• Welcome to the forum website of the Vincent H.R.D. Owners Club.

    Should you have any questions relating to the Vincent H.R.D. Owners Club, or Vincent H.R.D. motorcycles in general, please contact Graham Smith, Online Forum Webmaster by calling 07977 001 025 or please CLICK HERE.

    You are unrecognised, and therefore, only have VERY restricted access to the many features of this forum website.

    If you have previously registered to use this forum website, you should log in now. CLICK HERE.

    If you have never registered to use this forum website before, please CLICK HERE.

"D" Shaped Exhaust Port

davidd

Well Known and Active Forum Website User
VOC Member
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

Well Known and Active Forum Website 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.
 

davidd

Well Known and Active Forum Website 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

Well Known and Active Forum Website User
VOC Member
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)
 
Warning! This thread is more than 8yrs ago old.
It's likely that no further discussion is required, in which case we recommend starting a new thread. If however you feel your response is required you can still do so.

Top