I had an extra set of mufflers that I plan on modifying, so this weekend I cut them open to see what's on the inside.
The pipes inside the 550i mufflers go from 2.75" that dump into the middle chamber and there are two pipes that loop from the first chamber and go down to the third chamber and out the tailpipes. These pipes are about 2".
I plan to do something similar to the M5 mufflers like this:
We'll see how this progresses. As of now, I'm just brainstorming ideas. If anyone has any ideas feel free to let me know. I am trying to improve the sound, but at the same time trying to avoid drone. I would like to keep it as simple as possible, too.