Power valves

One simple question, setting aside the vacuum measurements needed, is there a simple rule to the standard Holley 4160 power valve "tuning"?

The holley currently has a 6.5 hg, what could I expect if I lowered it to a 4.5 or increased it to an 8.5?

Does the low end power increase if the "hg" is lowered or does the low end increase if the "hg" is increased?

1982 ford f150 4x4 auto, rv cam, .30 over, (a mild rebuild)

Generally, 6.5 is good for most applications, probably including yours with a mild cam. There are some simple equations you can use to estimate it, based on idle vacuum in and out of gear. If I remember right, you can take the idle vacuum out of gear, divide it by two, and add 0.5, and you get the supposedly right power valve. You can also take the vacuum reading in gear at idle, subtract two, and that should give you about the same power valve. If anyone reading this disagrees, let us know, because that is from memory, not a book or anything.

The number is the vacuum at which the valve will open. Cars with big cams and low vacuum readings use a lower number power valve so that the fuel gets introduced at a lower vacuum. If you go too high a number power valve, the engine will run lean, possibly surge around etc. If you go too low a number, or if you have a leaky power valve, the engine will get way too much gas and send out black sooty smoke and not be up to normal performance-wise.

Thanks

that is the answer i was looking for, the Holley web site didn't really make it clear.

Later

Kt