header diamter ?
 

on the bottom of the header...where it attaches to cat....does the diameter make a big difference....2" 21/4" 2.5"....can u actually lose hp by getting a 2" over a 21/4

what style header??? 4-2-1 or 4-1. a 4-1 with a 2.5" collector was my preference with my Honda.

4-2-1

YES! thats really where youre gonna gain a lot of power. a 2" collector is for bolt up applications to stock cats. its really restrictive. a 2.5" collector opens up things more an lets the motor breathe better, therefore making more power. its noticeable.

I did notice a difference going from my ebay 4-1 with 2.5" collector to a 4-2-1 2" collector. I did however cut most of the collector off and welded on a 2.5" piece but I still feel like I lost some top end power...but on the plus side it seems like my power band is much smooth and more drivable with my modified 4-2-1 with 2.5" collector.

the butt dyno theory may be true. i went from a 4-1 with a 2.5" collector to a 4-2-1 with a 2" collector and g the same thing, probably bc the extra back pressure made more low end tq....but the thing was a dog on top n fell on its face. this was on an ls motor. HOWEVER, my longtube hytech rep made a massive difference in power over the 4-2-1 on my lsvtec setup. maybe the higher cam profile likes the larger collector better.

ok....i appreciate the info