The rule of thumb for power valve size selection is to divide your highest part throttle manifold vacuum reading by two and select the closest size smaller. Example: 8" Hg of manifold vacuum would use a 3.5" Hg power valve. Holley power valves include gaskets (thread size: 1/2-28).
What does a power valve do? The power valve allows the carburetor to function much leaner for part-throttle fuel economy but once the throttle is in a wide-open position, vacuum in the manifold drops to almost zero and the power valve enriches the fuel mixture. When the engine is returned to basic part-throttle operation and vacuum rises, the power valve closes up, leaning out the mixture. The power valve is important because of its effect on everyday, around-town drivability.