What exactly does the butterfly valve in the exhaust do ? Is it a type of exhaust brake ? I've noticed when letting off the throttle it has the feel of a brake/compression release , then the motor picks back up just before idle . Just wondering ? 
What exactly does the butterfly valve in the exhaust do ? Is it a type of exhaust brake ? I've noticed when letting off the throttle it has the feel of a brake/compression release , then the motor picks back up just before idle . Just wondering ?
My understanding is it reduces noise below a RPM threshold. When I put my Arrow on, I removed it removing the two screws. Funny thing is I can't tell a difference in sound.
Joined Mar 2013
3K Posts | 161+
Naples, IT
Think we covered this or somebody linked to the .ms site regarding the flapper valve. It was proven that it is purely for noise mitigation down low. I'm gonna wait until the warranty is up before I drill the flapper valve servo bar out or anything.
Joined Nov 2013
1K Posts | 1+
Inland SoCal
It's mostly for the EPA sound test. The measurement is taken from the side, about two feet off the ground, with the bike passing the microphone in second gear at lowish revs. But there is some benefit at low rpm from the added back pressure. It will be leaner without it, and that would hurt power. That's when a fueling optimizer (like a Power Commander) would be a good idea. I suspect that the mileage would suffer, though. The tank is a bit small already.
Note that exhausts that aim upward can measure as quieter than ones that are more horizontal. Could help explain why the stock exhaust sounds so loud!
Note that exhausts that aim upward can measure as quieter than ones that are more horizontal. Could help explain why the stock exhaust sounds so loud!
