Darkhorse-It's very easy to say "nope, that won't work". Myself, I prefer to spend the time figuring out a way IT WILL WORK and I do believe that's the point of this discussion.
My past experience has been that replacing stock exhaust manifolds with headers provides a decent gain. If however, we're talking about replacing one set of headers with another supposedly better set, the gains will be much smaller, likely on the order of 5 HP, depending on the amount of tuning and engineering done by the manufacturer. Are all primary tubes of equal length? Are they tuned for bottom end or high RPM power (pipe length vs. diameter)? Are they a 180 degree design? What about the collectors and the exhaust piping? How's the fabricating work like bends and welds-neat and tidy or sloppy? I've built my own custom headers on a 4 cylinder Formula car and personally, the cost to HP equation for aftermarket headers just doesn't add up on our cars. Not IMHO. If you're going to go to all the trouble of installing headers you'd be much farther ahead cutting up the stockers and reworking them. That way there are no emissions issues/problems for those of us who must go through regular testing.
