I made one for a OS 46 VF rear exhaust once and really tourned on. 21000 rpm on a 10x7 prop.
A sreamer. Had it on a sweet stick. The fastest stick at the field
Not exactly sure how it works but I think it actually makes the engine think its running a lower rmp than it really is and gets fast. It was described to me in lamons terms like this so I am not exactly clear. BUT IT WORKS...

Shane