Furious nailed it on the head. Exhaust valve allows the engineers to give stock headers a larger diameter without sacrificing low end torque. Large headers are dynamite for top end, but you get diminishing returns as rpms drop.
I think it would be the same thing as the one on VW R32's? There is a mod referred to as the flapper mod. If it is taken off the car sounds better...
I've also read on another bimmer forum that some peeps took it off because it sounds deeper.
Uhhh yeah, that's because they're effectively doubling the final exhaust exit area. Sounds great, it's louder. But on an otherwise stock system, performance will drop. Drilling a bunch of holes in the muffler is likely to "improve" its sound as well, but not likely to improve performance.
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