power valve

[6packCuda]

Can somebody explain to me what the power valve on a carb. does?

[ChallengerHK]

As I understand it (and somebody correct this if I'm wrong), when you quickly go to full acceleration it results in a temporary lean condition.  The power valve provide an extra squirt of fuel to overcome the lean condition until the main circuit can catch up.

[Chryco Psycho]

the power valve is held closed by manifold vacuum so for example the PV is rated at 6.5" so at 6.5 " of vacuum & lower the spring overcomes the vacuum holding it closed & allows fuel to bypass the jet circuit inside the carb & enriches the amount of fuel available to the boosters / engine

[6packCuda]

So, with a lower power valve rating like 3.5, the power valve opens sooner than a 6.5?

[Chryco Psycho]

No later actually , as the vacuum in the intake has to drop more down to 3.5 " before the PV opens

[6packCuda]

Ok. Now I've got it. Thanks Chryco.

[moper]

good description. I've heard two things from direct factory Holley tech guys. One says the PV actully needs twice the vaccum rating (so a 3.5 needs 6" of vaccum to stay fully seated) to stay totally closed, and one who said the start to pass fuel at that level.  I'm more inclined to beleive the second guy...lol. But both were straight from Holley's tech line about 6 years ago when I called to settle a bet.

[6packCuda]

I had a guy do some tuning on my six-pack a while back and he took out a 7.5 PV and put it a 3.5. Does this sound right to you guys for a 451 with a mild cam?

[MEK-Dangerfield]

I had a guy do some tuning on my six-pack a while back and he took out a 7.5 PV and put it a 3.5. Does this sound right to you guys for a 451 with a mild cam?

The power valve rating is all based on what vacuum you are pulling at idle. With a mild cam, I would say it is high (14-18 inches). Given that, I wouldn't think the 3.5 is doing you any favors. 

  Mike

[6packCuda]

Cam is 284/.507 Any recommendations on what power valve I should be running?

[gkring]

I've always taken a reading in gear at warm idle and halved it.
For example your car idles at 10 inches in gear, use a 5 power valve. Too low a number will cause part throttle lean conditions, to high a number will cause a rich condition. More for driveability and part throttle acceleration as at full throttle your manifold vacuumn will be zero or very close to it. I've never found alot of magic in a power valve setting, way off will cause lots of issues but close and exact run the same.

[Chryco Psycho]

3.5 should be fine , take a manifold vacuum reading at idle in gear , drop at least 1.5 " below the vacuum reading
I bet the 3.5 will be right

[moper]

 

[PlumCraZRT]

Yea i've gotten the 1/2 of your idle vacuum reading in gear is the correct power valve sizing.  Got it from the tech guys at Holley.  Although that last post about their "tech guys" is starting to make me lose faith.  Not sure how much faith i had before either...