I wasn't talking about fuels anywhere beyond 87 and 93. I was focusing on fuels around 89 and 91. Below 87 isn't necessarily safe, and above 93 can't provide any real world benefit to a stock engine.

Due to variations in real world fuel quality, BMW can't design an engine for 91 and then specify 91, simple as that. You said that BMW specify 91 because they know that less than that will result in knock in unfavourable conditions. I was just pointing out that that isn't necessarily true. BMW design it for less than 91, and by specifying 91 they reduce or eliminate complaints from owners. If they designed it for 91, half the owners would be complaining that their vehicles didn't operate well on that fuel.

My point is that a specific local example of something that is called 87 or 89 can work fine, and can in fact be optimum, but not for all owners. It depends on a multitude of variables, including local fuel quality, engine condition, and ambient conditions.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
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2003 X5 3.0 Steptronic, Premium, Titanium Silver
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