I've recently started to try and tune my 850 DP, and I have a few questions. I have a pretty lumpy cam (Comp's XE295HL) and my vacuum at idle (~850) is 6 in Hg. The power valve that I'm currently using is a 5.0, so this seems about right. My intake vacuum increases up to about 20 in Hg with increasing rpm (I've only tested this up to about 3000 rpm). From what I understand, the vacuum normally decreases after some point until it reaches 0 in Hg at WOT. I'm guessing that the power valve is supposed to open up once the vacuum dips below 5 in Hg(?). So the circuit that the power valve is part of operates at high rpm?
Is this correct or am I way off?
I apologize in advance for the rookie questions...

You are absolutely correct. Vacuum drops when you open the throttle blades enough, as in heading to WOT.

Why not follow Holley's advice? They say divide your in gear idle vacuum in half: thats your power valve. I'd try their advice and go with a #4. (Those guys know a little about Holley carbs)

If you want to really know what the carb is "seeing" from the engine, get your hands on a long peice of hose and tape the gage to the windshield while you drive the car. You're really interested in how it changes around town at part throttle and light throttle. There is more to a PV selection than idle vacuum in gear. Holley has personally told me go 1/2 the idle vacuum in gear, and also go "a few numbers down from that idle vacuum reading in gear." Within a couple days of each other...lol.

Thanks for all of the input. My local parts store has a limited selection of power valves, so I'll probably have to order one (despite claims to be "performance oriented" it doesn't seem like many of the chain stores actually carry any real performance parts, unless you count Flowmaster mufflers and cold air intakes ). I'm probably going to go with the 3.5" Hg valve...

The power valve is in line with the main jet circuit. This circuit is not active at all during idle and part throttle crusing. You are using your idle and transfer slots during this driving / idling.

That being said, it is not correct to use any indication based off of how the car idles!

You really want to cruise the car down the road and see what vacuum it pulls. Find the lowest vacuum reading during cruising on level (light loads). Then take off between 1 and 2 inches and select a PV from there.

That way, when you open up the throttle blades during crusing, to go faster or up a hill or something, the vacuum drops, the PV opens and richens up the higher load demand until it recovers and is back in a light load situation.

This is the RIGHT way to go about this!

What throttle opening % do you have to be before you're able to start flowing fuel from the power valve?