Just like the title says: Which material is better to make header flanges from, and why? Assuming you use stainless pipes (which most will do), what would the advantages or disadvantages be to using each material for the flange for a turbo header?

 

make it out of the same material as the pipes.
with the same thermal coeffient it will work better.

 

I would use mild, the lower thermal expansion will help prevent gasket burnout and bolt problems. Even if you use stainless piping the different expansion rates won't cause any problems because of the way the pipe "grows".

Brad