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AN approach to the problem of selecting the optimum compression ratio and spark timing as related to the antiknock qualities of the fuel to be used, has been developed. First it is determined at points throughout the range of engine speeds how far the spark can be advanced before knock appears or power falls off. Then a distributor curve is drawn which is just safely under the curve of maximum permissible spark advance versus engine speed. This information is recorded for each compression ratio under consideration. Finally power output and specific fuel consumption at the spark timing indicated by the distributor curve are compared for the different compression ratios. It has been found that fuels of high sensitivity permit greater spark advance than fuels of low sensitivity.