Very often I thought about, why the stock exhaust system brings peak power only through such a narrow range of 1,500 rpm. Finally, I opened a ’74 exhaust pipe and one of the YPVS and saw the reason:
The simplest way to enlarge usable speed rage is to modify the inner pipe. The side holes in the front of the whistle must be closed. Put a sleeve of 0.7 mm sheet metal in pipe to close them. Then cut the slots about 16 mm in the front of the pipe and turn the six resulting metal tabs on the cylindrical sleeve so closely together that the front pipe is closed and about 16 mm shorter.
As result of this modification 21% of the shock wave is reflected at the 2nd baffle. It is 34 % with the YPVS. The
pressure wave finds its way to the second baffle plate, where it can be reflected. At the same time part of the pressure wave is reflected from the closed whistle. With my RD 350 air the full
torque starts unchanged at 6500 rpm, but from 5500 rpm on, the bike accelerates much stronger than before.