Original von Greggio:
I've often been curious about why a reed valve should be faster - I was reading Gordon Jennings' two-stroke tuning theory hand book and he is (was?) of the opinion that rotary valve, set up right and in good condition, was better than reed.
Can anyone enlighten me on this?
- A reed valve introduce a restriction in the mixture flow, and that causes a pressure drop, that decrease the amount of mixture in crankcase.
- To get the reed valve to work well you have to create a new flow passage, in the Vespa case, beside the crankshaft as well as reducing the cranks web. That volume plus the volume that is between the reed valve and the crankcase, lower the compression in the crankcase. This makes the differential pressure between ambient and the crankcase to be lower, and that will give a decreased suction capacity.
So, an engine with a perfect rotary valve it will be faster, but not very good at lower speed. To get a good running engine at all revs you have to compromise. This will give a lower top speed.
To get the perfect rotary valve, you have to do a lot of machining on crankcases and crankshafts, as well as testing., then do it all over again on new parts. Remember, there is two things that have to be right, inlet open and inlet close.
The advantage of the reed valve is, that the engine decide when to open and close at all different revs. When the pressure in the crankcase get lower, then the pressure after the carb, the reed valve open, and remains open until the crankcase pressure and the pressure after the carb equalize.
The worries about the opening and closing of the inlet are gone!
(See fig. 3.35 on page 74 in A. Graham Bell’s “TWO-STROKE PERFOMANCE TUNING” )
For “Land Speed Record” attempt, rotary valve is a good choice, but the every day speed-freak will be happier with a reed valve engine.