after you read the title your first thought/question is probably "why?" if your first thought/question isn't "why?" you probably have tried building for SCCA class STX.
STX (all of street touring) per SCCA rules requires Cats to be within 6" of their OEM location. I have personally challenged this via SCCA's official channel for rules changes, rejected, no rationale provided by SCCA. Headers are STX legal, but must conform to the aforementioned Cat restriction. this (obviously) eliminates off the shelf headers for ZHPs. essentially, the cat rule makes german cars the only one that cannot run header in Street touring...
I am getting real close to attempting to modify factory S54 exhaust manifolds to fit my M54. the cat locations are similar, the OEM manifold is tubular and though I have no way to compare the flow improvement, visually, it appears substantial. consider, they are designed for a 0.2 L larger engine spinning at ~1000 rpm higher = lots more air flow....
i should note that i am attempting this bc cost is low. S54 manifolds go for less than $200. reproduction flange on ebay ~$100.
plan would be to use either OEM or reproduction flange for M5X, cut off S54 flange from S54 manifolds, weld, then use M3 section1 tubing connection and reduce from 60mm to 50mm exhaust tube under the car...
below are things I have considered... and some associated risks. if you can think of something I have missed, please post to this thread!!!
-exhaust ports are round vs oval, based on reproduction flange available from kniseleyperformance, I think this will be a non-issue. note, per pictures on eatsleeptinker.com's spec E46 engine build, S52 manifolds fit M54. so the kniseley flange should mate fine.
-bore spacing, according to the forum posts, bore space is 91 mm for M54 and S54. this makes some sense.. but hate to trust forum posts...
-exhaust port angle, I have no way of knowing whether the angle of the ports on the head is same, but i think there is a chance to modify the angle by adjusting the S54 pipes where they will be welded to the flange..
stud/bolt access, this could be an issue as the flange bolt patters are substantially different. i am hoping... there will be room to access the nuts/bolts... (hope is not a plan, but its all i got)
-Cat space, i am assuming that if they fit an E46 M3 coupe, they should fit an E46 coupe/sedan..
-section 1 pipe on M3 is 60mm. it 'appears' to run in the same location as E46 non-M3. hope to be able to simply reduce using a small section of 2.25" pipe..
-O2 sensors, i don't see why this wont be plug and play.
pertinent thoughts? educated concerns?