I know around the 98 MY that they were kinda notorious for clogging anyways, enough that BMW put out a recall. I had already had one replaced, but one of them had started to rattle (I dont remember, old or new(er)), so I had it replaced. The exhaust note did get slightly deeper, but didnt really feel any difference in power. Within a few months the other one failed, so I was forced to replace the second one (really long story, had to be towed and everything), so now theyre both new. Having the 2nd one replaced once again gave a slight increase in exhaust note, so theyre more free flowing than the stock ones, but for the price vs stock, you cant go wrong.

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