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.