I hope Lohring will come back to elaborate on this...this post can be very constructive to many.

Scott,

I know I am still a little wet behind the ears, but it doesn't make sense that if you lower the cylinder to raise the compression and then raise the roof of the exhaust port that you would also lower the compression back to the factory setting. You raised only the exhaust port, not the cylinder. I have been known to be wrong in the past, but this time, my experience with outboard motors tells me otherwise.

Carlo,

When I was porting outboard engines, we would always leave a large radius across the top and bottom of the intake and exhaust ports. Straight edges always left a place for the ring to catch as it went by to open and close the ports. A large radius allows the ring(s) to close more gently than to just slam in to the top or bottom of the port. I know that in the Zanoah engine, the ring never loses contact with the intake side of the cylinder. Not true with the exhaust port. I realize that you have a lot more experience building Zanoahs than I do, and I know that you build a mighty fine engine, but I have to disagree with you on this point. If we left the ports square in the outboard engines, we were almost guaranteed that we would have a failure with in the first hour of run time.

Steve Seebold wrote:
Steve,
Not quite sure what your asking here ? But let me make a stab at it ?

You are right in your view that when cylinder in lowered, transfers are too
We then raise the exhaust port height more than cylinder was lowered ...
What we have done PRIMARILY is tighten up the squish clearance doing this !

Ideally we would want to re-cut the roof of transfer ports bringing them back to stock timing specs But doing so is quite difficult with these small engines, Doing so could lead to a major upsetting of the transfer flow direction and vertical alignment if done incorrectly.
If you have dentistry 90* tooling, can be VERY ACCURATE with a steady hand ? ... by all means go for it.

MANY have found that the transfer duration loss's are more than made up for with increases in transfer ports AREA. The widening and eyebrow mods do this.

Do keep in mind that in very high RPM engines with low compression ( Our Mod ZENOAHS ) By keeping the transfers slightly lower, the BLOWDOWN duration go's up ! ... This actually helps greatly in aiding the low to mid range torque of the engine.
* You can raise the transfers back up and get the blowdown less while upping duration a little giving a tad more peak HP, but engine will be a tad weaker in the lower RPM range.

?? Did this fill the gap of questions you had ??

There are two schools of thought on how to measure compression ratio in two strokes. Scott is referring to the effective compression ratio, that is the volume of the cylinder including the combustion chamber after the exhaust port closes divided by the volume of the combustion chamber. Japanese two stroke manufacturers popularized this way of thinking. Nearly everyone else, especially people that develop computer simulations to predict two stroke performance and four stroke engine manufacturers, use the geometric compression ratio. That is the total volume of the cylinder including the combustion chamber divided by the volume of the combustion chamber. The geometric compression ratio of Zenoahs is more than 12 to one. Zenoah recommends a fuel with over 90 octane.

Two strokes with high exhaust timing are harnessing the energy of the exhaust gases in tuned pipes to supercharge the cylinder. You are not loosing power by raising the exhaust timing. Many two strokes in karts and motorcycles have exhaust ports with over 200 degrees of open duration. That means the piston has little pressure on it for well over 1/2 the stroke. Pipes are everything in modern two strokes.

The problem with the Zenoah cylinder is that the maximum straight angle you can run from the exhaust flange to the cylinder wall is around 10 degrees. To raise the timing higher you need to sharply angle the last part of the exhaust passage. You still have a very thin area at the top of the exhaust port. Quickdraw incorporates very wide exhaust, intake, and transfer ports in their cylinders. They can run lower timings and still generate lots of power since the is the open area of the ports that matters. This involves the open duration and the port width. The M and D cast cylinder has port widths very similar to modified Zenoahs. It incorporates all the standard timing changes with an inner wall on the transfer ports and enough meat on the cylinder to avoid the compromises in modified Zenoahs. The exhaust has a nice downward slant, the carb is higher on the cylinder and has a shorter length intake passage. It is the way Zenoah would have built a high rpm engine that used a tuned exhaust. However, you need to remember that Zenoahs were designed to be very inexpensive to produce and run canister mufflers. They actually are quite powerful in stock form, especially for their cost.

Lohring Miller

Lohring,
That was a great explanation.

good on ya mate the first post i've read that's on the money.degree your engine to find your base timing machine the base to the required/desired compression height and redegree you engine to the timing you want to try and machine the ports for flow.then you can work on lightening the piston and raise the crank case compression too..heaps you can do!!!
cheers Ian