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raw stainless will brown slighly for abot 2" of the primary where it exits the head it's normal but thats all it will ever do. I believe the thorley system is coated mild steel which depending on climate and annual miles driven will slowly degrade and rust. it also depends on the quality of the coating used. A stailess steel header will disapate better and more evenly than a mild steel header so from a performance standpoint it's not a necessity but more of a cosmetic addition. Some claim they need it to lower underhood temps, But how much heat do you think the cast iron manifolds that came on the car generated or kept for a longer period of time than stainless? On a mild steel header coating is a must. That's enough of a rant for now sorry
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