I have been interested in doing something similar because I want the sound of the performance exhaust but not the price haha. I believe there was a post awhile back where someone cut open a bmw performance exhaust on a e92 335i. I'm sure there is difference between the 335i perf exhaust and the 135i perf exhaust, but maybe you can take some ideas from that. I'll try to find it. Good luck with though
