Actually its just the opposite with these motors…
F.I. Long-Tube Headers installed - Review/Pics/Vids/Dyno
but this is just based of one persons results… if your looking for some low end, look into getting the Motordyne intake manifold also.
Im not sure how the Z1/momentum headers compare to the long tubes, but seems it would be the same result, more high end. but I could be wrong. hope this helps