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A study was conducted to investigate the relative influence of charge cooling and autoignition chemistry on the greater knock resistance seen by alcohol fuels compared to petrols when operating under “Beyond RON” conditions in a Port Fuel Injection (PFI) engine. The methodology employed was that of a modelling study calibrated and validated using experimental data, with ethanol and iso-octane used as representatives of the alcohol fuels and petrols respectively. A two zone combustion model combined with an empirical knock model formed the centre of the modelling work, with the experimental investigation conducted on a boosted PFI engine. The comparison of knock resistance between ethanol and iso-octane showed that autoignition chemistry plays the largest role in the knock resistance advantage of ethanol. This dominance by autoignition chemistry is partly aided by PFI's poor use of the charge cooling capacity of ethanol.