[Rich911E]

S Man:

I would hazard a guess that I have read a few more patents than you have and I do not find them to be misleading. They can be quite exhaustive (my personal record is 171 typed pages) and it can appear confusing and wordy but there is a reason for it.

Racing engines do sometimes have D-shaped ports. It depends a lot on the RPM range in which the engine is to operate and the degree of curve necessary to the exhaust port. If you are free to design the engine with short exhaust ports and nearly straight shots from the valve face to the exhaust system you can deal with the radius problem in the exhaust pipe and make the curvature nice and long. However, if you have to make a 90 degree turn in 2 inches of exhaust port, the difference in velocity between the top and bottom of the port is substantial. As JohnC pointed out, it is to maintain a high exhaust gas velocity -- it you are running flat out all the time maintaining velocity at low and midrange piston speeds is not so much of an issue. However, if you are trying to maintain high velocity to scavenge the cylinder effectively, the D-port will provide an advantage. There is also a slight anti-reversion advantage to a D-shaped port exit into the round exhaust system when operating at low RPM because the exposed wall of the D port inhibits return into the port.

Rich