– Why are you supposed to put high octane gas in some (typically luxury brand) cars?
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The ocatane number of petrol describes how resistant it is to autoignition, that is to say how much you can compress it before it gets so hot that it ignites itself (this is actually the method that Diesel engines just normally use for their engines).
The reason this is important is because higher compression makes engines more efficient (and therefore for a given size also more powerful), so you want to go as high as possible in the compression, but if you go to high your air fuel mix will just ignite by itself, before the spark plug can, and that can cause a backfire, which is when the fuek ignites before the zylinder passes the neutral point, meaning that zylinder is suddenly blasted in the opposite direction it’s supposed to be moving (hence back fire).
This is also why leaded fuel exists, because this drastically improves the octane number of petrol.
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