it is ok to have intake port timing more then 160 degree?
from various reading, they said it is optimum on 160 and if bigger then engine will start too peaky and hard to start. is that correct?

according to jennings formula, about calculation of time area
I try to calculate intake port for Zenoah 30cc and 31cc with prt timing 160 and target RPM 18,000
it required 2.78cm square.

zen intake high is max 1.15cm with timing 160 and width max is 2.95cm (almost everlap with transfer port)
so intake window area is 1.15x2.95=3.3925cm2 and jennings said only 65% area is effective or in his word "Mean Port Area"
so 3.3925x65% = 2.2051cm2

from that calculation, we can conclude that we never can reach 18,000RPM with zen 30 and 31cc?

please someone help

Maybe with that calculation you can come to that conclusion, but the tach on the water has concluded they can and do hit 18,000 rpm and higher on heat racing oval courses, even with less duration and width.

Doug @ ESP

Jennings calculations show that the Zenoah etc. piston port engines are deficient in time area for induction (intake) which restricts the rpm where peak power occurs but that does not mean the engine cannot produce useable power past that rpm point. Peak rpm for max power is actually more affected by the time area of blowdown. i.e. higher rpm for peak hp would need either more time area for the exhaust or less time area for the transfers or both.
Dave

Something that I read about Luke Roy & Bill Husted that might be of interest.

In around 1987, Luke rebuilt the engine along these lines. The result was a rear exhaust front intake sideport racing engine which drew much of its fuel mixture directly through the inlet into the cylinder through what used to be the boost port, using the pipe rather than the crankcase as the pump. In this design, one can look through the carburettor venturi at BDC and see straight out the exhaust port! The main role of the piston-controlled crankcase pumping system (which still operates) is now confined to passing enough mixture through the engine into the cylinder for starting and run-up. Although the inlet timing was necessarily a compromise to allow the engine to be started, Luke says that it performed very well indeed, confounding those doubters who had previously been wondering if Luke had finally "lost it"! He just never got around to doing anything further with it at the time.

At the present time of writing (2013), the piston port concept is right back at the centre of Luke's thinking. He is currently considering the construction of a 10 cc tether car engine along these lines. In theory, Luke reckons that the ideal intake timing for running at full speed on the pipe is around 200 degrees (!), but experience with the 15 showed that a sideport engine with that timing won't start since there is insufficient pumping action from the crankcase. Luke reckons that the fix is some kind of flap valve in the inlet tract to restrict the timing to around 150 degrees for starting and run-up. In a tether car context, a centrifugal lever could be used to advance the intake timing to the desired 200 degrees as the car speeds up. Boy, would I love to see their faces if Luke ever gets this concept working competitively! Getting beat by a sideport engine—gotta smile at the idea! Knowing Luke as I now do, I wouldn't bet against him succeeding!

thank you all for your help

@dave
yes you are absolutely right, same like what I get, the calculation for zenoah is all show up defiecient
is there something wrong with jennings equition or maybe he make some mistake or forgot something?

you mention about blowdown, does more blowdown time mean more RPM we get?
what number should I aim for?

Im my readings "Mean Port Area" term seems odd since it doesnt account for port shape "time area" it seems has been the standard used for 42 years.

Jennings is correct but he is not saying that those time areas will not allow an engine to rev past a certain point. He is saying that the power of the engine will be deficient because of the low time area for the induction. You can set the rpm point where peak power is produced by changing the pipe length or changing the blowdown timing and so changing the blowdown time area. Blowdown timing is critical. Many times you see written that an engine with high exhaust timing has little torque and a narrow powerband but that is not necessarily the case . You can easily have an engine with low exhaust duration say 175 degrees that has low torque and narrow power band if the transfer timing is very low. 2 theoretical examples would be engine 1 which has exhaust of 170 degrees and transfers 110 degrees giving blowdown duration (timing) of 30 degrees this engine would be likely to have less torque and a narrower powerband than engine 2. with 180 degrees exhaust and 130 degree transfers which has a blowdown of 25 degrees. This is with all other things being equal such as port width. These changes in port timing are actually changing the time areas and so widening the ports will give the same effect as increasing the port duration. So for example you could have an engine that you modified and made the exhaust timing too high to drive a certain prop but by widening the transfers you reduce blowdown time area giving more torque and so more chance to drive the prop well.
On the induction time area calculation which you talk about you are correct to say that you need to widen the induction port as much as possible to be able to develop max power from the engine and ultimately this is the restrictive point on the Zenoahs RCMK's etc.
Of course this is all info to be taken into account but the best motors are probably developed by painstaking work on the dyno/test stand and by experience with on the water testing. For more info on time areas you could look at Design and Simulation of Two Stroke Engines by Gordon Blair pages 417 to 431 perhaps taking particular note of page 429 where those who widen transfer ports too much on Zenoahs etc could learn something. FYI the ISBN is 1-56091-685-0

Dave thank you very much your for very complete explanation

Seems like increasing the width would have the similar effect as enlarging the bore but not the stroke , very little.