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I was dissatisfied with the common suggestions for different port durations [with the "blowdown" being from exhaust opening to transfer opening] 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 go thru the transfers 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 The transfer of intake charge from crankcase to cylinder lasts till the positive crankcase pressure zeroes out. Looking at pressure traces from crankcases I saw that the pressure zeroes out at BDC at 3500 RPM or less and it bottoms out more or less at a time after BDC equal to an additional 3.2 degrees for every 1000 RPM over 3500. (example; at 10,000 RPM the pressure ends at BDC+20.8 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.  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. 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 and then design the pipe so its powerband middle is at the portings peak power RPM. Then the two will be in harmony with each other. How can the expansion chamber extend the powerband of an engine with limited porting? As the RPM exceeds the peak power RPM of the porting then eventually the power starts to drop off steeply when the transfer time is reduced to 80% of its normal time, due to the exhaust pulse overlapping it and delaying the transfer of intake charge into the combustion chamber. But if the pipe is designed so that its powerband begins at an RPM before that 80% happens then it can greatly extend (by about 1/3 more RPM) the RPM that the engine can rev out to, although the peak power will be very limited.
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. |