When I was in the process of adjusting the squish on a rev 28.5 using solder on both ends it was not even around the squish at all and the shape of the chamber was not uniform . After I matted / lapped the piston to the squish and and tweaked the shape is when I finally got it to its best performance to date, (using stick on sand paper, press and twist method).
Now I'm building a Zen cylinder and it has a more uniform chamber and matched squish band. I rounded the walls of the chamber a little and increased the volume so my questions are did I increase compression ? other question is if I added more volume should I go higher on the timming or keep it stock and tweak it ? I will be using a TK 1.6 crank and a 36mm rev piston. I sanded the top of the piston cause it comes with a rough finish and matted it to the sqiush band, this will give me a little more clearance up top so I can keep the cylinder base lower and piston ring ends on the intake nub .
I assume I can keep the exhaust roof timming stock in order to keep a greater compression distance since I increased the size of the combustion chamber and get a bigger bang!

If you want to run squish clearances less than .010" (2.5 mm) you need to machine the combustion chamber and the piston. I cut a 12 degree angle on both and take the minimum amount off each to just clean them up. I also machine a flat area on the piston. You will need to take a lot off the cylinder base to compensate. The combustion chamber volume ends up around 2.4 cc. That's a pretty low compression ratio, but I don't know how to reduce it in a legal NAMBA G class engine. If you have a head button engine you can run a lot more interesting compression ratio. Years ago we ran a 1.8 cc volume chamber with .002" (.5 mm) squish clearance in a Quickdraw. Jim's toroidal heads can go even smaller.

Lohring Miller

Zen cylinder with no base cut using a using a CY bottom with no base gasket the cylinder will ride the piston about .006. I added about .014 gasket and piston ring seems like it is in a safe position but I plan on using a Zen bottom. I will shoot for .012 min and .015 max on the squish.

Would I be better off keeping the stock compression distance since I made more volume to be compresed ?

Thanks for posting the photos of $ Bill Lohring. I'm testing a toroidal head that will be used with a gas, methanol mix. The button's total volume now is less than 1 cc. I'm using the semi surface discharge plug you are responsible for with a Mallory CDI coil. The spark plug is still .015" away from the piston crown at TDC. Still no detonation & no signs of death ash under the piston crown. I will be using the Hardinge CNC lathe at Aero Precision to generate some different chamber shapes instead of just using ball nose end mills for the toroid.

Jim Allen

It sound like you reduce the volume of the chamber if you are running a higher exhaust timing which gives you a shorter compression distance in order to maintain good compression.

Yes & no. You can only raise the compression to the point where the engine begins to detonate. The toroidal head design will not allow the detonation to take place, even when the compression is very high. Having the ability to raise the compression greatly improves the bottom end & mid range HP. It also improves the high end HP, especially in the over rev range.

Cut & try!

JA

I rounded the chamber walls a little there by increasing the volume so should I go with a lower tiimed exhaust and have a greater compression distance?

A smaller or greater compression distance means very little in any engine. What is very important is how good is the seal when the piston is moving towards TDC & when the expanding gas is pushing the piston downward. Since both events must take place in very short periods of time, the sealing devices (piston ring roundness, cylinder bore roundness, piston ring tension, bore finish & bore material, piston ring groove geometry, piston ring material, etc.) must be correct.

JA

Obviously there is a ratio that works better as far as compression distance to chamber volume. You have adjusted your chamber volume of 1 cc to work best the compression distance. I have over 2.4 cc to deal with and figure I need all the distance available.

I don't know about a ratio for a particular compression distance & volume, but the exhaust timing of the engine in question is 195*+. I run the highest compression that the engine allows without detonation.

JA