Car and Driver did a big article about 10 years ago, using various cars, running them with different octanes on a dyno.

A Saab 9-5 turbo lost as much as 15% of its power on 87 octane. But that was a turbo, and boost was cut to prevent detonation.

On a non-turbo, at most the engine is going to cut timing. This will hurt power a bit, but not anything like 15%. I'm going to speculate and say 5%, if that.

Considering how much people pay for cold air intakes and such, which at best will give you maybe 5% more power, a few bucks for a tank of gas might be a small price to pay for more power for some people.

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'02 X5 4.4i