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If your car is anything like the 2003-2005 M54 Z4 and 2006+ Z4M, the primary cats are in the headers. Any secondary cats are unmonitored. It's illegal to have the secondary cats removed, and many shops may refuse to mess with them due to potential fines. But, no one will really know if they've been removed, you will more than likely still pass a visual and sniffer as long as the primary cats are in place, and the car won't throw any codes if all the sensors are monitoring the primary cats in the headers (you will need to visually confirm).
Anything aft of the cats is fair game, as long as you don't break any noise laws.
The exhaust on the early Z4's is one piece from the headers back. Most mufflers are cut and clamp. You will need to put the car on the rack to confirm.
If you have both sets of cats in place, you can put just about any kind of muffler on the car without it sounding terrible. Once you start removing cats or resonators, the car will need a tuned muffler from a name brand company.
I personally wouldn't put a universal muffler on my car.
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