I have 535i 5 with a pretty large hole in the muffler. Although I kinda like the sound I really want to replace it. I have looked into replacing it with an aftermarket one, but feel I can make my own for quite a bit less money. I plan on not really making much more than an extra 100 hp, through out the cars life with me. That is a high number at that. Not really going for a loud exhaust either. I am now 30 and must be getting old because I think it is too loud now. When I was younger the first thing I did was remove the mufflers and or cat. I will be replacing the cat with a resonator or two as well. I don't have emissions testing were I am registered.

So my question is, what diameter piping would you suggest?

Should I stay duel till I get back to the muffler?


Thanks
Brandon