A8skyhawk

Which do you recommend:

a) Holley’s idle vacuum / 2, i.e, 11 in idle vacuum = 5.5 powervalve?

b) Cruising vacuum - 2, i.e, 11 in. cruise vacuum = 9.0 powervalve?

And why why would you recommend one or the other?

NeverTooOld

Pick a power valve that is 2" to 2-1/2" Hg below your idle vacuum in DRIVE.

chevymans 77

pick one that gives the performance you want,

When cruising around and you start mashing the go peddle is when the power valve will do its thing, you can pick one that makes the car run to your liking.

The power valve comes in way before WOT if easing into the throttle

Neal

HeadsU.P.

Two inches below the vacuum reading at "cruise speed". Which would involve running an extra long hose off of manifold vacuum to a point at the base of the windshield where you can monitor the gauge.

v2racing

Using half of idle inches of vacuum for the power valve size is wrong and always has been! Holley tells people to do this only because it's easy and they don't expect people to be able to tune them properly. The power valve has absolutely zero to do with idle. It is to enrich the main circuit when transitioning from cruise to power.

I use a wideband AFR gauge and a vacuum gauge to set the power valve.

I start with a power valve that is low vacuum like a 4.5. While watching the vacuum gauge during cruise, if you have a sensitive feel for what the engine is doing, you can feel when the engine starts feeling like it needs fuel, at least I can. I watch the vacuum gauge and take note of when I start feeling the need for more fuel and record that vacuum. It will be when you're giving it more throttle and the load is increasing on the engine.

My engine makes 9 inches at idle in neutral. In gear it makes about 7 inches. At cruise it makes 20 inches of vacuum. It starts pulling pretty good between 9 and 10 inches of vacuum. I put in a 10.5 power valve and it transitions beautifully from light cruise to good part throttle power and the AFR follows very nicely.

You need to get your main jet set for cruise before setting the power valve. I run between 14.2 and 14.6. Then after selecting a power valve, set WOT. This is controlled by the power valve fuel restrictors in the valve body. They are easy to make adjustable if yours aren't and it is needed. Lot's of info on this on the web.

I like to set my primaries with the secondary linkage disconnected and then set the WOT with the secondaries last.

Mike

427Hotrod

I like to drive it and look for a nice long hill. Without giving it throttle...hit the hill at 50-60 MPH and when vacuum drops the PV will open. If you get it right it will feel seamless as it gradually increase fuel and not lose power. If it's too low of a rating it will go flat and you'll have to keep pushing on the pedal which will drop vacuum more and finally get it open.

6.5 is about the std in most "hot rod" carbs. Some of the cams I use like 3.5-4.5's. I've seen some mild combos wake up well with 8.5's in them.

There's a lot going on to select the best and idle vacuum isn't it. Velocity of the intake tract, cam, exhaust, gearing, trans/converter will all impact what works best.

JIM

HeadsU.P.

Incorrect info.

Neil B

I use 6.5's for medium cams (230-250@.050 and 8-10" idle vacuum) and 4.5's for big cams (260-270@.050 and less than 8" vacuum). You can go with lower PV's per Holley's recommendation, but I've found it requires more pump shot and a richer idle mixture. Just my .02. Your results may vary. Play with your combo and see what works; PV's are cheap.