Summit lists 4 pages of different valves, many with different engine vacuum settings and flow rates.
I copied this from one of the listings:
Holley Replacement Power Valves
Holley replacement power valves are an important component in the power enrichment circuit of Holley's performance carburetors. They operate by using engine vacuum to control their opening and closing rate. When they open, additional fuel is supplied to the main system during heavy load or under full power situations to eliminate hesitation and bogging--ensuring the smooth delivery of power without any drivability concerns. They're available in many different vacuum ranges to time the system's operation to your specific requirements, and they come in your choice of single-stage, two-stage, and high-flow designs. A gasket is included.