I am really surprised that this info is not readily avaliable. I checked and the Hi-flows are in stock at a lot of suppliers, so there must be some demand for them. I have looked in Emanuel's Holley book, but it shows the old Holley std flow PV that had the little holes. And he did not say at what PVCR size the Hi-flow PVs become necessary. I'll save you all the long story here, but I think I need the Hi-flow PVs. I would like to know at what point they become necessary before I go to the trouble of ordering them and changing them.