When the PV is closed at low RPM, the chamber is open to the port through the left drum valve. This changes the tuning of the system, like having a longer header section on the pipe. When the PV opens at higher RPM the chamber closes and is out of the equation.
The effect you feel has a lot to do with what pipe you run with it. I notice it more with the GG pipe than with the FMF, which is more of a low/mid pipe anyway. So, the GG pipe with the LTR cover is a win/win setup!
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