In continued studies with full-scale engines on the laboratory dynamometer, the effect of the engine variables air-fuel ratio, ignition timing, and rate of exhaust gas recirculation on octane requirement increase and exhaust emissions was studied in two engines at two compression ratios. A lead-free base gasoline was used in all tests, and a test procedure previously shown to relate well with road tests was followed. In both engines, ignition timing and rate of exhaust gas recirculation were found to have appreciable effects on octane requirement increase. Air-fuel ratio had an effect in one engine but not the other. The effect of compression ratio was mixed.