Current methods for reducing emission of oxides of nitrogen (NOx) from the spark ignition (SI) engine employ dilution of intake charge with relatively inert gases which tends to limit peak combustion temperatures and pressures. Employment of intake charge dilution has led to reduction in engine power output and increased combustion cycle-by-cycle (CBC) irregularity. This investigation sought to determine the degree of increased CBC combustion variations experienced as increased amounts of charge dilution reduced emission of NOx. Emission of unburned hydrocarbons (HC) was also documented.It was found that increased CBC variations result from employing intake charge dilution as a tool to reduce NOx emissions. The significant aspects of the increased CBC variations were an observed increase in maximum cyclic pressure dispersion, a slower flame speed as reflected by an increased angle of occurrence of peak cyclic pressure, and increased variations in the crank angle of peak pressure.