Its tuned to run 91+ by running 89 it will probably have knock. The guys at BMW know what their doing. They tuned it for 91+ because they can get more power from it. If you really MUST run 89, you could try calling dinan or another tuning company and get it tuned for 89. The cost for manufactured tunes from them are 300+, so if you are trying to save costs it will be cheaper to run 91 than to keep modding to try to get it to run 89. If you live in a place where 91 is not available, might still be cheaper to buy your own gas tank and have 91 delivered to your driveway.
1997 BMW 540i