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Thread: power valves in racing carbs

  1. #1
    Senior Member MASTER BUILDER
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    power valves in racing carbs

    like to hear your thoughts on using a power valve in the front only of a racing carb, so you dont load up while driving around the pits. like to hear arguements for and against thanks
    chevy
    art

  2. #2
    Senior Member DYNO OPERATOR
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    OK, here's my thoughts. I really don't think you'll load up enough to matter at the track. If you run it a lot at home however, you'll load it up. I ran power valves in my sbc with a 950hp for just that reason. Since I went bigblock and a Dominator, I hardly ever fire it up at home. Just to unload and warm it for oil changes and valve adjustments, so I don't run pv's.
    The only downside I can see is the potential for a backfire damaging a pv or just one failing. They really aren't necessary on a race car if you can resist the urge to annoy your neighbors!!

  3. #3
    Senior Member EXPERT BUILDER
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    i agree,not needed as for the neighbors just rev it up and cleaner out if it starts to load up :lol:

  4. #4
    Senior Member DYNO OPERATOR
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    Quote Originally Posted by john858
    i agree,not needed as for the neighbors just rev it up and cleaner out if it starts to load up :lol:
    I like the way you think.....

  5. #5
    Member JUNIOR BUILDER
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    On the side of using one ( or both ), you will go through the burnout, staging process cleaner and driving through the pits / return road.

    For those that are concerned about damage during a backfire ... how often do your engines backfire ?

    I was talked into running them front and rear in a 9375 with the 3rd circuit blocked and I didnt have any issues with it nor have the 2 people who have borrowed it and used on 3 differant engines.

    I have a cup made up that fits over the rear PV on the float side and holds enough fuel to prevent it from going lean.

    I also know of a N/A Altered that runs approx.1.15 60' and ran power valves front and rear. He might jump in on this thread if he reads it.

  6. #6
    Senior Member MASTER BUILDER
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    using power valve in race car

    thanks so far for posts. my car is strictly a drag car and will never see the streets. also would like to say I am using a 3.5 power valve, which is supposed to take longer to open, so I can idle better, as opposed to a 6.5 or 8.5, which kick in a little sooner and are usually used on the street.I really dont understand how vacuum and the power valves work, but from what i read i know to use a real low number power valve for a drag car carb. can the last post who was in favor of the power valve explain this whole vacuum-power valve thing to me as it is like calculus to me, and i really dont understand it thanks again fellas
    chevy
    art

  7. #7
    Senior Member RACING JUNKIE jmarksdragster's Avatar
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    I have run with and without, really no difference in drivability on an engine with a fair amount of cam. While some get away with it in the rear, why use one? On a drag engine the secondaries are almost always closed or wide open. Just one more thing that can screw up. Primaries I would use on a milder street strip car, but on a race only drag car with a fair amount of converter, I would leave it out. You have no chance of potentially inconsistent metering without. A circle track or road race car, it's a different story.
    Mark Whitener
    http://racingfuelsystems.myfunforum.org
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    Good work isn't cheap, and cheap work can't be good.

  8. #8
    Member JUNIOR BUILDER
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    Power valve

    Street car leave them in. All out race car pull them out.

    Dave

  9. #9
    Senior Member EXPERT BUILDER
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    i just look at it this way, there is enough things that can go wrong with a racecar so if you can get rid of potenional problem get rid of it,even if it may never happen just one last thing to worry about.good luck

  10. #10
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    Power Valves are held shut by vacuum.

    When the Vacuum drops below the rated value of the Power Valve ie: 3.5", 6.5" etc. the Power Valve will open and stay open until the vacuum signal is above the rated value.

    Typical Drag Race cams have a ton of overlap causing a low Vacuum Signal to begin with thats why a lower rated PV is used.

    During the Burnout and Staging the Power Valve will likely remain shut and the Carb will only be receiving fuel from the Main Jets which are smaller ( than when run w/o a PV ) so it will be running leaner.

    The other guys are absolutley correct in saying that running without them is one less that can go wrong. I was just pointing out that they can be used as you asked for both sides.

    I will always run them in the Primary Side regardless. The Secondairy's I might try one ... I might not.

    A PV will not affect Idle ( unless it is damaged, ruptered by backfire ) regardless of the rated value as your not pulling fuel from the boosters at idle.


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