with an example of Zen 30cc engine

I have cut exhaust port till max width and height, almost reach transfer port from side to side and height also max if go further will starting losing power, at this point I only get about 18,500-18,800 RPM max, with better fuel octane mybe can reach 19,000

I am trying to reach higher RPM and I see the only way is by lowering transfer port to increase BDT
anyone have try this before?

with smaller transfer time and area, is that enough supply for top RPM?
how to determine minimum transfer time area and what formula I can use?

If you want to create high RPM, HP it will be necessary to increase time area amounts in the induction, transfer & exhaust ports of the engine. It will also be necessary to develop a tuned pipe capable of providing the necessary boost for high RPM, HP operation. This also applies to a carburetor & fuel system capable of providing the necessary fuel. In the case of very high RPM, HP turning the needle out will not be enough. There is no mention of the necessary mechanical parts necessary for reliable, sustained high RPM, HP, but if the engine fails mechanically, it will not be usable!

JA

Reducing transfer time area is not really the way to go. You need to increase Exhaust and BD time area by widening the exhaust port near the top. You can calculate and compare time areas with a program such as Bimotion Port and Pipe www.bimotion.se but you need a LOT of time on your hands to do this.
As Jim Allen says mechanical reliability is a big issue. If it is a 1mm stroker then running much over sustained 19,000rpm on the Zenoah crank will most likely damage the big end bearings and if its a 2mm stroker then the Zenoah crank will twist very easily.. At the very least with the 1mm, change to an RCMK crank, true it up carefully and then lighten the piston as much as possible and the same with the very heavy wrist pin.

Be careful when widening the exhaust port in any ringed type engine. Widening & then putting a large radius to keep the piston ring from catching doesn't help time area numbers much. A three window exhaust or a single bridge exhaust such as the type used on QD engines is much more effective. Be careful with the physical size & shape of pickets in the exhaust window because they can act like heat exchangers, heated by out going exhaust gases & then heating the boost charge in both directions. The closeness of the A transfers (those closest to the exhaust) & the direction they are aimed if incorrect can allow short circuiting of the incoming charge. Any two ports that are designed to be the same, but are on opposites sides of the liner must be the same size & have the same vertical & horizontal angles. Failure to do this will result in many strange & unpredictable gas flow patterns. This doesn't sound like a Dremel tool job to me.

JA

The fact that there is a good exhaust time area number for high RPM, HP in the exhaust window doesn't mean the window is working properly. Directly attached to the window is the exhaust nozzle. The nozzle's shape in changing from the exhaust window's shape to the final round shape of the exhaust nozzle must be considered, as well as, it's angle, the taper amount & the area. One thing that happens with bad port technology is poor scavenging flow that has turbulence. You want to expel the hot exhaust gases & not mix them with the fresh incoming charge. Port windows should, with their ducts, be able to provide a path for efficient flow. The inner radius, cross-section & duct length are all critical things that can work with or against any port window.

Suggested Reading: SAE Technical Paper #1999-01-3333, The Relationship Between Port Shape & Engine Performance For Two-Stroke Engines.

JA

Is this the correct SAE article?

Found a free copy. I will see if it can be attached.

___________________

Edit: looks like it can be opened from here

That's it Mr. CC.

I am with you dodol.
The 2mm cranks add quite a bit of transfer duration which hurts the power. On my latest 30.5cc builds, I lower the cylinder by using a .005" copper gasket under the cylinder. Then I cut the squish accordingly, adjust the compression (enlarging the combustion chamber), and port the exhaust, intake and transfer width accordingly. If your worried about what people call "free porting" (the ex side piston skirt bottom opening the bottom of the ex port at TDC, common with 2mm builds), you can fix that by using a RCMK piston which has .020" longer skirts than the Zen piston. So far, we have stopped twisting 2mm cranks by using the air cooled G290 sealed third crank shaft bearing and the Zenoah crank.

Here is my latest 30.5cc kit made like you like it. I suggest using the QD hot pipe, a Walbro WT 771 carb and a Zenoah 2mm stroked crank.
www.davesmotors.com/cc275

Doug @ ESP

thank you all for your suggestion

@JA, here I just want to do some experiment
I know metalurgy/mechanical failure can occur at any time
piston ring and crankshaft failure also in my consideration

@Dave, the top of exhaust port already opened about 70-80% of bore

I also create a small bridge in the middle of exhaust port to hold the ring, just like QD system
not much we can do to exhaust, it is already max in height and width

intake port also max width, if go further we will cut the transfer port border
to minimize short circuiting, I add material in place where short circuit may occur between intake to transfer and exhaust to transfer port

now have anyone try lowering transfer port mean lowering transfer time area, at the same time increasing BDT to push more HP or RPM?

You can fill the top of the transfers with JB weld and cut to get the timing you want.