"D" Shaped Exhaust Port

davidd

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Bill,

Here is a photo of the D shaped port in the Terry Prince head.

IMG_2047.JPG


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
 

bmetcalf

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In the '80's, US magazine Cycle had a story on tuning a Yam SR500. On the flow bench, they found that the port flow was detaching from the inside curve of the ports and causing a local vacuum and eddy currents. When they filled in the space with body filler, forming a "D" cross-section, the flow increased.
 

davidd

Well Known and Active Forum User
VOC Member
In the '80's, US magazine Cycle had a story on tuning a Yam SR500. On the flow bench, they found that the port flow was detaching from the inside curve of the ports and causing a local vacuum and eddy currents. When they filled in the space with body filler, forming a "D" cross-section, the flow increased.

Bruce,

I believe the Corvette engines use D shaped ports. Also there are some headers that are D shaped.

I know that the floor is supposed to help with flow. The theory is that the flow crowds the outside line of an exhaust pipe and the inside line becomes a jumble of turbulence. Thus, if you raise the inside line, even as a flat or floor, the inside line becomes more organized in terms of flow and moves right along. The flow data says it helps, but you would think there would be more utilization of the shapes if they made significant improvements.

David
 

vince998

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Work on Scavenge, and live with increased fuel consumption and flames from the exhaust. (this is the cheaper option and still increases engine efficiency)
Seriously though, modern engines work increasingly with exhaust gas recirculation (to reduce NX values) and are still getting 180BHP+ from a 2 liter motor (production engines that is)
 
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