Hello,
I was having an issue with my car under the following conditions - 6th gear, 50-60 mph (so figure below 2,500 RPM), 2/3 - 3/4 throttle, Comfort or EcoPro modes, ambient temperatures ranging from 55 to 75 degrees. The car would shudder pretty hard and it felt to me like it was detonating and the ECU was pulling boost, timing, etc until the detonation stopped. Then it was advancing everything back to max, detonating, pulling everything...and so on. No warning lights or limp mode, but I could envision the car going into limp mode if I allowed it to go on (which I didn't out of fear of hurting the motor). The issue was easy to duplicate so I was confident it wasn't a fluke.
To test the theory I filled my tank with 1/2 91 octane and 1/2 E85. Before doing so, I went searching for spark plugs that are 1 heat range cooler than the stock plugs. I found something interesting - looking at RealOem, there is a reference to spark plugs for cars without flex fuel preparation. Hhhmmm...ok. I'm not aware of the N20 being sold anywhere in the world as a flex-fuel vehicle. Could that imply that the car is already setup that way?
Anyway I couldn't find any plugs available except for the OEM parts so I quit looking. I also consulted with a friend who is a BMW Master Tech (and outstanding BMW race car builder) to see if E85 would hurt the car. He said he didn't think so and that I should give it a shot.
Now on to the E85 - WOW! The shudder described above is completely gone. My car feels so quick it is amazing. Even the idle has smoothed out a touch. Maybe it is all my imagination but there is a perceptible difference. Our cars should be the right configuration to take advantage of E85 - DI and Turbo with very sophisticated engine management. So perhaps the extra octane really is helping out.
On the downside, my fuel economy has taken a serious hit. Usually I average between 26 and 29 on my daily 22 mile commute in LA traffic. Since filling-up I am having trouble getting above 19. Some of that might have been due to my playing around trying to get the car to shudder. Some of it might be the result of spending less time on the freeway since the fill-up. Next tank I will attempt to more closely duplicate my routine commute. But I am expecting 20-25% reduction in economy. Time will tell.
If only there were more E85 stations around So Cal. Thus far it appears to be a reasonable substitute to $9/gallon race gas, which is (shockingly) easier to find at the pump than E85 is. But the lack of infrastructure makes using E85 very challenging.
Thanks for reading.