Hoping to start building my headers this weekend. The heads I'm using have a D shape exhaust port. All the tubing I'm using this round. The two options I think I have are to either let the D shape transition immediately to round tube that is larger than the pork welded to the back side of the flange, or to build up some sort of transition with Weld and grind. What's the proper way to go about this for the best result? Keep in mind that this is a challenge car and that I am a redneck with a welder and no fear of failure but no machine shop.
I'm not a doctor, but everything I've read or been told says smooth transitions are best, whether it is transitions from a small round port to a big round tube or transitions from a square (or d-shaped) port to a round tube or whenever something merges or changes direction. Smooth is better so shoot for smooth.
Can you deform the tube closer to the desired shape before welding it to the flange? Hammer, Vice, Press, etc?
Cut a long skinny V out of the tubing to make the circumference the same as the port. Weld it back shut and hammer (with great skill) the tube until it matches the port shape precisely. Weld it all up and smooth the inside with a die grinder. Done, perfect long smooth transitions. I'm not volunteering to execute this theoretical plan, for reasons.
Maybe? Hadn't thought about that to be honest.
Im guessing that the less mismatched the parts, the better, right?
What MazDeuce said. Its hammer tuning time. If you get really crazy, you can make a form out of hardwood thatyou could hammer into the end of the tubes to get the shape more or less correct before you attach it to the header flanges.
A quick google:
https://www.jalopyjournal.com/forum/threads/header-building-help-oval-at-flange-to-round-primary.514961/
That said, I don't believe it is that important at the flange itself in the grand scheme of things since flow will be limited and shaped by the ports themselves and whether the collectors are properly configured to properly leverage the pulses to help pull exhaust down the tubes.
Look at what happened when they tested headers with huge dents in the sides of the tubes and found very little difference in power output, its really about the length of the tubes and how they join together at the collector that seems to make the largest difference.
RossD
MegaDork
1/12/18 1:55 p.m.
Before going all crazy with being smooth and nice transitions, lets review the youtube video of them denting the headers and seeing negligible changes in on the dyno.
So, for a v6 long tube, what kind of collector design should I be looking for?
200hp is the factory rating for the engine. 6500rpm max.
And hardwood is genius! Got a couple of pieces of maple that should be perfect. And easy to work with.
Honestly, if i have to make headers to make the engine fit, id like to make the best i can.
Driven5
SuperDork
1/12/18 2:03 p.m.
So the heads have D shape and the Tubes have round shape. Is it possible to make the the flange connecting them transition from D shape to round?
Or, just do what we did. Weld round tubes to the outside of the flange.
Done.
I don't think we even cleaned them up that well (we probably should at some point).
From what I have seen of the factory exhausts, ANY header is going to be better than the factory 3400 exhaust....
If you're looking for the absolute last 2 horses and 3 torques, then make them D-port to match. Otherwise, just leave them round. Countless dyno tests show that it really doesn't make much difference.
The way we used to do it was we had a piece of steel that was ground down to the D-shape of the port and we whacked the tubing onto it and hammered the tube into the D shape.
The other option is to buy the proper D flanges. Get the tube roughly bent so it will go in the hole and use a drift and hammer to push the tubing out into the D shape.
pres589
PowerDork
1/12/18 3:33 p.m.
I like wv's suggestion of slicing off the factory stuff a bit past the flange at the head and then running out with your own pipe. Probably more important to get a decent collector implimented and then maybe the pipe length. Port shape, as long as you don't neck down anywhere, is probably not that important.
This thing not getting turbos now?
In reply to pres589 :
I think that is version 2 or 3.
Yup. Eliminate variables and build in stages. Stage 1 is stock engine (minus some porting and headers). Stage 2 is boost. Or nitrous. Not there quite yet....
Im overthinking this......
Somewhere, Andy Nelson's spidy senses are tingling.
In reply to Dusterbd13 :
Plus then you can say "Stage 1 engine, Stage 2 suspension" in your build description. It gives your car more street cred.
iceracer said:
Leave them as is.
anti reversion 
From David Vizard's book How To Build Max-Performance Chevy Small-Blocks On A Budget:
clshore
New Reader
1/13/18 4:43 p.m.
Curtis said:
If you're looking for the absolute last 2 horses and 3 torques, then make them D-port to match. Otherwise, just leave them round. Countless dyno tests show that it really doesn't make much difference.
The way we used to do it was we had a piece of steel that was ground down to the D-shape of the port and we whacked the tubing onto it and hammered the tube into the D shape.
The other option is to buy the proper D flanges. Get the tube roughly bent so it will go in the hole and use a drift and hammer to push the tubing out into the D shape.
This^^
A tapered piece of steel with the D-shape ground in.
Use heavy grease or lubricant to make it easy to extract the 'plug' from the tubing.
In reply to clshore :
that D form jig is in Ron Fournier's Metal Fab book iirc
Is the flat of the "D" top or bottom?
Either way, you can use a clamp to squeeze the pipe into the rough radius of the "D" weld along the sections that match up.
Then heat the misaligned bits of tube with an O/A torch and gently tap the tube with a hammer to shrink it down until it also lines up, weld/repeat.
Hammer technique will make or break you, so some experiments may be necessary.
A light hammer with a big flat face will move/redistribute material, a heavy one will dent it.
