According to the Holley web site you select the proper power valve by taking a vacuum reading at idle with a warmed up engine. Then subtract 2 from your vacuum reading and divide the result by 2. That will give you the correct power valve to use. I did that with my stock 8BA flathead. Vacuum reading at idle is 20. Subtarct 2 = 18. Divide 18 by 2 = 9. However, my Holley 390 carb came from Holley with a 6.5 power valve. Which power valve should I use? I bought an 8.5 but haven't installed it yet. Will an 8.5 power valve open sooner than the 6.5 valve? For some reason , I am having a mental block on this. Thanks.
Ill disagree with an 8.5 opening sooner It will open at 8.5 inches of vacumn...Heres what I would do.Bolt on the holley and check vacumn redings again,If its low go 2 incremates at a time 8.5 then 9.5 then ten,Youll get it dialed in better that way,Also buy a spring kit for a vacumn advance........
The valve will open when the vac drops below 8.5, that is why there is a spring on it, vac holds it shut.
The way i have always done it,Is to hook up a vac gauge either tape it to windshield or if your hose is long enough put it inside the car.Drive the car and lay into the throttle at a steady rate, see what the vacuum drops to and choose your power valve with those figures.
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