Project SR E30: Part 8 – Fabricating a Custom Stainless Steel Oval Exhaust

Let’s have a quick chat about back purging. A lot of welders don’t back purge when they weld stainless and claim it is fine and never have an issue. Of course, shielding the back of the weld on stainless will absolutely make for a stronger weld as it will eliminate sugaring. The point people usually like to argue is it is not strictly necessary to back purge an exhaust for it to last.

There is a good reason why roll cages are made from round tubing. Round tubing is inherently strong in all directions due to it’s shape. Oval pipe is different. Due to its shape, oval pipe is weak in the vertical direction, which is the direction that inherits all of the force when you bottom out your car doing sick jumps.

Mr. Kojima himself reported that they had multiple issues with the oval exhaust cracking on Dai Yoshihara’s Formula D S13 until they started back purging it. For this reason, we will be back purging this whole exhaust.

Kapton tape sealing exhaust pipe for back purging
Before we can back purge, we have to seal the pipe we’re welding. There are 3 welds on this piece. We are only welding one at a time so we seal the other ones with Kapton tape to prevent the argon from escaping. Kapton tape is perfect for this application as it can withstand very high temperatures.
Back purging exhaust piping for TIG welding
For sealing off the end of the pipe we simply use bunched up foil. Nothing fancy. We like to add a bit of Kapton tape on the ends to help it seal. It’s important to add a vent hole in the foil to allow the argon to push the air out of the pipe for a complete purge.
Capping off exhaust pipe for back purging
For the hose connection, we simply wrap the hose in foil tape and bunch it up tight to create a seal. It’s not perfect but it works.
Back purging an exhaust pipe
One of the most overlooked parts when back purging is the angle of your part in relation to the gas flow. Argon is heavier than air, so it wants to sink to the bottom of the pipe while the air wants to rise to the top. In order for the argon to be able to push the air out, the gas inlet has to be placed lower than the vent hole. Propping up your part at the correct angle to achieve this is critical for a proper purge. You must also be cautious of the gas flow when you are rotating the part to weld the back side. Use whatever you have at hand to prop it up correctly.
Back purging argon gas flow
Now that we have everything set up, we can purge the pipe. You will need a dual flow regulator for this. We like to run the back purge at 15 CFH. For a small pipe section like this downpipe, 60 seconds of purge before you weld is plenty. We will scale up as we weld larger and larger sections.
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5 comments

  2. Looks great but any reason to do it in 1 piece? Seems like its going to be a bear to remove later on.

    Reply

    1. At the time, we couldn’t find any decent oval exhaust flanges that wouldn’t have taken away from the ground clearance.

      That being said, the exhaust is actually pretty easy to remove.

      Reply

  3. Nice work, I was gonna see if you can check out the treehouse racing lollypops. they are smaller in physical size and gain more clearance. Your build reminds me of one of my friends who went KA-t in an e30 and it ran pretty gnar.

    Tip: might want to 2-piece the exhaust for when you need to change out the guibo, subframe bushings, and other rear maintenance.

    Reply

    1. Yeah we’re definitely going to switch to a heim-style lolipop bushing down the line. I’ve gotten that burning polyurethane smell a couple of times already.

      Reply

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