Can someone explain what a power valve really does. And the differance size.

The size relates to the amount of vacuum that causes the PV to open. As you go WOT the amount of vaccume diminishes. When the vacuum drops below the number on the PV it opens. The PV is part of the carb's power circuit and supplies additional fuel beyond what the jets provide.

Check out www.camaros.net/techref/ (http://www.camaros.net/techref/) under Series II for more Holley information.

The power valve will add fuel under certain conditions. I will try to over simplify how it does the job it is supposed to do.

A typical example would be if you were cruising down a long flat strech of highway where the carb would be feeding from the main circuit and the power valve would be closed. The engine is operating at a high manifold vaccum and there is no need for any additional fuel.

Now, you start to go up a slight hill. Without you moving your foot at all, the engine load will increase and because the load in increasing but the carb has not moved, the vacuum will drop a little. Depending on the power valve selection, it will open as the vacuum drops alowing additional fuel to go to the metering system. The added fuel from the power valve will richen the mixture so you will have more power to climb the grade.

As you get to the top of the hill and go down the other side, the load decreases and the vacuum rises causing the power valve to close again. It does this without any help from you at all.

There are other cases where it helps but basicaly it richens the mixture under high load conditions and closes under steady cruise or idle conditions. I can not think of a reason to remove it so leave it there and let it do the job it is supposed to do. Big cams might need to go a little lower on the size number while less radical engine combos should stick with the "out of the box" setting or maybe go up a size or two until you feel it go lean as the load gradualy increases.

The power valve has a number on the side that tells you at what vacuum setting it will open and close. If it says "65" then it means it will open at 6.5" of vacuum. If it says "70", then it will open and close at 7.0" of vacuum. The power valve that comes with the carb from the factory is usualy pretty close to optimal so no need to stray very far from the original size unless you have a good reason to. If you need to know what your carb has for a power valve originaly, you can look up the list number from a PDF file on Holley's web site.

Do a search for the "Bob2000" web site. It has a very exhaustive set of instructions for Holley carb tuning steps. That is the best instructions I have ever seen poseted anywhere. graemlins/thumbsup.gif

-Mark.

Thanks for the lesson on powervalves? I have a 750 HP holley on a 383 AFR 210 heads 13.3 compression.comp cam solid roller 276-284 duration @.050 .630 lift. Motor is not running yet So I do not know the vaccum. What powervalve would you think I should start with . Thanks

Here's a link to what holley says about power valves...

http://www.holley.com/HiOctn/TechServ/TechInfo/CI-121.html

Here's more of the power enrichment circuit the power valve is part of...

http://www.holley.com/HiOctn/TechServ/TechInfo/TI-222A.html

orange,

start with the original one for that carb or go with a 6.5 as a very rough starting point. I would tune in the order shown on the Bob2000 web site to get it nailed down.

-Mark.

Try starting with a #2.5 PV. With your big cam, you will not be able to use a PV that opens much higher than 3 or 4" -- I'm guessing you'll have 5-6" vacuum at idle.