Originally Posted by nospam
91 is recommended, but 89 is allowed by BMW on the N20 engine. It won't void your warranty and shouldn't damage anything, you'll just sacrifice performance.
And reduce gas mileage.
When using 89, the engine will adjust to fire the plugs later, to avoid knocking. Because 89 octane fuel doesn't burn
faster than 91 octane, it only ignites
faster, using 89 leads to less complete fuel burn.
So reduced performance and reduced mileage. But sure, it will run on 89.
In order to not only change the ignition point but also the burn cycle so you get the same mileage out of 89, you need to adjust the stroke depth and compression. This is a mod you do when you buy the car, by walking across the street to the Honda dealer instead of the BMW dealer.