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The octane rating has to do with the flash point of the fuel. The higher the rating the harder it is to ignite. A spark has no trouble igniting the air/fuel mix but what we're trying to avoid is the pre-ignition of the mixture. When the piston compresses the mix in the cylinder it starts out at a certain volume and then gets crushed anywhere between 8 to 11 times as small(that's the compression ratio, eg 8.9 to 1). The air/fuel mix gets really hot just because it's getting squished(that's what gasses do) and if the temp reaches the flash point, BANG it explodes before the spark plug can do the job. The down side to this pre ignition is that the piston has still not completely finished the top of the stroke and now must do so in the face of an explosion. That means that the carefully timed explosion event which should occur when the piston has just started down(lots of energy) is early and the explosion energy that should be carried to the crankshaft via conecting rod is lost and even can contribute to slowing down the piston or in a bad scenario bend the rod or blow a hole in the piston. The flash point is reached easily in high compression engines, 9:1 to 11:1 and not so easily in low compression engines, 8:1 to 9:1. You need not worry if you run 91 as recomended but you should only run lower octane if you run in cooler weather. Conversely don't waste your money on the highest octane fuel either just because of the advertising hype. You should only buy as high a rated fuel as your car needs to run without pre ignition, any more is wasted if you have a lower compression motor. Turbo owners should go with the highest they can find because the air gets hot by being compressed by the turbo and then again in the combustion chamber.
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