– Why are you supposed to put high octane gas in some (typically luxury brand) cars?
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The otto cycle engine has a compression step before ignition. The higher the compression rate is, the more performance you can take out from the engine assuming the same cylinder number, volume and everything else.
But there can be a problem, because the fuel can start burning (or detonating) without ignition, during compression. It means that the piston is still moving towards the inside of the cylinder (compressing), while the fuel is already trying to expand and forcing the piston backwards. However the piston has no chance to move backwards before it would finish the compression movement and reach the endpoint first. This is called knocking and extremely damaging to the engine.
To avoid that, you must use fuel that can tolerate the pressure, in other words it doesn’t start detonating in the compression phase. The normal 95 octane gas has a given compression tolerance so these engines can compress at around 12:1 rate or so. (In older times and even nowadays in some countries you would find 93 octane that would go with around 10:1 compression rate or so.)
If you want a stronger engine, you can either pack more cylinders in it, or make the cylinders bigger, or give them larger compression rate. Strong cars actually combine all of these. But if you compress more, you need larger octane number, such as 98 or even 100 (for a super sport car).
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