This work focuses on the prevention from knock in the case of SI engines supplied by natural gas network. The effect of two inert gases (N 2 and CO 2 ) adjunction is experimentally studied. The added quantities are between 0% and 25% in volume for N 2 and between 0% and 15% for CO 2 . A significant increase of the Knock Limited Spark Timing ( KLST ) is measured in all the cases. A twice-higher augmentation is noted when CO 2 is added compared to N 2 for an equivalent volumetric concentration. The overall augmentation varies between +1 to +6 °CA depending on the operation. Finally, a law for predicting the KLST augmentation implied by the adjunction of inert gases is deduced from all the measurements.