I was dissatisfied with the common suggestions for different port durations for different top RPM because they didn't take into account the exhaust port shape and size which determines the length of time that high pressure exists in the cylinder. That pressure has to be near zero before the intake charge can transfer into the cylinder. At high RPM if the transfers begin to open while there is still more than 5psi in the cylinder then there is unwanted mixing of exhaust gases and intake charge and the transfer of intake charge is delayed.
MINIMUM NEEDED TRANSFER TIME-AREA
Using the full data from 250cc and 138cc engines I was able to come up with a formula that gives the peak power RPM. Above that RPM the intake thru the transfers is inadequate and thus limits the power.
The transfer time, when there is positive pressure in the crankcase, lasts till the pressure zeroes out. Looking at pressure traces from crankcases I saw that the pressure zeroes out at BDC at 5000 RPM or less and it bottoms out more or less at a time after BDC equal to an additional 5 degrees for every 1000 RPM over 5000. (example; at 10,000 RPM the pressure ends at BDC+25 degrees.) So that is the end of the "raw" transfer time (which began when the transfers opened). This timing can be extended a little by the return diffuser wave of the expansion chamber if it is designed with its steepest angle right before the belly which delays the return wave peak.
My conclusion about all this is that there is a certain amount of transfer time and area that is minimal for an adequate filling of the combustion chamber (above the exhaust port). As RPM goes too high there is not enough transfer time-area for the fuel mixture to transfer completely from the cases to the cylinder. So the RPM at which the power starts to decrease is just past the engines peak power RPM.
How does the expansion chamber help or hinder the engines peak power RPM? Imagine two hill-like power graphs. One is for the engine and one is for the pipe. Only if the two overlap correctly will the final result be the maximum power/powerband available from the engine. So it is really important to design the porting for the desired engine peak power RPM (which results in an actual peak power at about 400 RPM more) and then design the pipe to end its powerband about 1000 RPM higher than that. Then the two will be in harmony with each other.
Can too high an exhaust port be detrimental? Yes. I have done many tests with many different porting arrangements and when an exhaust port has a longer duration than needed it has less power and sometimes even less peak RPM. Why? Because the higher the port, the less trapped cylinder volume there is above the port (making the actual engine size smaller) and the more intake charge can be lost out the port.