Read a few threads about this and wanted to try it out. There's a few ways people have modded these. Some just pulled out the stuffing in the middle can and wrapped the perforated pipes. Others welded in pipes in place of the perforated ones. We went with another method. Pictures below.
This is what the left and right cans look like inside if you've never seen them. They criss-cross thru the center of the muffler.
We cut out the pipes and re-directed the Y-pipe straight thru bypassing the center of the can.
Last step is to weld on the titanium tips and throw it on the car..
Painted it black..
Video E90 E92 M3 ACM Exhaust Mod vs. Magnaflow and Challenge Sport X-pipe:
Video 2 E90 E92 M3 ACM Exhaust Mod vs. OEM Exhaust:
Video 3 ACM Modded OEM Exhaust E90/2/3 M3