Holley power valve size

There are two ways to check what size power valve to use...
1. Go down 1.5" or 2" from what the vaccuum guage reads....
2. What ever the vaccuum guage reads divide that number by 2 ond thats what size power valve you need or go down to the nearest size...

Which one of these methods are more accurate? Because if my vaccuum read 8" then I would need a 6.5" power valve if I used method 1. Or I would need a 3.5" pv if I went with method 2.

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I went with the divide by two method, seems to work great but maybe the other way would work too...ha ha I read the Holley book over a couple times and there is no real set standard or a super specific explanation of how the power valve works in relation to the difference in a couple inches of vacuum.

 

I always divide the reading by 2. There is some tuning choices here in your case. Just depends if you need to enrich the engine earlier or later. IMO the 6.5" is too close to your idle vacuum of 8". I would use the 3.5" so that it would stay closed until you were into the throttle more.
You must have a fairly radical cam profile.
Let us know how it works out.
Greg

 

What he said. I'm sure you know this already, but the numbers on the power valve (3.5, 4.5, 5.5, etc.) indicate when they start to open in inches of engine vacuum. As soon as you crack the throttle, engine vacuum drops. At wide open throttle, there is zero vacuum. The "divide by two method" activates the power enrichment circuit at half the inches of vacuum you have at idle (and in gear if auto trans). With 8" of vacuum at idle, 6.5" will be opening up just about every time you begin to accelerate/open the throttle. A 3.5" is a great place to start as you typically want the power valve to only open during a heavy load or hard acceleration, but the engine may tell you otherwise once you get it out on the road for testing. Good luck!

 

1.5-2 inches below(in gear if its an automatic)

 

I did the divide by 2 method on a 3x2 set up w/94's. Ran ok but ended going 1 number higher. Seemed to run better and the primary carb had more mixture adjustment at idle.

 

Thanks for all the input....

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