After 160,000 miles, I believe it is time to replace my 1990 325i exhaust components. My exhaust makes a healthy groan now and I would rather have a quite setup. I was wondering if I could just go to Midas and have them put in a universal cat and muffler setup. I know it will be a lot cheaper than the stock stuff, but I also wanted to know if it will affect the engine performance in any way and will in affect fuel economy.