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Model fuels composed of isooctane, n-heptane, toluene, ethanol and metyl tertiary butyl ether (MTBE) were tested in a modern high speed engine with electronically controlled injection and closed loop, λ=1 control system. The fuel compositions were chosen to represent a spectrum of straight run, catalytically cracked and reformulated gasolines. All fuels were blended in such a way that they had the same research octane number (RON) of 95 as determined by testing in a CFR engine according to the standard ASTM procedure D-2699. 5, 10, 50 and 90 percent energy release and maximum pressure crank angles and their variations, as well as knock probabilities, intensities, locations and characteristics, were determined for all the fuels at two speeds, 25 and 50 rps, at full loads. The fuel composition has a weak effect on the energy release parameters and their variations, but a strong effect on knock.