<P> Higher octane ratings correlate to higher activation energies: the amount of applied energy required to initiate combustion . Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition or detonation . </P> <P> The compression ratio is directly related to power and to thermodynamic efficiency of an internal combustion engine (see Otto - cycle). Engines with higher compression ratios develop more area under the Otto - Cycle curve, thus they extract more energy from a given quantity of fuel . </P> <P> During the compression stroke of an internal combustion engine, as the air - fuel mix is compressed its temperature rises . </P> <P> A fuel with a higher octane rating is less prone to auto - ignition and can withstand a greater rise in temperature during the compression stroke of an internal combustion engine without auto - igniting, thus allowing more power to be extracted from the Otto - Cycle . </P>

Is us 91 octane equivalent to australian 91