Or is this a myth. Anyone had a dyno before and after?

 

Slowing, eliminating as much reversion as possible will bring back the torque curve to a lower RPM. Read up on MC exhaust reversion.

 

They might add low end by limiting reversion there where its likely to occur, but restrictions like this rob mid and upper rpm torque and HP. There are better ways to deal with reversion. Some exhausts do real well with it without restrictions, others not so well. Reversion is the result of the cams and pipes not playing well causing low end torque loss.

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