Ok, say you have a 6.5 PV installed. When does it open? Does the vacuum have to reach 6.5" before it opens or does it open when 6.5" inches of vacuum are lost from what it is at idle? Say I pull 16" at idle, does it then open 9.5"? Just confused of what I've heard from different people.

 

At zero vacuum it's open; it closes when the vacuum reaches that number. If you look at it the other way, it's closed at all values of vacuum above that number, and opens when vacuum drops to its number. Idle vacuum has nothing to do with that. If your car idles with 16" of vacuum, and it has a 6.5 PV in it, then when the vacuum drops to 6.5", it opens. If it idles at 8" and has a 6.5 in it, it opens at 6.5. If it idles at 8" and has a 9.5 in it, then it will be open at idle, which makes it near impossible to tune.

Its purpose is to richen the mixture when you give it lots of air (push the pedal alot).

I find it best to use a large PV number, about 4-5" below idle vacuum; and lean the jets until the car almost starts to surge at cruise, or until it sags at tip-in while cruising, and fatten it back up just enough to cure that. Sometimes I'll start out with one PV number, and need to step it up a notch or 2 as the jets get dialed in. If you set a Holley up that way it will give you the best idle and gas mileage. Then tune the secondaries for best mph in the 1320. Don't try to tune a carb for ET.

 

Thanks for the info