– Why are you supposed to put high octane gas in some (typically luxury brand) cars?
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The engine compresses air fuel mixture in the pistons before it ignites it with the spark plug. The more the engine compress the air mixture the more efficient the detonation and the more powerful the engine for a given engine size. High performance engines therefore tend to compress the air mixture more then regular cars. However if you compress the mixture too much it will detonate by itself even before the spark can ignite it. This is called engine knocking and will cause the explosion to push the piston down the wrong direction. This both reduces performance and can damage the engine. Higher octane fuel have a harder time igniting so it will handle more compression before self-detonation. So it will not knock as easily as regular fuel. You can also put high octane fuel in a regular car but this have a normal compression ratio and does not suffer from the knocking so it will have minimal effect. In fact it can be harder for the spark plugs to ignite the high octane fuel and you may end up with reduced performance and it may damage your spark plugs and oxygen sensor over time.
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