We are often asked about the nature of Holley’s Power valves and what exactly they do, being Holley distributors with over 65 years of serving Northern California we take the time to explain Power Valves.
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 sys- tem’s operation to your specific needs.
Each Holley power valve is stamped with a number to indicate the vacuum opening point. For example, the number “65” 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 be 1/2 the intake manifold vacuum reading taken. EXAMPLE: 13” Hg vacuum reading divided by 2 = 6.5 power valve. If your reading divided by 2 lands on an even number you should select the next lowest power valve. EXAM- PLE: 8” Hg vacuum reading divided by 2 = 4 power valve.
Since there is no #4 power valve you should use a 3.5. 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 throt- tle 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.
If you have a carburetor older than 1992 (or you have experi- enced an extreme backfire) and expect a blown power valve, use this simple test.
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TEST: At idle turn your idle mixture screws (found on the side of the metering block) all the way in. If your engine dies the power valve is not blown.
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!
Many 4 barrel carburetors are equipped with a power valve depending on the carb list # and some may come with 2 power valves. The valves are numbered by the amount of engine vacuum in inches at which they will open and allow for more fuel to be added.
Stock engines have high vacuum readings (10-18 inches at idle) and the Holley power valves with higher readings like 6.5 to 10.5 will work. Holley makes a performance style “standard” flow or the “high” flow power valve. The latter has larger openings for more fuel flow with bigger engines. “Single stage” power valves are available in 1″ increment sizes from 2.5″ thru 10.5″
Many people are quick to remove the power valve, but if the power valve is removed and plugged, the main jet size must be increased 6-10 jet sizes to make up the required fuel amount lost by the removal of the power valve. Reconnecting the power valve circuit without proper tuning may cause part throttle fuel economy to suffer and create an overly rich condition which may lead to plug fouling.