Port shapes

[evilgenius]

I build Harley engines and on the twin cam heads there are a lot of different ideas about port shape. they all have a D shape on the intake and some guys are dividing the D on the intake and the exhaust port narrows at the exit. the engine is EPA so i think they do this to keep the head temp high. the after market heads from Harley have a D shape in the exhaust too but it still seems very restrictive I think that some of that might be to create reversion to preheat the chamber. But it seems to me this would be better accomplished with a good 2 into 1 exhaust. dose anyone have any thoughts? ??? ???

[gofaster]

The D exhaust serves to raise the floor, broaden the short turn radius, and act as an anti reversion dam.

[evilgenius]

I understand how it would broaden the short turn radius but how dose the D shape act as anti reversion dam? and in the Harley head don't you want some reversion?

[gofaster]

The D faces a round exhaust pipe, making a ledge. No, you never want reversion!

[msj442]

the d shape is most common on an aftermarket head where the short side is raised along with the center side of the cylinder floor being higher than the opposing side. if you look at a good flowing exhaust port it is tilted whenever possible. the roof is raised on the center side also along with the floor.

jmo, max