1100G Engine D vs. Round Exhaust Ports

[Guest]

When I had my 1100 apart over the winter--and spring--and part of the summer, I bought a spare head to have the valve grind and other service done to. The head I got on eBay had the D exhaust ports. I had not seen the round port yet, so I know nothing about a difference between my stock head (round exhaust ports) and the D port head. Some say that the D exhaust ports, in spite of their appearance, flow better than the round ports. Is this true?

 

[bitzz]

Kind of an open ended question, so I'll give you an open ended answer.

Typically if you're porting for flow you'll end up with a "D" shape with a flat floor.

... not always. Laminar flow, turbulence, fluid dynamics etc etc
If you look at it from the side, the top of the valve opening, the back of the seat, is flat. That's where the "floor" starts.

 

[Nessism]

There are threads on this in the archives if you feel like searching. Going off memory the D-port heads have more port volume and will generate lower port velocity compared to a round port head for an otherwise stock engine. Big bore w/header and better intake may favor the D-ports though.

 

[Guest]

There are threads on this in the archives if you feel like searching. Going off memory the D-port heads have more port volume and will generate lower port velocity compared to a round port head for an otherwise stock engine. Big bore w/header and better intake may favor the D-ports though.

I'm searching posts right now. The result I have found so far is discussing an 1100 D exhaust port head on a 1000, so not apples to apples. I would be surprised is the D port had lower port velocity on the exhaust side on an 1100 given that the exhaust appears to get squeezed pretty tight going past that big lump on the floor of the D exhaust port. I'm not an aerodynamicist, but it seems like the big hump is put in there to increase velocity on the exhaust side.