340 Intake Manifold Casting# 2531915 - Upgrade

i just watched a video ...i think it was that Holdner guy. He replaced a stock cast iron intake with an airgap and there was no change in hp on the dyno. Also watched a video where they replaced an airgap with the edelbrock rpm and no change on dyno hp. I think it was a chev small block producing 450 plus rpm. There are lots of 10 second 340's running cast iron intakes.
I don't know what it is about aluminum intakes but for some reason guys can't change out the stock intake fast enough.