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The effects of fuel octane have been assessed on the efficiency and emissions of a high compression ratio (ε=13) spark ignition direct injection (SIDI) engine. Under low load stratified operation (1200 rpm, ∼20% load), a low octane fuel (RON=84, comprised of toluene, iso-octane, and n-heptane) yielded higher brake thermal efficiency and significantly lower hydrocarbon emissions than a base gasoline (RON=91). The indicator diagram for the low octane fuel showed evidence for two stage heat release, suggesting the presence of spark induced compression ignition (SICI). These results suggest that higher efficiency under low load stratified conditions can be obtained with lower octane fuels that undergo SICI combustion. The effect of fuel octane under high load was assessed at WOT with a high RON model fuel (RON=103, comprised of toluene, iso-octane, and n-heptane).