i have noticed on heads how the exhaust port is often a squarish, or oblong, or anything but perfectly round, and i only see perfectly round header primaries.

so i imagine that when the exhaust gasses go though the port, the walls of the headers dont match the port walls, doesnt this create unnecesary tubulence? if the two holes were the same shape wouldnt this increas exhaust velocity and there for hp and torque at all rpms, especially lower rpm's

or am i just crazy

 

If the header is slightly larger than the port, especially on the bottom (short side radius or SSR) it can help prevent reversion or backflow into the engine. Some headers have the same shape as the port, others are round as you said.

I'd try to line up the top of the header opening with the top of the port and have the mismatch at the bottom for best anti-reversion. In no case should the header opening project into the port in the head. Minor grinding on the header might be required, especially if the heads were ported.

As to whether you are crazy, that's probably better asked in the Lounge or on www.nutzycoocoo.com.

 

You're right they should match, but so far it's unpractical to produce D-shaped tubing for building headers.

 

In one of the old 'Head Tech' threads, one of the head guru's fed answers on the topic. He stated that the top of the port exit and header entry can match, but for reversion purposes (as noted above), the bottom should never match. Reversion is a killer. You want to avoid this if possible.
I had made a set of 1" thick spacers/adapters to bridge the port of the head to the header. The sides of the adapter ports were tapered from being matched at the top to the offset at the bottom to help with reversion. The car pulled noticeably harder from 6500+ Just thought I'd share my experience

 

so it was a stepped reversion lip? sounds like a good idea

 

Take a look at this picture of the LS7 Exhaust Manifold/Header http://vorlon.case.edu/~aap8/gallery.../LS7_Engine_26. Obviously, Chevrolet is addressing this exact issue.

Steve

 

We're dealing with less reversion with an OEM cam than we would with a more aggressive after-market grind, so it may not be much of an issue. There are more ways of limiting reversion, but I'm told the port exit to manifold/header entry step is the most effective, as is not 'port-matching' the intake manifold larger than required for optimum flow. A lot of the pro's go as far as designing the intake and exhaust runners in such a way that limits backward flow. Valve angles are part of the equasion. In many cases, a 'stepped' header can also help. Scavanging is very important. Stepped headers, high-velocity merge collectors, x-pipes, and 'boom tubes' all help with this.