Sleeve porting considerations:
Power band requirements will dictate the optimal port timing. High RPM corresponds with high ports.
• Pipe design is crucial to working with port timing. A narrow power band can use a pipe that makes high peak torque values and longer port durations are possible. A wide power band demands a less aggressive pipe and requires lower ports to work suitably.
• Exhaust port durations are the key timing factor to determine first. Regardless of pipe, the exhaust timing will establish the first reference point in time or ratio for the events to occur. Transfer ports are secondary due to the design of the pipe, and variables from pressure in the crankcase.
• Blowdown time is the next factor to consider in timing. The higher the RPM, the more time is needed, and the less time is available for the transfers to pass air/fuel in. Again, the pipe and crankcase pressure will determine the minimum blowdown for a given RPM.
• The above stated, Transfer timing can be adjusted with more flexibility when tuning factors such as crankcase pressure and pipe design. More is better in this case up to a point where blowdown is compromised at higher RPM.
• Aiming the flow can become a critical factor in power. Roof and wall angles have a “sweet spot” where the flow is balanced to provide the best input pattern avoiding excess losses out the exhaust.
Transfer Port Air Flow:

Twin streams of incoming charge emerge from twin transfer ports flanking the exhaust port, and angle back across the piston crown and slightly upward, joining into a single stream at a point approximately two-thirds of the way back from the exhaust port. This stream is deflected upward by the rear cylinder wall, and then it sweeps up to the top of the cylinder to be directed back down the forward cylinder wall-moving the residual exhaust gases out of the exhaust port as it advances in that direction. There is some turbulence generated by this activity, which is unfortunate because turbulence promotes the very kind of churning and mixing that should be avoided. But the turbulence is minimized when the flow is symmetrical, and there will therefore be less dilution of the fresh charge trapped in the cylinder at exhaust-port closing. Skewing either transfer port to one side, or lifting the upper edge of one slightly higher than the other will badly upset the scavenging pattern. Still and despite the fact that high horsepower numbers make good conversation, power range is going to be an extremely important consideration until such time as we have transmissions providing infinitely variable ratios. So the best scavenging system is one that has bulk flow capabilities while maintaining a high degree of flow control.
The proper direction of the scavenging streams is important for reasons beyond the reduction of turbulence and fresh charge/exhaust products mixing. Cylinders need very wide ports to avoid excessive timing durations, which mean that the ports must be crowded together too closely to entirely avoid the dangers of "short-circuiting" the charge. Having a high delivery ratio avails the engine nothing if the mixture streams emerging from its transfer ports are allowed to divert from their intended path and disappear out the exhaust port. This danger increases as the transfer ports are crowded closer to the exhaust port; too, a degree of crowding is almost inevitable. It should be understood that this proximity is acceptable only when determined effort has been made to direct the scavenging streams sharply towards the rear cylinder