Now I've read two differing recommendations.

One theory suggests a power valve with a rating of one half of the idle vacuum. Such as with a vacuum reading of 13" you would want 6.5" power valve.

I've also read where you want a power valve with a rating of two less than your vacuum reading. What is the correct way to select a power valve?

When using these procedures is it the same for both primary and secondary power valves?

Front and rear is to be the same.

Power valve If you have to change the power valve then use an adjustable or socket. Place in a vise with wood on each side of the vise jaws so you don't damage it. Tighten to the following specs. For power valve with multiple drilled fuel opening, torque to 40-50 inch pounds. For power valve with 2 rectangular fuel openings, torque to 100 inch pounds.

What size Power Valve to use?

To decide what size, use a vacuum gauge and measure the vacuum at idle and subtract that reading by 2.

Example: if idle reads 10 then you need a 8.0 or 80. If the number is not on the side where the wrench grabs it, then look at the front face. The first digit is at 2 o’clock and the second digit is at 5 o’clock. Example: first digit=6 and second digit=5. You have a 65 power valve. When the engine vacuum goes down to 6.5 then the valve opens up and lets in more fuel in the carb.

Holley only uses a PV on the primary side. The only way to dial in the correct one is to drive the car. If you get a light dead spot transitioning from light to medium throttle..you need to go up a number. 6.5" is a good place to start.

Holley only uses a PV on the primary side. The only way to dial in the correct one is to drive the car. If you get a light dead spot transitioning from light to medium throttle..you need to go up a number. 6.5" is a good place to start.

Not sure what Holley your using but it sounds like it's a 750 3310-2 which doesn't come with a rear metering block. If your carb has a secondary metering block then don't forget to change the rear PV too.

Im using an 850dp, its been a while since i've had it apart myself, but im fairly certain it has two power valves.

I'll double check timing tomorrow and take a vacuum reading.

Whats the school of thought on half the vacuum reading? If most are recommending 2 less than the vacuum reading? More outdated info from holley...or?

Lots of Holley carbs, including the 3310-1, (8.5 and 10.5) use different PVs in the secondary side. Both vacuum and mechanical secondaries.

Which ones is a matter of tuning, but it should be at least 2 less then your vacuum at idle in gear, so it doesn't open while just sitting at a light in drive.

Good place to start is the factory specs.
http://holley.com/data/TechService/Technical/Carb%20Numerical%20Listing.pdf

I have about 9" of vac and use an 7.5 front and 6.5 rear, the 7.5 front takes care of a slight tip in hesitation at cruising speeds. its an 850 4781