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Making any comparisons against the stock M2 N55 exhaust need to take into account the oem valve operation which closes at "high" rpm (we can only assume it's noise emission related) effectively halving the muffler output diameter.
I don't know that anyone has ever tested/measured the affects on power of the valve open/closed, for the aftermarket manufacturers that may negate any performance increase..
Early days when I tracked with the valve operating on oem settings the valved side was always a lot cleaner indicating it spent a bit of time closed. I ended up keeping it open by pulling the fuse & now with BM3.
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