There is a lot of mystery surrounding power valve selection for Holley carburetors. Once you have an understanding of how power valves work and a basic vacuum guage, it is very simple to select the correct power valve for your application.

Most four barrel carbs come with a power valve based on list number and application. These are select to work on the majority of vehicles that the carb will be used on, and will get you in the ball park, but may not be ideal for your particular setup.

Many people believe that power valves cannot be trusted to work becuase if the engine backfires through the carb, it will blow out the power valve. Most Holley carbs now come equiped from the factory with built-in power valve blowout protection. If your carburetor does not have this feature, you can purchase an easy to install kit from Holley (item # 125-500) to prevent this problem.

Some carb tuners will remove the power valve and replace it with a plug (Holley item # 26-36). This should be done with extreme caution, and only on vehicles that spend the majority of their time on the race track. When the power valve is plugged, the main jets must be increased from 6 to 10 sizes to correct for the amount of fuel lost by removal of the power valve. In addition a low rpm rich condition will be created that can lead to plug fouling and decreased part throttle fuel economy.

To determine the proper power valve for your setup, you will need an inexpensive handheld vacuum guage. *SAFETY NOTICE* When checking the vacuum on your vehicle, please make sure the wheels are chocked and the emergency brake is activated and properly working. Allow the engine to come up to normal operating temperature before taking the reading. On vehicles with automatic transmissions take the reading at idle speed with the engine in drive. On manual trans equiped vehicles the reading should be taken at idle while the transmission is in neutral. Use a power valve that is rated at 1 to 2 inches below the reading on your vacuum guage. For example, if your guage shows 8 inches of vacuum at idle you should select a power valve rated at 6.5 or 7.5.

Holley standard flow power valves are available in ratings from 2.5 to 10.5 inches (in 1 inch increments) and also in 1.0 and 5.0 inch ratings. High flow power valves are available in ratings from 2.5 to 10.5 inches (in 1 inch increments) with the exception of those rated at 7.5 and 9.5 inches which are not currently available.