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Originally Posted by Tempus
IIRC, Octane is just a measure of the branch chain hydrocarbon isomers. I can't remember the formula for it though.
Higher octane ratings just mean that there are literally a higher % of the octane isomers and opposed to the other thing (heptene?) which is still flamable as all hell but tends to explode (engine knock) early and easily.
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Pretty much. Octane rating doesn't actually measure octane content it rates a fuel's ability to withstand compression without knocking relative to an octane/heptane mixture. 0 is the equivalent of burning pure heptane. 100 is the equivalent of burning pure octane. This is how you can get octane ratings above 100. Confusingly there are also two octane scales (RON and MON), and it's not always abovious which is used. Europe, Australia and most other countries display the "RON" octane rating on pumps while the US and Canada use the average of the two (also called DON). This means the same fuel in the US would have a rating on the pump about 5 points lower than in Europe or Australia.