When modifying an engine and raising the exhaust timing I also widen up the exhaust port a little bit. But how much is too much? What is the maximum width of the exhaust port before getting trouble with the piston ring?

Andre

I assume you're talking about Zenoahs. I don't find that widening the exhaust much in a Zenoah is really needed. The big restrictions seem to be on the intake side and widening the transfers as much as possible is the first modification. The next is to start raising the intake timing by trimming the piston skirt. This gives greater gains than widening the exhaust.

See the article on the CMB 35 for exhaust modifications we tried on that engine. Here, we did open up the side exhaust ports and carry them over the transfers even more. This engine has much better breathing than a Zenoah due to the disk valve intake.

Lohring Miller

Andre,

Your GREATEST problem area on exhaust ports and rings is ... the shape of the roof and outside top corners.

The piston ring needs to be first compressed back into it's groove at the outside corners first, then progressively compressing the ring at piston rises in bore with middle/top of port being the tallest area.

If you shape your Exhaust ports roof in this manner, ring clipping will be minimal allowing the largest possible area. (Within reason of course :-\ )

Thanks for the input, lots of informations to think about for the next engine.

Lohring, of course you are right, I talk about Zenoah engines. When you are tlaking about shortening the piston skirt to get a higher intake timing, what do you think is a good value (rate) to cut?
I have read the information about the cmb35 modifications, very interesting. And when I remember correct you wrote some time ago about drilling a small hole at the underside of the exhaust port into the sleeve, making a channel into the case from the crankcase to this hole to get abetter lubrication of the piston in this area. Is this also an option what will work on the Zenoah engines?

Andre

I'm not convinced that the hole below the exhaust helps in either the CMB or Zenoah. The CMB seized because the liner was out of round above the exhaust port and had no clearance. Zenoah pistons seize when they get hotter than the water cooled cylinder. This can be fixed by taking .1mm (.004 inch) off the diameter in the ring area.

The engines get harder and harder to start as you raise the intake timing. They also loose low end power. A safe place to start is .4mm (.015 inch). I have taken as much as 1.5 mm (.060 inch) off, but I don't recommend anything like that much.

Lohring Miller