I've always wondered why nissan engineers specified that the A-series head will have square ports. It sounds especially silly when you realise that its starts round at the valve and the exhaust manifold has round tube, so why would it transition to a square right at the edge of the head??

I stumbled on an explanation while playing on the net last night. This dude reckoned that flow out of the head wasn't impeded much, but the return flow was. I'm not sure if I agree with him yet.

Has anyone out there in Datsun land ever ported a head so the exhaust ports were round again??? I'm curious to see what effect it has.

i like these sorts of questions too!

most of the l4 & l6 had square port exhaust as well
although some later heads were round port exhaust

round must be better? because i think all the FIA L series heads had round exhaust ports.

probably easier to cast without needing machining aftewwards (or less machining) so cheaper.

Lots of the old skool V8s have square ports hey (I think on the intake tho?), it ain't just datsun - its bound to be cause of cost savings

I just checked on the Datrats website and you are correct - the FIA L-series head appears to have round exhaust ports. But Nismo A-series manifold gaskets still have sqaure/rectangular ports.

I can't really imagine how a square exhaust port is cheaper to manufacture than a round one. I've certainly seen US heads with rectangular intake ports and I think round exhaust ports ?!?

Some non-crossflow heads are designed with pairs of intakes and pairs of exhausts together E-I-I-E-E-I-I-E, which is in contrast to E-I-E-I-E-I-E-I. The main advantage I see is that each intake port is heated by only one adjacent exhaust port not two. This helps keep the intake charge temp down. So I could understand the engineers making the ports rectangular if it means you can jam them closer together and leave more space between exhaust and inlet ports. But it would not appear to be the motive in the design of the A-series head.

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.

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

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!

Benny you beat me to it,

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

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