While pricing a new cam - the DC engine book says I need these springs.

if the valve is wide open and passes all the air the port can flow then
will opening the valve further accomplish anything?
IR3333- in a way you are correct. You can only flow what it will flow. BUT the time it takes to close the valve and open it is not diffrent and allows for more air to move. Slightly I grant you but it is diffrent. Years ago we would install 5/16" chevrolet valves in the heads.... not much from the Chrysler 3/8" stems but it did flow a bit better.