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Engine tests were conducted on a production 2.5-liter V-6 engine to investigate the effects of spark plug tip designs on a 4-cycle SI engine of current technology. The data suggest that cyclic variation can increase when the ground electrode faces the primary intake port. Lean-operation limits were extended by the use of J-gap spark plugs as compared to surface-gap and ring-gap spark plugs at the conditions tested. The surface-gap type spark plugs lose some energy as the arc traverses the surface of the insulator. Voltage requirements decrease for reversed polarity at the part load conditions tested but increase at wide open throttle.