Both devices are rotary type induction valves used on two cycle engines, but they are very different in their design. Rotary valves are placed inside the crankcase cavity & are driven from the crankpin. Zimmermann valves are placed in a separate cavity outside the crankcase cavity & are driven by the crankshaft. The idea behind the Zimmermann valve is that the valve is isolated from the pressure fluctuations in the crankcase, which should result in lower friction losses. This is not the only type of induction valve that can have much lower friction losses. The bell valve has the lowest friction losses of any of the present induction valve designs. It also has the best time area numbers possible without having to resort to aggressive openings & closings.

Jim Allen

It often comes down to costs on commercial engines. For that reason shaft rotary valves dominate glow size engines and piston port intakes dominate weedeater size engines. I don't know how to build a reed valve that will perform better than a piston port in a 26 cc engine. I haven't seen any other attempts that work better than a piston port. However, the Zimmerman valve and other rotary valves have serious advantages over a piston port (and in my opinion a reed valve). The whole engine has to be designed to take advantage of the better breathing.

The CMB 35 we worked with wouldn't stand the rpm the better breathing made possible. Mike Bontoft's ingenious Zimmerman valve Zenoah was fancy frosting on a cheap cake. It worked very well, but couldn't compete with the best piston port engines. I wish I had some experience with the promising Zimmerman valve CMB 27. However, I'm not optimistic that CMB understands the principles of modern two stroke design. It's strange since the best modern two stroke is also Italian.

Lohring Miller