Comprehensive single-cylinder engine data for three combustion chambers were analyzed statistically in order to quantify the effects of engine operating conditions and chamber geometric variables on combustion characteristics, and of combustion on engine performance stability. Operating condition variables of interest were air-fuel ratio, residual fraction (internal plus external EGR), spark timing, engine speed, and fueling level (trapped fuel per cycle). Geometric parameters of importance were chamber “openness” and squish.Combustion and engine performance stability were found to be related such that engine stability was improved when combustion variations were reduced, as would be expected, and/or when the combustion event was shortened. The combustion, in turn, was affected by both the engine operating conditions and chamber geometric characteristics. The chemical factors -- air fuel ratio and residual fraction -- affected both the length and variability of combustion (on a crank angle basis). The physical factors -- chamber geometry, spark timing, engine speed, and fueling level -- affected primarily the length of the combustion event and secondarily the steadiness of combustion. Currently, the most practical approach for improving engine stability appears to be shortening of the overall combustion event. Adjustments of physical factors, particularly chamber openness and turbulence-generating features, provide a good opportunity for achieving this reduction.