A single-cylinder engine with a transparent piston crown was tested with three intake geometries which produced different levels of swirl, and with two spark locations. A heat-release analysis was performed on the accumulated pressure-time data. Using this data base, relationships of maximum cycle pressure and its location in the cycle to the placement of the burn in the cycle are developed for different burning rates as a means of diagnosing sources of combustion variability.Among the significant observations were that combustion variability for operation away from the lean limit or incipient misfire is not strongly dependent on flow conditions at the spark plug and early flame behavior, but more dependent on the cyclic repeatability of the overall cylinder flow pattern. In the region of incipient misfire the early flame process becomes a significant source for variability. The onset of significant variability in IMEP corresponds to a condition where a collection of cycles shows the production of an increasing number of abnormally late burns as the spark is advanced. Additional spark advance results in the occurrence of misfire at approximately the frequency of the late-burn occurrence.