This paper presents theory and experiments results for a noncontacting method of measuring reciprocating IC engine torque nonuniformity. This method yields an index of torque nonuniformity which is potentially useful for detecting abnormal engine operation and has important application for control and diagnostic functions. The nonuniformity index can be computed for each engine cycle if desired or averaged over many cycles. The method also has the capability of detecting intermittent abnormal operations.This paper explains the nonuniformity index and describes noncontacting instrumentation with which it can be measured. The results of experimental measurements of the nonuniformity index in actual operation of a vechicle equipped with an automatic transmission are presented. The statistical distribution of this index is shown to be such that abnormal engine operation can be reliably detected even if such operation occurs infrequently.