If you increase the crankcase volume, you lower the Helmholtz frequency of the inlet system, so the engine wants to producve its maximum torque at lower revs. But the exhaust pipe has to agree, otherwise: no deal, unless you restore the original Helmholtz frequency with a longer inlet timing.
But that was already damn long to begin with. It would make carburation quite moody, so that's about the last thing I would do to a dirt bike.
Did I test it? Yes. Not with the original Rotax inlet tract, but with an elongation piece between carb and disc. It led to the following story (that you can find, together with a lot of other short stories, in my FOS tips & concepts):
https://1drv.ms/u/s!Atyzb5b7jtWNmVcX...5eKjL?e=nxuw41
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In all inlet systems the inlet flow accelerates as long as the pressure upstream of the inlet tract is higher than the downstream pressure.
When both pressures are equal the flow has reached its maximum velocity and because mixture keeps entering the crankcase, the case pressure rises above the upstream pressure and this rising pressure gently slows the flow down to a standstill (or suddenly instead of gently if the inlet port closes too soon, which can happen in both piston port and rotary induction systems).
In any case, the pressure at the crankcase side of the inlet tract will then be higher than the pressure at the bellmouth, and mixture will start flowing back towards the free world. The extend of this backflow may be so small that it is not noticeable at the bellmouth, but it is happening nevertheless. Reed valve systems do it just as much as other inlet control systems.
I call this phenomenon bounce-back and it should not be confused with the blow-back that occurs when an inlet port closes too late.
Bounce-back mixture never really made it into the crankcase; it returned at the closed door.
Blown-back mixture initially entered the crankcase but then reversed its flow direction because of the rising case pressure before the case was closed.
This rising case pressure resulting in flow reversal has rather little to do with the piston moving down after TDC. It can even happen before TDC if the Helmholtz frequency of the inlet system is too high for the engine revs (low revs, big carb diameter, short inlet tract, small case volume).
By the way, bounce-back may be hardly noticeable, but on the other hand it can be even more vicious than blow-back.
As an experiment I once put a 200 mm elongation tube between the carb and the rotary inlet cover of a 125 cc Rotax.
It lowered the Helmholtz frequency of the inlet system so much that inlet flow velocity was still near its maximum when the inlet disc closed. Bounce-back was so severe that within seconds the dyno room was completely fogged up with mixture. It frightened me to death; a spark would have been enough to blow the roof off.
Physics is something you study; not many shortcuts, or as Wobbly would put it: no free lunch (he must be thinking with his stomach).
But I've got something better to offer: logic. Anyone can do it and in my opinion logic is more important than mathematics and physics together.
Originally Posted by 190mech
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thank you ! 
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