Welding tubes for an exhaust manifold - What's the best way of configuring the tubes to the flange?

[St0RM33]

Hello. I have a small question, which should be obvious to someone familiar with fabricating exhaust manifolds.

Regarding the exhaust port flange; Would you rather machine your flange to the OD of the exhaust header tube, pass it through the flange and weld it around on the skin of the tube, or would you rather machine the flange to the ID of the tube and weld it on the edge of the tube?

The obvious choice i believe is that it depends on the tube size you can get, obviously the one matching closer the area of the exhaust ports, since the second option allows you for porting the flange for better port matching/transition to the ID of the tube (assuming a bit oversize). The first option would be easier on fabrication though since it will be a snag fit to the flange which helps it positioning, plus i guess it will has slightly better support (for a turbo application). Welding on the skin of the tube though i guess it will have a higher tendency to crack, right?

What do you recommend and why?

Kind regards,
Ken

To mod: If you believe this topic fits better a different forum section, please feel free to move it, thanks!

 

[MikeHalloran]

Where do you get heads with round ports these days?


Mike Halloran
Pembroke Pines, FL, USA

 

[JayMaechtlen]

You can form the end of the tube to match the port shape, right?
That's done in production...

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[St0RM33]

Mike, you don't I just didn't mention it since it just makes everything more difficult to explain

Jay, yes you do, although my question is regarding not the port size or shape, my question is that if it is better to pass through the tube through the flange and weld it, or just weld it on its end to the flange.

Made a quick visualization in NX, as you can see both ports have the exact same size tube but one is passed through the flange while the other is bonded to the face of the flange:

http://i.imgur.com/Idl0n5T.png

 

[Tmoose]

What are the flange and pipes going to be made of?

I think typically the tubes pass thru the flange and are welded on the gasket face. My take is that detail would ease fixturing and manufacturing considerably.

http://www.fordmuscleforums.com/att...22-used-long-tube-hooker-headers-102_1136.jpg

http://cdn.speednik.com/files/2014/11/sema-2014-hooker-headers-new-blackheart-exhaust-systems.jpg

http://photos.motoiq.com/MotoIQ/Features/Hooker-LS-S13-Headers/i-zRKbtsM/0/L/IMG5631-L.jpg

I think a popular modern technique is to build the weld above the flange the flange face, and then mill the welds flat, but still proud of the flange to form a small sealing ring.

In an old article a premium hot rod header builder advocated additionally finishing the outboard flange/tube joint with a bronze welded fillet to provided a low stress detail with no chance of undercutting. Stahl, maybe.

 

[EdStainless]

The last ones that I saw built the tubes were flared and welded to the outside of the flange.
The flare radius provided strain relief. And they were not round either.
In order to minimize cracking the header must be flexible. The more rigid that it is the higher the thermal stresses will be. You have to choose one, either deformation or cracks.

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P.E. Metallurgy, Plymouth Tube

 

[St0RM33]

Tmoose, i have started doing this project and basically i am going with 304ss flanges and 316/L pipes. I wanted 321 pipes but they are impossible to source locally.
Yes same guessing i had; passing them through the flange is better when considering thin tubing. However i am going with Sch10 pipes (~2.76mm thick) and may even go up to Sch40 (~3mm thick) pipes after all. In this case the pipes need to be pressed on the vise to get the oval/slot-type port shape. Due to the design it would be really difficult to pass them through the flange. Plus in this case it is better to weld them at the edge so you can get full weld penetration which will make a stronger joint. Also this design uses gaskets (MLS in my case)

EdStainless, that's the right way to do it

 

[EdStainless]

Ken,
Use thinner tubing. In applications like this that rapidly heat and cool you only have two options, to allow distortion or to suffer thermal fatigue cracking. Better to use thin tube and allow for distortion.
Sch 5 is considered heavy for this (we supply a lot of 321 for such applications), most of this tubing is under 1mm wall.

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P.E. Metallurgy, Plymouth Tube