the knock sensor on the 93-onward m50tu will allow the use of 87 or 89 by retarding power if it senses knocking. before i put my eat chip in, i could run 87 pretty good, but my engine was only used for 40k miles before it was put into my car. i'd think that cars with a fair number of miles on the motor will ping when ran on 87 or 89, and as he was asking for things to increase his power, he might be hindering it in this way without knowing it. also, m50tu has a compression ratio of 10.5:1, m50 has 10:1, empirical engine knowledge might suggest that running 87 or even 89 in an engine like that would make it pre-detonate or retard (in his case).

Quote Originally Posted by Dick Schneiders
The pre-vanos ones do, like both of my 1991 525i's, but his 1995 is the M50 with Vanos. I think that I remember seeing that they have a knock sensor or something that eliminates the need for 91 octane.

Dick Schneiders