[jluetjen]

I'd venture that JohnC and Warren are both correct. Ideally, both the intake and exhaust ports would be a straight shot right to the valve. One of few breathing compromises which I've seen in 911 heads when compared to other heads is that the ports have to make something resembling a ~75 degree turn. There is where I could imagine the D shape port providing a benefit (operative word is imagine!).

Lots of engine have sharper angles that that, not to mention (as Warren has) push-rods, water jackets, etc. In classes where it is allowed, it is not uncommon for people to weld material to heads and then recut ports with a considerably reduced bend. If you can get the angle down to 45 degrees or so I guess there is a significant improvement.

So I propose that you're both right!

- John