Originally Posted by e30_kid89
Before and after dyno sheets?
Car & Driver did it properly with an E46 M3 years ago. They tested around 10 cars and the ONLY one to gain power with 94 versus 91 was the S54. Gain was 8-10rwhp, can't remember now.
That said I never use it in mine. I can't feel the difference and Shell 91 no ethanol gets better mileage.