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I don't know why people think the valve placement cuts off the exhaust route through the muffler. It functions exactly like our OEM functions.
When closed exhaust is blocked from completing the route through the "loud" route. All exhaust must route through the part of the back box (muffler) that reduces sound the most.
When open the route through the muffler is still there but there is a direct route as well that uses minimal sound deadening material in the case of OEM or straight pipe in the case of Dinan.
Exhaust back pressure will NOT increase nor stay the same when the valve is opened. It can only reduce because the preexisting route is still open as well as the easier/louder route.
Same setup/mechanics as OEM.
Lastly, I doubt very many M40is on here have actually modded up to the level that changes in the muffler design would net even noticeable improvements outside MAYBE a few HP on a dyno. Y'all worrying too much about stuff that doesn't matter at the power levels most everyone here is at.
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