I don't come here often, but I've been watching this thread. Seeing the picture in post 121 brings this thought to me. The V-bands themselves aren't meant to do 100% of the sealing. It's near impossible to keep that surface 100% flat when welding it on. And because stainless grows so much it's hard to keep things like that lined up. Testing it at room temp will give completely different results vs. 900 degrees. The best way to seal stainless tubes (other than welding) is to have a tube inside a tube type joint. On a V-band, some come with a little locator ridge that is meant to keep the two flanges aligned, but because the fit is usually sloppy they can't do much sealing. I don't like that style I like the simple flat flanges. With the flat flanges you can let the tube pass completely through the first flange and part way into the second.
locator left, flat right.
Here is an exagerated view of how the flat flanges should be used. The arrow indicates the direction of exhaust flow. Notice the first tube passes through into the second flange. Ideally the fit between the OD of the tube should be tight to the ID of the flanges. Two things happen here.
The first tube is usually hotter than the second flange, so it expands and seals that way. This is also how slip joints from Burns and SPD are designed to work. They sell complete slip on collectors that work just fine in turbo applications.
The second, more minor thing, is exhaust gasses have to pass back upstream to make it out of the flange.
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