Most car manufacturers issue restrictive, tight cast iron manifolds that's great for keeping heat to an optimal temperature so that your catalytic converter can work at peak temperatures. We all know how tight and cramped engine compartments have become, which is why headers are compactly designed for maximum space. In addition, headers are resonance tuned to help the engine breathe for maximum exhaust flow. This is why intake, header, and exhaust system combinations work so well. Get in it and push it out.
Headers assist in evacuating the most burnt gas while helping the engine pull in fresh fuel.
That's why I,H,E systems work so well and synergistically sync to produce the quickest exit of gases from your engine, resulting in more horsepower. There are many headers available in the market. You've got Neuspeed, DCSports, RSR, A'pexi, OBX, Pacesetter and the list goes on, but note that all headers are NOT created equal. The resonance tuning, fit and finish, and quality are reason enough to be concerned that you're getting the most for your money. Dyno graphs are great test lab indicators as to the validity of each companies claims, and we do mean claims.