Re: What size Power Valve?
[Re: 70sixpkrt]
#2420633
12/18/17 10:12 PM
12/18/17 10:12 PM
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Joined: Jan 2003
Posts: 12,301 Benton, IL.
DaveRS23
Special needs idiot
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Special needs idiot
Joined: Jan 2003
Posts: 12,301
Benton, IL.
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Holley has said for many years that half the idle vacuum is the recommended opening point for the power valve. Why that has never been updated is beyond me. First off, there is no vacuum signal to the power valve at idle. So, it's relevance to the power valve is questionable. More important is the cruise vacuum. Going from 15" to 6.5" before the PV opens is a long way. Too long in my opinion. I personally have never seen a PV open too soon on a muscle car, but I certainly have seen them open too late. So if it were me, I would put a power valve in that opens somewhere between 8.5 and 10.5. No matter what the idle vac is.
Master, again and still
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Re: What size Power Valve?
[Re: DaveRS23]
#2420809
12/19/17 03:35 AM
12/19/17 03:35 AM
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Joined: Aug 2003
Posts: 43,315 Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,315
Bend,OR USA
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All the people I know, a lot of them, that had problems with the power valve opening while in gear at idle had the primary throtte plates adjusted so they where on the transition circuits, not on the idle circuit only Been there, done that, more than once
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: What size Power Valve?
[Re: DaveRS23]
#2421246
12/20/17 12:39 AM
12/20/17 12:39 AM
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Joined: Aug 2006
Posts: 2,537 God's Country Maryland
GODSCOUNTRY340
master
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master
Joined: Aug 2006
Posts: 2,537
God's Country Maryland
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Holley has said for many years that half the idle vacuum is the recommended opening point for the power valve. Why that has never been updated is beyond me. First off, there is no vacuum signal to the power valve at idle. So, it's relevance to the power valve is questionable. More important is the cruise vacuum. Going from 15" to 6.5" before the PV opens is a long way. Too long in my opinion. I personally have never seen a PV open too soon on a muscle car, but I certainly have seen them open too late. So if it were me, I would put a power valve in that opens somewhere between 8.5 and 10.5. No matter what the idle vac is. No vacuum signal to the power valve at idle, sorry but that is INCORRECT. The power valve has full manifold vacuum to it at idle. If it didn't it would be open. The power valve gets it's vacuum from a port in the baseplate, not the carb boosters. Once the manifold vacuum drops below the power valve rating the spring in the valves opens it. Just look at the chamber in the carb for the power valve vacuum side and notice the port at the bottom of that chamber and where it goes, it goes directly to the baseplate and subjected to the manifold vacuum.
Last edited by GODSCOUNTRY340; 12/20/17 01:10 AM.
I love the smell of Deer guts in the morning, it smells like... VICTORY!
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Re: What size Power Valve?
[Re: 70sixpkrt]
#2421257
12/20/17 01:08 AM
12/20/17 01:08 AM
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Joined: Aug 2006
Posts: 2,537 God's Country Maryland
GODSCOUNTRY340
master
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master
Joined: Aug 2006
Posts: 2,537
God's Country Maryland
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You're right, the PV dumps fuel into the boosters when it opens at it's rating number and there's air rushing through the boosters. But when a PV blows the diaphragm gets ruptured and now fuel is being sucked through that rupture into the manifold through the same vacuum port that supplies the PV chamber, causing an over rich idle problem. Back to the OP's question, you need to use a vacuum gauge to determine the PV rating you need. Hook it up, go for a ride and do a few easy throttle accelerations, don't fully mash the pedal to the floor cause your vacuum will drop to zero. Watch the vacuum gauge and notice what it drops to. If there's no hesitation your PV is most likely the right size, if there is any hesitation you'll want a PV with a higher rating than you now have. (if you have a 6.5 and there's hesitation go to a 7.5 and test again.) I'm not trying to bash Dave, He's a very bright guy and very helpful on here.
Last edited by GODSCOUNTRY340; 12/20/17 01:15 AM.
I love the smell of Deer guts in the morning, it smells like... VICTORY!
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Re: What size Power Valve?
[Re: crackedback]
#2421269
12/20/17 01:30 AM
12/20/17 01:30 AM
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Joined: Jan 2003
Posts: 3,685 South San Francisco, Ca
70sixpkrt
OP
master
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OP
master
Joined: Jan 2003
Posts: 3,685
South San Francisco, Ca
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Back to the OP's question, you need to use a vacuum gauge to determine the PV rating you need. Hook it up, go for a ride and do a few easy throttle accelerations, don't fully mash the pedal to the floor cause your vacuum will drop to zero. Watch the vacuum gauge and notice what it drops to. If there's no hesitation your PV is most likely the right size, if there is any hesitation you'll want a PV with a higher rating than you now have. (if you have a 6.5 and there's hesitation go to a 7.5 and test again.) I have a vacuum gauge hooked up and says 15". I'll drive it next week and feel for any hesitation. I had my center carb apart due to a stripped accelerator pump screw and was just wondering about the right power valve.
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Re: What size Power Valve?
[Re: mopargilley]
#2421379
12/20/17 11:57 AM
12/20/17 11:57 AM
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Joined: Nov 2003
Posts: 36,041 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
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A bit out of my area (so FWIW) but when dealing with "hesitation" I think you'd want the A/P to be correct also, immediate tip in/correct nozzle/correct pump cam (holley).
live every 24 hour block of time like it's your last day on earth
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Re: What size Power Valve?
[Re: 70sixpkrt]
#2421406
12/20/17 12:57 PM
12/20/17 12:57 PM
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Joined: Jan 2003
Posts: 12,301 Benton, IL.
DaveRS23
Special needs idiot
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Special needs idiot
Joined: Jan 2003
Posts: 12,301
Benton, IL.
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Definitely one of my clumsiest responses to say that there is no vacuum to the PV at idle. I oversimplified to the point of being incorrect.
Sorry about that.
The point that I was trying to make was that the cruise vac is usually more relevant to the PV opening than the idle vac. And that it is better to have a slightly early opening than a slightly late opening.
Master, again and still
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