POWER ENRICHMENT SYSTEM

The power enrichment system supplies additional fuel to the main system during heavy load or full power situations. Holley^® carburetors utilize a vacuum operated power enrichment system and a selection of power valves is available to“time this systems operation to your specific needs. Each Holley^® power valve is stamped with a number to indicate the vacuum opening point. For example, the number 6.5 indicates that the power valve will open when the engine vacuum drops to 6.5" or below. An accurate vacuum gauge, such as Holley^® P/N 26-501, should be used when determining the correct power valve to use. A competition or race engine which has a long duration high overlap camshaft will have low manifold vacuum at idle speeds. If the vehicle has a manual transmission, take the vacuum reading with the engine thoroughly warmed up and at idle. If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up and idling in gear. In either case, the power valve selected should have a vacuum opening point about 2" Hg below the intake manifold vacuum reading taken. A stock engine, or one that is only mildly built for street use, will have high manifold vacuum at idle speeds. To determine the correct power valve the vehicle should be driven at various steady speeds and vacuum readings taken. The power valve selected should have an opening point about 2" Hg below the lowest steady speed engine vacuum observed. Most of the popular Holley^® "Street Legal" and "Street Performance" carburetors incorporate a power valve blow-out protection system. A special check valve is located in the throttle body expressly for this purpose. This check valve is designed to be normally open but will quickly seat to close off the internal vacuum passage when a backfire occurs. Once closed, the check valve interrupts the pressure wave caused by the backfire, thus protecting the power valve.

THE TRUTH ABOUT POWER VALVES USED WITH HOLLEY^® CARBURETORS

There still seems to be a lot of misconception about Holley^® carburetors blowing power valves. Nothing could be further from the truth. Holley^® performance carburetors built since 1992 have utilized a power valve check system that effectively eliminated this infrequent problem. Consisting of a spring, brass seat and check ball, the check ball system is 100% effective protecting the power valve diaphragm from damage due to engine backfire. The power valve check ball is designed to be normally open but quickly seals to close off the internal vacuum passage when a backfire occurs. Once closed, the check valve interrupts the pressure wave generated by the backfire, thus protecting the power valve diaphragm. There is no way that the power valve's diaphragm can rupture due to an engine backfire.

How to ensure your Holley carburetor is using the right power valve for your engine.


Holley carburetors have a power enrichment system that provides fuel to the main power circuit during heavy loads or under full throttle situations. The vacuum operated power enrichment system is controlled by a Power Valve that times the operation to your engine's specific needs.

How it works:
The power valve opens at low vacuum, such as at wide open throttle, and directs more fuel into the main power circuit. The valve itself is a small rubber diaphragm with a small coil spring. When opened, it allows fuel to flow through a calibrated opening in the metering block called the power valve channel restrictor. This restrictor determines the amount of additional fuel delivered to the engine.

Problems:
The incorrect size power valve, or a blown out power valve can cause problems such as poor fuel economy, black smoke emanating from your exhaust, dark or fouling spark plugs and a poor idle. If you suspect that your carburetor has a blown-out power valve, you can perform this simple test.

1) Check the manufacture date of your Holley carb.

Performance Holley carburetors come with a power valve blow-out check valve built in. It prevents damage to the power valve in case of backfire. Holley carbs older than 1992, however, may not have this check valve built in.

2) Test it using the idle mixture screws

If you still suspect the power valve is blown out, start your engine and allow it to idle and get to normal operating temperature. Then, turn the idle mixture screws all the way in. If the engine dies the power valve is not blown.

High Performance Engine Power Valve Selection:
High performance engines with modified cylinder heads, long duration camshafts and single plane intake manifolds may require a change to the power valve. To find out which power valve your high-performance engine needs, you can perform the following procedure:

1) Hook a vacuum gauge to an intake manifold vacuum port.


2) Warm up the engine and note the vacuum reading at idle. Automatic transmission vehicles need to be in the Drive position, while manual transmission vehicles can be in Neutral.

3) Divide the vacuum reading in half. The number will determine the correct power valve.


Each power valve is stamped with a number that indicates the correct vacuum opening point. For example a power valve with the number #65 stamped on it, will open at 6.5 inches of engine vacuum. As an example, a vacuum reading at idle of 13-inches, is divided by two and results in a 6.5 inches of vacuum. Therefore, you should have a #65 Holley Power Valve installed in the carburetor.


If you divide the vacuum reading and it falls on an even number, you should select he next lowest power valve number. For example a vacuum reading of 8-inches, divided by 2 and you come up with a number of 4. In this case you would use a #35 power valve.

Finally, if your engine produces 13 inches of vacuum or more, the stock power valve that the carburetor is equipped with from the factory, is sufficient.


Holley power valves come in a range of orifice sizes. The higher the number the more fuel is added.

Text by: DS Media Relations
Photos taken from Holley Carburetor Installation And Tuning DVD