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Old 01-13-2013, 02:13 PM
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vintage42 vintage42 is offline
BMW CCA 1405
Location: Louisville, KY
Join Date: Jun 2007
Posts: 3,750
Mein Auto: 1997 BMW Z3 1.9 5MT
Originally Posted by Barko View Post
... Owner said he always ran 87 oct in her. So I thought I would put some 89 in her. Wow what a difference. I lost 8mpg with 89. Went back to 87 and mpg came back....
The Owners Manual says the car is rated (for official horsepower and mileage) on 89 octane, can tolerate 87 octane, and will have increased performance on 91-93 octane. I doubt that using the 89 octane the engine was made to run on could have been the sole cause of the loss in mileage. It could have been variations in driving or filling or even a miscalculation. For consistency I would try a few tanks of 89 back to back, and then a few tanks of 93, before concluding that 87 was best. The 87 octane reduces power as the knock sensor is reducing ignition advance to prevent pinging. This may not matter in a commute.
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