Nigel,
Both air and water cooled engines will benefit from a 90% outlet purely from the overall efficiency aspect. The air cooled cylinder, which is always struggling to shed its heat load, will benefit from the smaller volume duct because of the reduced surface area absorbing less heat into the barrel. More heat and acoustic energies then remain available to be used by the exhaust to create more power, which will in turn creates more heat, and if dissipation rate is exceeded by generation rate then events can become terminal! Its a right old merry go round? Predictable thermal control will always be available to a water cooled engine. But any small amount of heat that an air cooled engine can shed, or not absorb, is to be greatly welcomed.
The exhaust port post is a bit of thinking out loud on my part and is another topic we chatted over at the recent Midlands Meeting. The object is to maximise blow-down area without compromising piston ring survival. The elliptical top edge has a small blending radius to keep things safe and the straight sides enable the transfer ports to pulled in tighter to gain a touch more useful area. Also shown is a half width, raised floor and does show how much extra cylinder area is exposed. This shape is not a new one and has been used successfully in a lot of engines.
The formula to find the actual radius blend point can have any bore diameter in-putted together with port chord width. As before, the port sizes used, for the 54mm bore, are just numbers and are not recommendations.
Apologies for the jumbled nature of the whole thing but I haven`t had a chance to do a decent re-write, but if you want to, you can work through to make some sense of the options available!
Trevor
Home 
Sun Nov 30, 2014 10:48 am
" />
" />