I did back to back dyno testing on exhaust manifolds on my FAST 484 in Hemi. This was over 20 years ago so I don't remember the exact numbers but it only picked up very minimal horsepower. Somewhere in the five to eight horsepower range.

[img]http://i.imgur.com/boeexFms.jpg[/img]
31 Plymouth Coupe, 392 Hemi, T56 magnum
RS23J71
RS27J77
RP23J71
RO23J71
WM21J8A
I don't regret the things I've done. I only regret the things I didn't do.
"Wise men talk because they have something to say; fools because they have to say something. ~ Plato"