Ok. My 1991 535i has an aftermarket POS muffler. It is a single in, dual out. I am wondering if it is causing to much back pressure? Going from dual pipes to a single inlet? What are the effects of to much back pressure? Engine rattle? Exhaust leaks at the seals? Engine still runs at midway or just a little past (Texas heat, already in the 90's). Besides it sounds like a bumble bee in heat.....

Any info is appreciated.