Abstract: An improved process and apparatus for measuring fuel octane numbers in a knock test engine of a multiplicity of fuels, using at least one fuel of known octane number as a reference, wherein fuel feed means, memory means, and calculating means cooperate so that the fuel feed means selects the fuel source and the fuel feed-rate for that fuel source, the memory means receives the knock intensity values generated in the test engine and the corresponding fuel feed-rate settings, the calculating means (a) determines the approximate feed-rate setting that corresponds to the maximum knock intensity, (b) provides a series of feed-rate settings bracketing said approximate feed-rate setting and generates a corresponding set of knock intensity values, (c) determines the refined feed-rate setting that corresponds to maximum knock intensity, (d) sends said setting to the memory means for storage, and (e) from the knock intensities that are generated during subsequent comparison of the fuels of known and unknown octane numb