[jhirasak]

The nice part about a Holly with vacuum secondaries is that the secondaries only open after you have sufficient air flow through the primaries. When adjusted properly you should not have any bog during acceleration and they only operate when the engine demands more air flow than the primaries can provide. As a result, they are more economical on fuel than a Holly with mechanical secondaries. Under WOT acceleration, if you install a small screw in the slot of the mechanical linkage between the primary and secondary shaft, you will assist the opening of the secondaries while the AFR is still slightly rich due to the accelerator pump shot. This is why a four barrel with vacuum secondaries are more suitable for street applications.