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RAPID development and widespread use of aircraft fuels of high knock rating make advisable a study of the best means of utilizing these fuels for high specific power output. The response of an hypothetical aircraft-engine cylinder to changes in supercharger compression ratio (boost) and cylinder compression ratio is developed theoretically and outlined by means of charts. The explosion pressures are calculated and the resulting information used to establish a criterion of cylinder performance. Results of some cylinder calibrations on high-octane fuels show the calculated relations to be fairly well established. The effects of (1) high mean effective pressures and (2) high crankshaft speeds on the detail design of the engine are analyzed. The conclusion is reached that, in order to obtain a balanced design of minimum weight for extremely high specific output, it is advisable to use both speed and high mean effective pressures.