AFAIK stainless steal is just more hardy when it comes to the elements and exhaust temperatures. You will probly have a different exhaust note from the two aswell. ohh and SS is shiny..
mild steel can last longer if you spray paint it with some high temp paint to protect it from rust, there wouldnt be any difference between ss and mild steel in terms of gas flow as they are both smooth on the inside, but ss is sexier
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