– Why are you supposed to put high octane gas in some (typically luxury brand) cars?
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High performance gasoline engines have higher compression ratios, which increases the risk of fuel autoignition and engine knocking (which, contrary to popular belief, are actually two different albeit related phenomena). To counter that, the fuel is refined to be more resistant to autoignition under compression. The measure of this resistance is called the Octane Number.
However, the fuel isn’t “better” in-and-of-itself. Putting 89 or 91 in a car that’s meant for 87 isn’t going to do anything other than burn a hole in your wallet. In the opposite direction, though; butting 87 in a car meant for 91 (or higher) will *absolutely* do damage to the engine over the long term. So basically always use the fuel rated for your engine.
There are a few situations where changing octane rating can be useful, though;
1) Older/beat up vehicles will sometimes have fueling problems that will increase knock. Thus, it used to be that once you got above 100-150k miles you might need to switch to the next highest ON of fuel if you didn’t want to actually fix the problem.
2) If you’re in a situation where you absolutely need to run the engine in conditions where it’s going to be prone to overheating, using a higher octane fuel will help mitigate knock issues.
3) If you slap an aftermarket turbo on your engine, you typically want to switch to the next highest ON rating of fuels available.
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