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.
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.
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.
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.
