For the tuned vespa engine this is the best induction sistem. It is true that rotary induction has still some margin on high rpm over the reed induction, but now the gap between the two is really narrow. The thing is that with a reed induction you will have a better power curve available giving you a better chance on the allready limeted gearing choice.
The thing that makes rotary induction better at high rpm is because the reed valve block and its petals restrict flow in the port.
The change in port section can easily induce gas flow turbulence and making the chamber volume bigger and using a larger valve will not necessary improve flow since it could create more turbulence reducing the total flow.
To avoid this the effective valve area should correspond at a slight restriction of the carburator bore area ( around 0,8 times). This because turbulence is much less in a nozzle than in a parallel or expanding passage.
One area that causes problem is the sudden volume increase which reed blocks produce immediately after the carburettor. This reduces air speed and the pressure differential existing between the carb side of the reed valve and the crankcase. And is the pressure differential that determines reed petals lift and their responsiveness.
A not really elegant attempt in reducing this problem is by inserting a plastic stuffer in the reed block entry area.
The big game is to play with the volume proportions between reed block area the chamber block area the carb bore and the cranckcase volume reminding that every engine will have its preferd combination of proportions.
technologi
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