How are Holley power valves rated for a given inches of vacuum number? Is that vacuum when they just start to open, or when it's sufficiently open so that any PVCR up to, say, .080" is not being restricted by the valve seat?

I just put an 8.5" PV in my 950 (71 PMJ and .066" PVCR) and there is no change to the wideband AFR until the vacuum drops to 7-7.5". The AFR doesn't drop all the way (from the 14.5 at cruise down to 12.5) until 6" or less.

My first thought is an inaccurately made (or marked) PV, but the 7.5" that was in there required even less vacuum and was too lean at part-throttle.

I'm not sure. All I do is take the vacuum at idle say 13 and get a valve half that 6.5 and I'm good to go never had any problems.

It's the point where it is supposed to start opening. They can be off a bit.

There is usually some lag between vacuum reading and observed AFR changes. Shouldn't be much though.

I disagree with that "half the idle vacuum" advice, which is rarely right but has persisted for decades. I suspect that Holley just tells people that who don't want to get deeper into carb tuning (but with a mild-to-moderate cam with 13" and an OOTB Holley carb that's too rich to start with, it's not far off). I drive my car on the street and I want the best driveability and mileage I can get, given the 272@.050 cam in my 451 4-speed

A much better measure is "cruise vacuum minus an amount to be determined" usually about 4" but every combo is different. For example, my idle vacuum is 8", and a 4.5 or 3.5 PV would be way too late-opening. Cruise vacuum is 15" and I don't need enrichment until about 8" since it drops so fast with just a small movement of the throttle, and a minimum of power is needed for "normal" street driving

Remember, there is very little if any relation between idle vacuum and PV opening. I've had a 10.5" PV in mine and (although it was open at 8" idle) it had no effect on the idle AFR at all.


Thanks... These were near-steady state readings accelerating as little as possible, going up hills, at an RPM low enough that there wasn't a lot of torque (but high enough to be fully on the mains). The opening point is quite apparent when the gauge swings from 14.5 down to 12.5

It does appear that the PV is off a bit, and I think the 7.5 was equally inaccurate since it was coming in too late. Time to get a PV tester adapter and I already have a Mityvac! Meanwhile I have a 9.5 and a 10.5 if I want to exchange a bit more gas for street manners.

If you need it earlier, on the right track with a higher rated number.

Are the two stage units still out there? Haven't used one of those in decades.

I 100% agree with the 1/2 idle vacuum not being the best way to do things. Will it usually get you a PV number, sure. Correct, not likely.

Although that is what Holley has said for many years, a lot of tuners feel that it is outdated and do not use it. It us often too late of an opening point, especially if the idle/cruise is a bit lean. I personally like a clean idle and cruise if I can get it and that means that I usually end up with a 9.5PV. There is not much wrong with a PV opening early, but certainly a late opening can be very noticeable.

The rating opening point varies a little bit. Some PVs begin to open at the rating while others are well on their way. And things are usually happening very fast at that point, so exactness in not usually necessary.

If you really want to know when they open you need to check them with a vacuum source like a Mityvac. Mityvac used to make a little power valve tester, I had one back in the days when I worked on carbs.

Looks like a rubber funnel.

As the PV’s age and the diaphragms get hard, it will totally change the required vacuum to activate them.

Starts closing, it’s open till the vacuum is less than the # , say 6.5, below it’s open, above closed, no middle ground. Nothing more than a on-off switch. The PV is a valve has no bering on how much fuel flows thru it, holes are bigger than the restriction.

Good point which is why I tested them before using them. What I found by testing each one is that the stamped number is just a rough guide. You actually need to test it and watch it or else you don't know what it will do. Some of them don't work.

It's a spring, so a 7.5 valve is fully open at 7.499 inches? Not a chance. They don't open like a light switch.

Maybe because the vacuum runs past the opening point when into a WOT situation, it seems as an on/off deal.

Normal cruise conditions the PV should be closed. Engine off, yes it should be open. Idle, could be either open or closed (should not be on main circuit at all).

Like one said, number don’t mean squat till it’s tested at its closing point. Like to see you measure 7.499 inches of vacuum. Because it’s not a 350 pound valve spring. I would test the PV then measure engine vacuum in gear, so it’s open, then in neutral, it’s closed, so engine wouldn’t load up and have Crisp throttle response. Normally these cams only had 6-8 inches of vacuum.

I may misunderstand the above posts, but there should be no signal to the power valve on the idle circuit. So the opening point doesn't matter at idle. You could have a 9.5 PV with 6" of vacuum at idle and the PV should not be adding fuel. Now, if the blades are open enough at idle to be tickling the main circuit, then the PV could very well be adding fuel, but then there are other problems with the carb settings to begin with.

The cruising vacuum is a far better indicator of the necessary opening point for the power valve. If you are cruising at 14", 15", or 16" of vacuum, it takes a lot of throttle movement to get down to a 6.5 opening point. Too much in most cases. Frankly, a 2 stage power valve would be ideal in a lot of cases, but ones that we would need are just not available. And look how much discussion there is on the basic power valve and it's opening point.

The power valve is part of the main circuit and is separate from the idle circuit when things are correct. I have no idea what the value of the idle vacuum has to do with the opening point of the power valve. And again, there is no real problem with a power valve opening earlier than really needed. But a power valve that is opening late can cause real driveability issues.

The PV is open to manifold vacuum under the base. Vacuum holds it shut when idling or high vacuum (above #)
I’ll add the main jet is at the bottom of the channel leads to the Venturi, the jet is a metered opening, when power valve opens it adds another metered opening to the fuel in the channel. The low pressure signal at the venturi will then have more fuel going thru it hence RICHER.