the combustion process of a spark ignition engine result in cycle-by-cycle (CBC) variations of the cylinder pressure development histories. Many investigators have postulated that these pressure variations are due to CBC variations of the gas motion near the spark plug at the time of ignition. This investigation was undertaken to determine if such a correlation does in fact exist.The CBC combustion variations of a CFR-RDH engine were examined in terms of the CBC pressure development histories. A constant-temperature hot-film probe was used to determine the velocity variations from cycle to cycle of the motored engine in the vicinity of the spark plug at the crank angles at which ignition would take place under firing conditions.The standard deviations of the gas velocity near the spark plug were correlated to the standard deviations of the crank angle at which maximum pressure occurred for different operating variables. The correlation was made using a model which assumed that the time required to burn the same initial volume of charge each cycle was dependent only on this velocity.With this model, a correlation of 0.975 was obtained between the standard deviation of the velocity and the angle of occurrence of maximum pressure. This led the authors to conclude CBC combustion variations are caused by CBC variations of the gas velocity in the vicinity of the spark plug at the time of ignition.