Holley Double Pumper® carburetors are a perfect performance upgrade for hot street cars and race vehicles. Double Pumper carburetors are an American icon when it comes to performance carburetors. From their roots as the premier carburetor on many factory muscle cars to powering thousands of vehicles across the finish line, the Double Pumper is a great choice whether you are burning up the street or the quarter mile. The Double Pumper is designed for use on lightweight vehicles with a manual transmission or an automatic transmission with a high stall and low rear end gears. The Supercharger Double Pumper with a tumble polished shiny finish and features dual accelerator pumps for additional fuel under initial acceleration; mechanical secondaries for great performance; dual feed fuel inlets for constant high volume fuel delivery and a manual choke for easy control of engine warm up. These carburetors that are intended for use with superchargers, and have a unique capability that allows their power valves to reference the intake manifold vacuum below the supercharger. This allows the power valve to operate as it should, based off intake manifold vacuum. 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 vacuum, which has no bearing on the engine load.