i have just finished porting my zenoah and i'm wanting to increase my compression. what section do i need to mill of to get my compression up.
thanks heap

Only way to gain compression is too lower the squish clearance.
I would not worry about going after more, a lot of tuners including myself go as far as too flat top the piston further reducing compression.
Once engine is at high speed and tuned pipe boost is strong, your compression gets a boost from the super charge effects of the pipe

On my last cylinder I matched the cylinder head and piston top to eliminate the ledge on the edge of the squish area. Even though I took as little as possible off both, I needed to take about .008 inch more than usual off the base of the cylinder to get .010 squish clearance. The more you lower the cylinder, the thinner the top of the exhaust port gets. I'm not sure that this cylinder is any better than my previous attempts. Without being able to reduce the combustion chamber volume, it isn't really possible to increase the compression. The Quickdraw head volume is at least .4 cc less than a Zenoah.

Lohring Miller

If you lower the cylinder to raise the compression, don't you also have to raise the intake and exhaust ports to maintain proper timing?

Also, if you raise the exhaust port to let more out, doesn't it make sense that you should also raise the intake to allow more fuel in?

Steve Seebold wrote:
Steve,

Original question was HOW to increase compression, And the ONLY way is to lower the cylinder tightening up the squish band clearance while slightly lowering EX timing too. Raising the roof of exhaust the amount cylinder was dropped would reset the timing value and compression back to stock specs.

Let me make a statement that sets the record straight

ANYONE who is selling MODIFIED RACING ZENOAH's and makes a claim their engine mods include RAISING COMPRESSION is making a FALSE statement !!

Racing spec mods require roof of exhaust port to be raised much more than cylinder gets lowered using thin copper or no gasket. The resulting compression ratio becomes LESS ... Period !

As stated above, ONLY if cylinder was lowered and EX port left alone would you have Increased compression.


As to RAISING the intake allowing more air in ? .... The intake timing operates opposite the exhaust because you are using the bottom of the piston to control the ports timing values, not the top.
So ... When you lower the cylinder you automatically increase the intake timing duration.

Scott

What Scott says is true and good information. I think it is important to add that compression should be refered to instead as squish clearance...would be a more accurate term. (the actual clearance between the piston and the top of cylinder...measure at the outside perimeter of the dome). The actual trapped compression ratio is what Scott is referring to and is in fact reduced when the exhaust port height is increased since the TCR is actually the compression built up inside the cylinder on the piston's upstroke AFTER the exhaust port is closed.

FTR, mods such as 'Flat topping' the piston, as done by some, is another thing that actually reduced TCR.

To add to what Scott said, I raise the transfers the same amount I took off the bottom of the cylinder to keep the same intake timing. I still take some off the piston skirt on the intake side, but the amount is reduced by the amount I needed to take off the cylinder base. I raise the exhaust port a lot by sharply angling up the last part going into the cylinder. There are a lot of different philosophies on how high to go and whether the port top should be flat or arched. The metal left at the top of the exhaust limits cylinder life and is influenced by how much you take off the base of the cylinder. That's why you don't want to take much off the cylinder base. The only way to seriously raise a Zenoah's compression is with a removable head button cylinder such as those sold at www.davesmotors.com/goped-geo-sport.htm . That makes the engine a GX engine in NAMBA. It still won't have the power of a Quickdraw.

Lohring Miller

To further educate those willing to follow this subject, let me elaborate further some of the dynamics evolved in a racing caliber tuned 2 stroke engines.

At the design level, head shape, volume and compression ratio on a 2 stroke differ from a 4 stroke.
In a 4 stroke compression ratio is based upon the FULL STROKE of piston from BDC to TDC.
In a 2 stroke compression ratio is based upon the point in which the piston dome closes the roof of the exhaust port, and the Compression ratio is based upon the stroke left over from this point to TDC

** So if we had both a 4 stroke engine and a 2 stroke engine of the same displacement, with the 2 stroke engine having 180* ex timing, The ACTUAL trapped air to get compressed into the volume of the head in the 2 stroke engine would only be HALF that of the 4 stroke. The head volume between both engine would need to be adjusted because of this.

The really cool part of this is that we use 2 strokes !! ... And we have many tricks to boost the compression without doing it mechanically with smaller head volumes. ( This statement relates too one piece designs that CANNOT be changed )
The tuned pipe fitted on most racing 2 strokes are in essence SUPER CHARGERS .... by design there divergent cone is a megaphone and actually draws out a greater volume of air/fuel than engines actual displacement. Then as the return wave gets back to the exhaust port just before the port closes, a pressure wave returns some of this drawn out excess air/fuel and stuffs it back in on top of the ascending piston.
** What we have now is a trapped volume of air/fuel that has greater volume than what we would have without the pipes help. We are talking CAPTIVE COMPRESSION, we just trapped more volume !! The actual Compression pressure just went up

Basically the reason why tuned pipes increase HP. My intent tho is too help you under stand that loss's of static compression ratios are overcome at higher engine speeds. By starting out with lower compression a MORE RADICAL stronger boosting pipe design can be used.
There is a point tho when the pipe boost can OVER CHARGE the volume of air/fuel trapped, then the captive compression volume with the engines compression ratio can go too high actually hurting performance

Ignition timing, Fuel octain rating, Engine temperature also effect the point of maximum working compression pressure any engine with tolerate before performance degrades. The RPM in which you need peak power at, degree of over rev beyond power peak and mechanical limits of the engine itself all need to be addressed in the search for excellent performance from your engines.

It is interesting that some fully developed Zenoahs can run competitively against the billet engines, even tho billet engines have much higher compression.
You do see tho the pipe design on the billet engines differ from the better running Zenoahs. There in lyes what I am talking about having pipe design and boost strength being able to bridge this gap in a lower compression engine still making high HP figures.

Just FYI ....
Scott

Scott,
That is very good information. While I am one who does not agree with the flat top pistons or the sharp angled exhaust ports, I do agree with most of what you have said and think it is great information for the do it yourself guys.
In my opinion, I am of the firm belief that we just can not get enough static compression in the Zenoah style engines. Being limited to the as cast dome we have limiting factors to work with. People believe that they can just keep lowering the cylinder to raise the static compression ratio but there are a lot of other things to consider when doing this. I will not get into all of the factors here, but I know most of the time anything tighter then .016 squish will only hurt performance and it has nothing to do with compression ratios. I have done quite a bit of testing with these theories and have proved it to myself over and over that with the port timings that are common in our Zenoah style engines that flat topping or doing anything to decrease the static pressure only hurts torque and makes the power curve sharp and not broad. With that being said there is more then one way to skin a cat and everyone has there own way of doing things and I am all for new ideas.
I also agree that the fully developed Zenoahs can be very competitive against there billet brothers.
I applaud you for always taking the time to help. Knowledge is power and you do a great job of helping others with it, keep it up man.

The main reason Zenoah powered boats can race with Quickdraws is that the power required goes up as the cube of the speed with the same design boat. That means that a 7 3/4 hp Quickdraw will only push a boat 7% faster than a 6 1/4 hp Zenoah. That's 4 mph at 60 mph. Small driving and setup errors add up to more difference. The billet engines tend to be more fussy and often aren't running at their peak. This performance difference does show up clearly in SAW boats. That difference in power should be good for 7 mph at 100 mph, about the difference between the Quickdraw and Zenoah records. No one has got a Zenoah powered record of over 100 mph yet.

Lohring Miller