[Dionysus]

Seems extreme to void the entire warranty - my dealer told me that non-genuine parts only had the potential to void the warranty for associated parts and that's only if you have a problem. BMW (whether this is UK or AG), however, not the dealership, makes warranty decisions.

In my experience BMW can be particular about warranty claims - my car had a visible screw in the tail light from new. The dealership could not find a way to remove it (so no way I could have put it there), but it still had to be validated by BMW to be processed as a warranty claim.

Does the Eisenmann back box modify the back pressure, etc. (i.e. could be associated to future engine problem)? Plenty of members here with non-genuine mods. I'm assuming you need to cut the OEM exhaust so cannot restore if you do have an issue that BMW could associate to the Eisenmann? If your dealership is saying it's OK I would go with that (I would be tempted to get it in writing, even if only via email) as it's up to them what they choose to declare if an associated warranty claim comes up.

I had a similar discussion with my dealership regarding the Akrapovic exhaust (not considered genuine accessory by BMW), however, the same (and more expensive) exhaust with BMW branding (M-Performance) is fully covered. The MPE exhaust has been fully validated by BMW - if there were any critical changes required, I would have expected Akrapovic to make the same changes to their version, so it's all a bit of a game IMHO (really can't see BMW having in place a commercial agreement with Akrapovic preventing them from making critical changes to the Akra version). The Eisenmann is a different matter though, so comes down to how risk adverse you are (it's not as though you're fitting non-std DPs). Does Eisenmann offer a warranty, and, if so, what does it cover (would doubt it's as extensive as Alpina or AC Schnitzer - may be worth checking out their offerings, although no doubt more expensive & may not provide the performance/sound you want)?

Reminds me of the run-flat position on non-Ms; some dealers are adverse to them stating that warranty on associated parts (suspension/steering bushes, etc.) could be effected. BMW did sell non-run-flat equipped 19s on the E60, however, it could be argued that lower profile=stiffer sidewalls on 19s (which, IMO, had better secondary ride than all the run-flat options), so not the same as changing your 18s/17s/16s to non-run-flats. Plenty of owners still did it though.