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I don't think its a great idea to keep your flap open. It's there to balance back pressure from low to high RPMs. Opening it permanently equals less back pressure at low RPMs, less flow velocity, and a negative impact on how your engine produces power.
Surprised the US version of the perf exhaust doesn't have one, but maybe its internals are designed differently. The 335i version has one, both euro and US.
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2015 F15 X5 25d Space Gray | Pure Excellence | Dakota Leather/Brushed Aluminium | Anthracite Head Liner | Fender Flares | Connected Drive + Real Time Traffic
2012 F30 335i Sapphire Black
2006 E92 325i Titan Silver
2000 E46 318i Cosmos Black
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