Port Timing and Peak Power RPM


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.

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 usage video.

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.

Port Sizes and Peak Power RPM

I offer this info as an "update" to the outdated ideas of the time*area formulas that Yamaha engineers developed for their GP engines decades ago which seemed to only be relevant to those engines.

This method revealed to me that engines with a single exhaust port and dual transfers are unable to acheive enough engine peak power RPM. That is due to the long exhaust pulse because of limited port size, and because the typical dual transfers are too limited in area. So the revealed peak power RPM is about half of what it should be. This limits their power at RPM above the engines peak power RPM. The most you can do in that case is try to make the red and green graphs cross 1.00 at the same RPM. All modern racing two strokes have a dual or triple exhaust port. This allows the exhaust pulse to exist a shorter amount of time so there is less interference with the transfer time. And the transfer area now is much more than in years gone by.

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.


Expansion Chamber index page