Originally Posted By: JHZR2
If you have emissions testing (or even if not, nobody anywhere likes to sit behind an obnoxiously stinky vehicle), you should at minimum ensure you find a cat with a CARB rating.
My 91 BMW was having issues with high NOx emissions. Everything ran perfect, so it was resolved to be a catalyst issue. IIRC failing was around 1200ppm, and my OE cat was on the cusp. Swapped in a manga flow and I barely passed. Somewhere in the 1170s, as I recall. Two years later, failed again.
Bit the bullet and got an OE converter, I passed with 14ppm.
So the OE converter was around 100x better.
The BMWs Boysen exhaust weighed about twice what My jeep's system weighed. About 275 K miles it failed emissions for NOx, I replaced the original O2 sensor and treated the fuel with some pink glop that was designed to help jet liners burn cleaner. It worked and I got another 3 yrs out of the cat. Your experience with after mkt cats exactly meshes with tales from the E 28 forum.