THE WORK REPORTED in this paper is part of an on-going study of cyclic variability in spark-ignition engines. In order to analyze knock and its variability from cycle to cycle in a meaningful way, an algorithm has been developed to characterize the severity of the bulk pressure change associated with knocking combustion. This algorithm depends on the third differential of pressure over the period when autoignition might occur. A large negative value of third differential indicates the abrupt pressure rise and narrow pressure peak commonly associated with end gas autoignition. This knock diagnostic has the advantage of working on a low frequency data acquisition rate (1 point/CA° or less) and of producing consistent results in spite of high frequency noise on the pressure signal such as might be caused by resonance of a spark plug-mounted pressure transducer.The proposed Knock Indicator is applied to data from a low speed Waukesha C.F.R. engine and is compared with several other pressure trace parameters to establish its validity as a knock diagnostic. It is shown that the Knock Indicator consistently predicts the degree of abrupt pressure rise associated with end-gas autoignition and thus is a suitable algorithm for monitoring the severity of knock as well as its occurrence. The method is suitable for use within most research programs which use routine engine cycle analysis as it adds knock severity measurement without any additional equipment or operational constraints. The low computational requirement may also make the method useful for future on-board combustion diagnostic systems.