I just finished installing a dc sports 4-1 header to replace my cracked oem manifold. I can definately hear and feel the difference in how the motor flows. I don't have any other mods to go with the header, nonetheless my motor feels smoother than before. I think that the gaping wound in my oem manifold was creating turbulence and hindering exhuast flow while being annoying loud. Eitherway i'm pleased that my teg no longer sounds like a chevette and has smoother output. As far as hp #'s go I couldn't begin to say, but it feels better.