.....the cams, the cams are why. gsr cams have more lift and duration then b16/b17 cams do. but then, ctr and itr cams are more agressive then that even. and im pretty sure that the valves and springs are stronger too. but, a gsr head does not flow as well as a b16 head does NA, but, under boost, a gsr head is more ideal because of the shape of the combustion chamber and i think the ports are different too, but, they also yield a higher compression. so, in my opinoin a true poor mans itr should atleast have the same valvetrain, other wise its nothing like an itr/ctr head