Replacing the N54 mids by the N55 mids on the 1M = significant reduction of 'drone'.
See these threads:
Benefits of N55 mids on the 1M:
- BMW OEM part;
- easy plug and play / bolt-on install (same shape and connections) and ditto removal (if ever required to);
- cheap mod for what you get in return: normal price is sub $400 (call around because prices seem to range - BMW OEM part, so your BMW dealer can certainly get them for you);
- significant reduction of drone;
- a little extra growl on the sound spectrum (if you want a louder, raspier noise, get Maddad mids instead of N55 mids);
- less restricted fumes flow (due to the absence of secondary cats) - a little extra oomph (few extra hp);
- ECU will not throw codes at you (reason: primary cats and sensors are sitting in the downpipes connected to the mids); only if you go for catless downpipes (loud !), the ECU will protest and, hence, needs to be reprogrammed (which voids the warranty);
- no undesirable results (too loud, too smelly, risk of trouble with authorities and neighbors)
N55 mids = BMW OEM part # 18307599743
For the OEM bolts and gaskets part #, see these earlier posts of Dackel:
In the event of replacing the N54 mids by the N55 mids, definitely keep
the N54 mids somewhere for possible future re-install (1M resale). As a matter of fact, due to the presence of secondary cats, the N54 mids are ± five times more expensive than the N55 mids !
N55 midpipes (left - no secondary cats) versus N54 midpipes (right - secondary cats):