Hmmm...!!!

Back in the 70's when the Chrysler Direct Connection [now called Mopar Performance] people developed the W series of performance heads for the LA series of small block engines they found that, for a given cross sectional area, the D shape, with a flat floor flowed more exhaust gas than any other shape, & this shape was adopted for the exhaust ports in these heads.
I believe that some Chevrolet heads, particularly big block heads, copied this basic shape for some of their performance models.
[can anyone confirm]

The Datsun A series engine has a flat floor in the exhaust port too. Perhaps those clever little Datsun Engineers were much more clever than we have yet given them credit for.

EDIT
OK, just read Benny's post. It looks like more than one group has come up with the same result.

Benny, Thats interesting about the D-shaped port. Did it say that shape is best at the edge of the head or throughout the port length? I'll look for that book. Right now I'm in the middle of "Race Car Vehicle Dynamics". Its an awsome book too.

Thanks for replies everyone. Sounds like you went to some effort to get an answer, Tom. Thanks.

So, has anyone actually ported the exhaust ports different shapes and tested it??

Interestingly when you read the turbo books, they suggest that you induce turbulence right after the turbine outlet. This is because turbulent air travels a shorter distance to get out of the exhaust, than what swirling air would, as the swirling air would travel along the outside edges of the exhaust...

Theoretically it makes sense, but in practice you simply turn the boost up slightly, and forget that you exhaust gas swirls...

I would have come to the same conclusion. After all, a square hole with width x has a greater cross sectional area than a round hole with diameter x.

However, if you had a round and a square port with the same cross sectional area the round port will flow better. There is less dimensionally induced turbulence in the circular hole.

Ok I checked with someone who would know these things; I hope I explain this right , it's a combo of cost and flow. For production work the squre shape gives more flow given the crudity of production casting.
This would explain the differant shapes on low volume FIA heads etc.........as more time is spent cleaning them up.
Also with modern equipent you can cost effectively get any shape you want.

Tom

Benny you beat me to it,

That book is fantastic, andwell worth a read, some great hands on proven work in there...

These engines were designed way before the days of CAD, could have just been so save confusion.

You can stick a square peg in a round hole, if you hit it hard enough!

I have a copy of "Four-Stroke Performance Tuning "
by A. Graham Bell

And I do recall a chapter on port shaping. It stated that the best shape for exhaust ports is a "D" shape with the flat section as the floor of the port. I guess a square port like the one in the A-series head is a step in the right direction of performance . From memory it had something to do with gas velocity is better with the flat floor port.

BTW the book is a great read and rather easily understood. All is explained from induction to exhaust and everything in between, aswell as gearing.

benny

Here's another question: Why do so many engines, regardless of brand, use square exhaust ports? No guessing now. Let's see some actual answers say from the manufacturer or the encyclopedia or other authoritative source.