Cycle-to-cycle combustion variation and engine emission levels are now recognized as effective indicators of engine performance. A system was devised that directly indicates combustion variation by measuring the work produced in a combustion chamber for every power stroke; it also continuously monitors engine emission level. Using this system, then, the effects of spark plug and ignition system variables on engine performance were examined. These criteria were employed in investigating the effects of spark plug gap size, gap location, electrode size and gap geometry, and the effects of spark timing, spark duration, and spark energy at various engine air-fuel ratios. In addition to studying each variable independently, statistically designed experiments to investigate the interactions between the variables were conducted. Although all the variables had measurable effects on engine performance, the most significant were spark plug gap size, gap location, spark timing, and spark duration.