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      12-29-2012, 11:36 PM   #54
Grovsnus
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Drives: Le Mans blue X1 35i
Join Date: Sep 2012
Location: CT

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Quote:
Originally Posted by conceyted View Post
The quote in the manual is actually:

Recommended fuel quality
BMW recommends AKI 91.

Minimum fuel grade
BMW recommends AKI 89.
Correct - and with the minimum non-recommended octane rate, the engine will adjust by delaying the ignition to avoid knocking, and this gives less complete burning and increased fuel consumption.

Quote:
Originally Posted by conceyted View Post
I always understood that to mean you should run AT LEAST 89, but the higher the better. You mention the unavailability of 91 and mixing 89 and 93 together. For me, if 93 is available I am going with that. And on the rare occasion that something like 95 is available, that is what I would choose.

I can't think of any reason the car would rather 91 over 93.
In earlier days, you could get increased sooting and catalyst clogging by running on too high octane rated fuel. This should be no problem today, but there are a couple of reasons:
  • Some unscrupulous gas providers increase the octane rating in "premium" fuels by increasing the amount of ethanol. Yes, it does increase the octane rating because ethanol ignites less quickly than gasoline, but it also reduces the efficiency. If you buy from "top ranked" vendors on BMWs list, this should not be a problem.
  • 93 doesn't give you any improvement (whereas 89 gives you a detriment), and 91 costs less.

Anyhow, don't run on 89 unless you're willing to live with increased fuel consumption and less power, and don't run on 87 except in emergencies (or you can't get 91, in which case you can use one part 87 octane plus two parts 93 octane or one part each of 89 and 93).