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Holley 0-80573S 750 double pumper is a supercharger-specific carburetor. The power valves reference intake manifold vacuum below the supercharger,allowing the power valve to operate correctly. A power valve provides further enrichment to the main metering system under load (low vacuum) conditions. Without this external referencing, the power valve would be "reading" the supercharger boost pressure, which has no bearing on the engine load.