I'm looking for some flanges for an exhaust mod, and was curious what flange ID I should look for. From a welding standpoint, is it better for the flange ID to be larger than the pipe OD? Or smaller than the pipe OD? If smaller than the OD, this would affect exhaust flow slightly, I would think...

 

if you make the flange smaller... it will restrict it but then you can "port it" back to the right size. if you have it the same size you can just weld the seem on the ouside or "butt" weld the inside. If its slightly big you might blow holes (if you arent used to welding) and it might be a little harder to place it exactly where you want.