Flame kernel growth and cylinder pressure data were simultaneously obtained from an optically-accessible, square piston, SI engine. Flame kernel growth was measured using simultaneous, orthogonal, Schlieren photography, while cylinder pressure was measured using a piezoelectric pressure transducer. The data were analyzed on a cycle-resolved basis to determine the correlation between cyclic fluctuations in flame kernel growth and cylinder pressure.The engine was operated at 875 RPM with premixed, prevaporized, stoichiometric isooctane in air. The engine, designed with ported intake and exhaust, was fired every tenth cycle to ensure complete scavenging. Tests were conducted with and without nitrogen dilution, while ignition timing was fixed at 25° BTDC. With 0% dilution the percent variation in the maximum cylinder pressure was 8.5%, while with 10% dilution the percent variation increased to 14%.The correlations between the fluctuation in flame kernel radius and the fluctuation in maximum cylinder pressure and maximum cylinder pressure rise rate were found to increase with increasing flame kernel size from a value of approximately 0.45, when the flame kernel radius was 5mm, to a value of approximately 0.75, when the flame kernel radius was 10mm. The correlations were somewhat greater for maximum cylinder pressure than for maximum cylinder pressure rise rate, while the effect of dilution was negligible.