General Notes on Engine Timings by A. Florez

High exhaust degrees duration = more power and rpm at top end
High transfer degrees duration = more power and torque at low end (watch out because if to high run time gets hurt)
High boost transfer duration =extension of the peak power curve
Advancing Induction opening = more low end (up to certain limits)
Retarding Induction closing = more high end
Bigger diameter on inner passage = more overall power = more fuel consumption past certain limits.

With regards to Induction timing, little changes in opening timing will not produce appreciable changes in performance. The factor that has a considerable effect on an engine performance is the closing timing. Closing 20% later will not affect high end, but will help low end because it limits "blow back" through the carb....past 20% you’ll loose low end and increase high end.

Rotary Valve induction engines are relatively insensitive to opening timing. If there is a rule, it’s that the opening timing is best established right at the point of transfer closing timing if you want a very wide power band engine.
If you want maximum power at the expense of loosing low end power, opening timing should come earlier from 130º to 145º BTDC, which means that the intake and transfer periods will have overlapping. The opening point has influence in power output, but as said earlier it is relatively insignificant as compared to closing timing.

The best closing timing for a wide variety of engines is 65º ATDC

With regards to exhaust and transfer timing and area. Widening an engine’s exhaust port, increasing It’s time area value without actually increasing it’s open duration, has much the same effect as raising it’s height and thus increasing both time and area. That is to say, widening the exhaust port increases the engine’s rpm’s at which max power is realized, while reducing low speed power. Same pattern is to be observed in increases to transfer port time-area, though in the opposite direction.

With respect to Crankcase volume: The ideal crankcase volume has to do with the compression in it, giving that way the necessary force to the movement of mixture through the transfer passages and ports and into the combustion chamber. It also has impact on the aspiration of new mixture charge because it affects the suction and speed in the induction passage. Of course it is related to carburetor size.

Crankcase volume is a pretty complicated thing. Smaller crankcase volume will give you more internal pressure, but it is limited to the volume of mixture going through crankcase. For engines with very wide power band and highly restricted carb, crankcase volume needs to be smaller. For engines with narrower-short power band, and working on higher RPM, crankcase needs to be bigger. It is a general concept and in every engine I’ve studied it is different. I personally learned this the hard way.

AFM