Single-Stage Power Valve Standard Flow 5.5 In. Hg - Holley Replacement Power Valves Are An Important Component In The Power Enrichment Circuit Of Holleys Performance Carburettors. 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.