Octane number indicates the ability of gasoline to resist engine knocking when burned in an engine. It is determined by comparing the gasoline to mixtures of iso-octane and n-heptane, with iso-octane rated at 100 and n-heptane rated at 0. A higher octane number means the gasoline is better quality and less likely to cause knocking. Factors that influence octane number include fuel composition, with branched alkanes having higher octane than straight chains. Common octane ratings for Indonesian fuels are 92 for Pertamax and 88 for Premium. Tetraethyl lead was previously used to increase octane but is toxic, and has now been replaced by methyl tertiary buthyl