The power valve is a key component of the power enrichment system of Holley performance carburetors. The power enrichment system supplies additional fuel to the main system during heavy load or full power situations. Holley utilizes a vacuum operated power enrichment system and a selection of power valves is available to time this system's operation to your specific requirements. Each Holley power valve is stamped with a number to indicate its vacuum opening point. For example, the number 65 indicates that the power valve will open when the engine vacuum drops to 6.5in Hg, or below. An accurate vacuum gauge, such as Holley P/N 26-501, should be used when determining the correct power valve to use. A competition or race engine which has installed a long duration, high overlap cam will have low manifold vacuum at idle speeds. If the vehicle has a manual transmission, take the vacuum reading with the engine thoroughly warmed up and at idle. If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up and idling in gear. In either case, the power valve selected should have a vacuum opening point about 2in Hg below the intake manifold vacuum reading taken.