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Originally Posted by roadkillrob
It has nothing at all to do with emissions or it could not be legally disabled - PE exhaust would need to say for off road use only if disabling the flap messed up emissions and a non functional flapper would need to set the check engine light if it was part of the emissions requirements.
It is purely to pass the noise tests at certain RPM's that sound levels are measured at, then can open to allow more flow and better sound at higher RPM's.
Torque - no difference, that is controller by how quick boost is built and how high boost pressure is, that is controlled by the wastegates.
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PE exhaust is after market. All car manufacture tests are done on stock. Better emmisions is good for BMW, no body cares what you install after that.
PS. BMW tech told me its to heat up cats. Everyone who done gold tee mod done it for abit more noise that's why everyone is saying its to reduce noise.
I know some cars use butterflies to control noise, others to regulate back pressure, it just might not be the case with 335i