Everything else the same, headers will be louder than cast iron manifolds. Some headers of thicker stock will be quieter. Note that those thicker headers command a commensurately higher price, not only because of the tubing used, but also generally thicker, more precise flanges, as well as other quality issues. When I went from hipo manifolds to tri-ys, I can't say I saw much in a commanding improvement in performance, but i did see a significant increase in decibels.
Tom Kubler, Long-time Mustang Enthusiast & San Antonio Mustang Club Founder
Tom Kubler, Long-time Mustang Enthusiast & San Antonio Mustang Club Founder