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  • What is a Manifold Referenced Power Valve?
    Nothing will kill a blower or Nitrous engine quicker that a lean condition. You want plenty of fuel available for the engine to use .There is a thing you need to know about the power valves on a roots style blower engine. The power valve is installed to keep the engine from loading up and running rich at an idle. On a normally aspirated engine the engine vacuum at idle will hold the power valve closed. When you step on the gas the throttle plates open and the engine vacuum drops as you accelerate. When the vacuum drops below the rating of the power valve, it snaps open and richens up the main system. On a blower with the carb mounted above the rotors there is constant vacuum all the time even under wide open throttle. The power valve will never open and you will have a lean condition. To remedy this there is a modification you can have done that is called manifold referencing the power valve. You plug the vacuum feed hole in the baseplate for the power valve. Then you drill a hole in the side of the main body into the hollowed out vacuum chamber for the power valve. You then insert a vacuum nipple in this hole. You will run a vacuum line to the lower intake manifold from the new vacuum nipple. Now you will have vacuum on the power valve at an idle, and when you hit the gas as the boost builds, it will force the power valve to open and richen up the main system. This can be done by most carb modifiers or even yourself. We offer quite a few different size blower carbs with this already done. Consult you local Holley dealer or our Techline for the correct application.