Modification of the Exhaust Whistle

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 YPVS’s first baffle has a centre hole of 32.5 mm und eight holes around it of 14 mm
  • The first baffle of the 250/350 air has also a centre hole of 32.5 mm but only 4 holes around of 10 mm
  • Distance between first and second baffle is 65 mm with the YPVS and 80 mm with the 250/350
  • The YPVS baffle has an outer diameter of 88 mm, the 250/350 air baffle has 85 mm in diameter.
  • The most important difference is, that the inner exhaust pipe of the 250/350 reaches through centre hole of the first baffle and thereby makes the second baffle ineffective. In contrast the YPVS, where the inner pipe ends before the 2nd baffle.

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.

Figure: Whistle from the front - closed and open
Figure: Whistle from the front - closed and open

Image: Whistle in action (stock)- inner pipe in action (modified)

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.