Perform the same function, not same damn thing. Isn't an exhaust manifold one opening for all exhaust ports while a header (or headers, whatever) are 4 seperate pipes, in the case of a 4 banger? The theory being that these pipes are designed in a way in which the exhaust gases from each exhaust port will enter the main exhaust system at different times. While in an exhaust manifold all the ports are dumping gases into one large pipe which causes the gases to become disturbed and ends up restricting the air flow.