Port Timing, Peak Power RPM, + my Excel Porting Calculator


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 the transfer of intake charge into the cylinder is delayed.

TRANSFER TIME-AREA?
Previous calculators depended on the calculated time-area of the ports. This is inaccurate for many reasons. The most important thing is the loop flow of intake charge. When there is excess time due to low RPM then some of the intake charge is lost out the exhaust port. When there is too little time then the flow doesn't reach the exhaust port and so it's unable to push out as much spent gases from the previous cycle. There is a resonant RPM for each engine depending on crankcase pressure and loop distance. If the engine is designed so that the resonant RPM is the same as the wanted peak power RPM then the maximum power possible is acheived. My porting program calculates the % delivery ratio for each 250RPM up to the max RPM.

delivery ratio

How does the expansion chamber help or hinder the engines peak power RPM? Imagine two hill-like power graphs. One is for the delivery ratio and one is for the pipe powerband. 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.

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.

Click here to read about the effects of different exhaust port shapes.

Click here to read the basic rules about engine porting.

Click here to read about using the Porting Calculator. Click here for its two usage videos.

You can find the actual peak power RPM of your bike by riding uphill and noting the speed at which the power starts to lessen. You can put a bicycle digital speedometer on any motorcycle and then calibrate it by wheel diameter. Use this formula after measuring rear wheel outer circumference in meters (.0254 x inches) and crank rotations per each rear wheel rotation in same gear that was used going uphill. (Do it with the spark plug out and the bike up on a stand.)
RPM = (KPH x crank rotations) / (wheel circumference x .06)
MPH = .62 x KPH   KPH = 1.613 x MPH


Click here for a list of my for-sale spreadsheet calculators for 2 stroke design, including this one.

Click here to read of the best method to measure/calculate port durations. It takes into conideration the lengthening of the connecting rod when it gets real hot which lessens the durations.

Click here to see my list of the most basic porting tools.

The old fashioned Yamaha time*area formula has these errors:
1) the port areas of transfers and intake was used to calculate peak RPM
2) the average exhaust port area was used instead of blowdown area. For single exhaust ports this can be compensated for with the correction factor, but not with bridged ports and those with auxiliary ports, both which have increased blowdown area in relation to total port area.
3) the ignoring of CCR is a huge error because that pressure determines the speed of the transfer flow which determines the resonant RPM of the delivery ratio.


Expansion Chamber index page