Bill,
Here is a photo of the D shaped port in the Terry Prince head.
The stated purpose for this is to prevent reversion. Large over lap cams often allow exhaust gas that is in the pipe to curl up the inside of the pipe and enter the combustion chamber. This contaminates the incoming mixture. It causes what is known as "megaphonitis." I call it both, but it happens on straight pipes just as easily.
The D is supposed to impede the exhaust gas reversion. I think it is also fair to say that the 1-5/8" stub attached to the 2" pipe is also supposed to do something similar, which is why it was never seen as an impediment to higher performance.
I suspect the reversion problem could be attacked by higher gas velocity in the pipe in concert with better intake tuning. The gas velocity through 2" pipes is quite slow. There is little momentum. The intake tract can be tuned just like the exhaust by creating a negative pressure area near the intake valve to draw the fuel charge in just before the intake valve opens. It requires lots of dyno time. The 2" pipe, however, works incredibly well at boosting performance from stock, which is why it is so popular.
David
Here is a photo of the D shaped port in the Terry Prince head.
The stated purpose for this is to prevent reversion. Large over lap cams often allow exhaust gas that is in the pipe to curl up the inside of the pipe and enter the combustion chamber. This contaminates the incoming mixture. It causes what is known as "megaphonitis." I call it both, but it happens on straight pipes just as easily.
The D is supposed to impede the exhaust gas reversion. I think it is also fair to say that the 1-5/8" stub attached to the 2" pipe is also supposed to do something similar, which is why it was never seen as an impediment to higher performance.
I suspect the reversion problem could be attacked by higher gas velocity in the pipe in concert with better intake tuning. The gas velocity through 2" pipes is quite slow. There is little momentum. The intake tract can be tuned just like the exhaust by creating a negative pressure area near the intake valve to draw the fuel charge in just before the intake valve opens. It requires lots of dyno time. The 2" pipe, however, works incredibly well at boosting performance from stock, which is why it is so popular.
David