Power Valve ???
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Power Valve ???

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    Senior Member Widetrack's Avatar
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    Default Power Valve ???

    OK, I'm no carb. guru but I am trying to get the best run out of a Holley carb. I have looked and listened to a lot of peeps on the boards to try and figure out a proper setup. I am running a 4781-6 Holley 850 on a 468 with a pretty healthy cam and it has power valves front and rear and my vacuum runs around 8-9 inch. at 1100 RPM. Should I be running power valves or not and what size would be recomended for this vacuum? I do know to jet up if not using PVs! WT

    Coby

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    Quote Originally Posted by Widetrack View Post
    OK, I'm no carb. guru but I am trying to get the best run out of a Holley carb. I have looked and listened to a lot of peeps on the boards to try and figure out a proper setup. I am running a 4781-6 Holley 850 on a 468 with a pretty healthy cam and it has power valves front and rear and my vacuum runs around 8-9 inch. at 1100 RPM. Should I be running power valves or not and what size would be recomended for this vacuum? I do know to jet up if not using PVs! WT

    Coby

    I suggest running power valves unless it is just a drag race piece. Part throttle drivability and fuel milliege will suffer with out them.
    With your vaccuam (depresion) reading I would run 6.5's front and rear.

    IMHO

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    Senior Member Widetrack's Avatar
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    That is what is in the carb now. I was looking to test this weekend and get the jetting set with plug readings! Hopefully LMAO! Thanks Warp WT

    Coby

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    E-7 Sheepdog (ret) SmokinLowriderSS's Avatar
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    I agree with Warp.

    Figure on jetting up 6 to 8 sizes by the time you get it right again after blocking the power valves off, and then spending weeks jacking arround with accelerator pump sizes, squirter sizes, air bleed sizes, just to get it to drive as well as it did before you decided to make it better.
    "As democracy is perfected, the office of president represents, more and more closely, the inner soul of the people. On some great and glorious day the plain folks of the land will reach their heart's desire at last and the White House will be adorned by a downright moron."
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    Senior Member ol guy's Avatar
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    Quote Originally Posted by SmokinLowriderSS View Post
    I agree with Warp.

    Figure on jetting up 6 to 8 sizes by the time you get it right again after blocking the power valves off, and then spending weeks jacking arround with accelerator pump sizes, squirter sizes, air bleed sizes, just to get it to drive as well as it did before you decided to make it better.
    I personally have dumped the power valves many times and have never had an issue. At the same time I dump the power valves I go one step up on the squirter and two steps on the jet size. If you are only pulling that vacuum at 1100 rpm when you crack the throttle the vacuum drops and power valves go for a nap. Back fire the motor good one time and they go tits up. IMO M

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    I thought when the vacuum drops the power valve opens to add fuel. And don't all holleys built after like 1990 or so have power valve protection for back fire?

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    Quote Originally Posted by River Rat 005 View Post
    I thought when the vacuum drops the power valve opens to add fuel. And don't all holleys built after like 1990 or so have power valve protection for back fire?
    You are correct, on both counts.

    Also a kit exists to retrofit it to older carbs.


    Widetrack, what is your MAX vaccumme at cruise? Power valves are ususally advised to be at 1/2 this value.
    Say, your best cruising vac. is 14", then the basic "starting point" is recomended to be a PV of 7.0.

    6.5 is close enough.

    if your max cruise vac. is only 8 or 9, then you should try a 4.5 or so.
    "As democracy is perfected, the office of president represents, more and more closely, the inner soul of the people. On some great and glorious day the plain folks of the land will reach their heart's desire at last and the White House will be adorned by a downright moron."
    H.L. Mencken

    OBAMA: Some people deserve this.
    The rest of us are being dragged along kicking and screaming.

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    Senior Member ol guy's Avatar
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    Quote Originally Posted by River Rat 005 View Post
    I thought when the vacuum drops the power valve opens to add fuel. And don't all holleys built after like 1990 or so have power valve protection for back fire?
    hEY RAT, it would be my thought and thats it. The power valve kicks in as vacuum comes back up and with that said, With a big carb and especially a large cam with a butt load of overlap the vacuum stays in the very low range. Just to think the equasion out. If you where a carb builder would you want to dump alot of feul to a motor with zero vacuum and for that matter what would that do to the efficiancey of how the motor is and how it will respond to a throttle change in that situation. For example, alot of feul that has not had a chance to atomize and mix with air to become a more volatile product for the combustion chamber. Sorry I'm ranting on the silly edge. M

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    Cantard 71hallett's Avatar
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    Quote Originally Posted by SmokinLowriderSS View Post
    You are correct, on both counts.

    Also a kit exists to retrofit it to older carbs.


    Widetrack, what is your MAX vaccumme at cruise? Power valves are ususally advised to be at 1/2 this value.
    Say, your best cruising vac. is 14", then the basic "starting point" is recomended to be a PV of 7.0.

    6.5 is close enough.

    if your max cruise vac. is only 8 or 9, then you should try a 4.5 or so.
    Nothin, didnt read the whole thread.
    Last edited by 71hallett; 05-30-2008 at 08:52 PM. Reason: didnt read
    Quote Originally Posted by gn7 View Post
    It would be a balmy 85* in Steel's shop if he would move a little faster

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    For what it's worth, depending on your throttle linkage I'd leave the secondary power valve out which is easier if you have progressive linkage.ou

    If you have a hard leaving boat the secondary power valve will unload. They don't make power valve extensions but they do make jet extensions.

    Sleeper CP

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    Quote Originally Posted by ol guy View Post
    hEY RAT, it would be my thought and thats it. The power valve kicks in as vacuum comes back up and with that said, With a big carb and especially a large cam with a butt load of overlap the vacuum stays in the very low range. Just to think the equasion out. If you where a carb builder would you want to dump alot of feul to a motor with zero vacuum and for that matter what would that do to the efficiancey of how the motor is and how it will respond to a throttle change in that situation. For example, alot of feul that has not had a chance to atomize and mix with air to become a more volatile product for the combustion chamber. Sorry I'm ranting on the silly edge. M
    From Holley's tech site:
    Holley
    carburetors utilize a vacuum operated power enrichment system
    and a selection of power valves is available to “time” this system’s
    operation to your specific needs. Each Holley power
    valve is stamped with a number to indicate the vacuum opening
    point.
    For example, the number “65” indicates that the power
    valve will open when the engine vacuum drops to 6.5" or
    below.


    In a "zero vaccume" condition, there would be NO fuel flow anyhow, because there would be NO fuel pulled from the main venturi circuit, so your "dumping excess fuel" simply does not, cannot happen.
    When you are running flat out, with only 1-2" vac, you WANT lots of fuel flow, and you can do it one of 2 ways. Bigger main jets, or all 3 "jets" (the mains and the PV in a Holley) flowing.
    Same for running 3" vac. dragging that 35' 5th wheel or Fountain up a steep hill, you are tryibng to make max power, and want full fuel flow, same 2 choices for how to do it.

    All the power valve DOES is add fuel to the main metering circuit, like having a "third jet".
    When vac. signal drops, so does fuel flow thru the main metering circuit jets, purely due to physics of differential pressure. The low-vac. condition opens the power valve to allow more fuel flow to fill in this "hole" in the gasoline flow. As vac. comes back up, the pull on the main circuit increases, pulling harder from the jets, and the PV closes off.

    The PV only flows fuel when a heavy load is holding engine RPM down much lower than the throttle opening would (such as pulling a trailer uphill, a WOT start against an auto trans tq. converter (or a prop), and at wide open throttle. Otherwise it sits there, held shut by engine vaccume.

    The fuel it passes goes to the main boosters, not anywhere else (unless a diaphram is ruptured, in which case it leaks constantly out the vac. port in the bottom.

    I was incorrect here, I said cruising RPM, should have said IDLE vac. level. Again, straight from Holley Tech.:
    QUESTION How do I tell what size power valve I need?
    ANSWER To properly size a power valve, take a vaccum reading at idle and if it is above 12" for a standard transmission a 6.5" will be safe to use. For automatic transmissions take a vaccume reading in gear at idle and if the vaccum is below 12" divide that in half for proper size. Example 9" of vaccume in gear at idle will require a 4.5" power valve.
    "As democracy is perfected, the office of president represents, more and more closely, the inner soul of the people. On some great and glorious day the plain folks of the land will reach their heart's desire at last and the White House will be adorned by a downright moron."
    H.L. Mencken

    OBAMA: Some people deserve this.
    The rest of us are being dragged along kicking and screaming.

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    Cantard 71hallett's Avatar
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    Thank you.
    Quote Originally Posted by gn7 View Post
    It would be a balmy 85* in Steel's shop if he would move a little faster

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    Senior Member ol guy's Avatar
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    Quote Originally Posted by SmokinLowriderSS View Post
    From Holley's tech site:
    Holley
    carburetors utilize a vacuum operated power enrichment system
    and a selection of power valves is available to “time” this system’s
    operation to your specific needs. Each Holley power
    valve is stamped with a number to indicate the vacuum opening
    point. For example, the number “65” indicates that the power
    valve will open when the engine vacuum drops to 6.5" or
    below.

    In a "zero vaccume" condition, there would be NO fuel flow anyhow, because there would be NO fuel pulled from the main venturi circuit, so your "dumping excess fuel" simply does not, cannot happen.
    When you are running flat out, with only 1-2" vac, you WANT lots of fuel flow, and you can do it one of 2 ways. Bigger main jets, or all 3 "jets" (the mains and the PV in a Holley) flowing.
    Same for running 3" vac. dragging that 35' 5th wheel or Fountain up a steep hill, you are tryibng to make max power, and want full fuel flow, same 2 choices for how to do it.

    All the power valve DOES is add fuel to the main metering circuit, like having a "third jet".
    When vac. signal drops, so does fuel flow thru the main metering circuit jets, purely due to physics of differential pressure. The low-vac. condition opens the power valve to allow more fuel flow to fill in this "hole" in the gasoline flow. As vac. comes back up, the pull on the main circuit increases, pulling harder from the jets, and the PV closes off.

    The PV only flows fuel when a heavy load is holding engine RPM down much lower than the throttle opening would (such as pulling a trailer uphill, a WOT start against an auto trans tq. converter (or a prop), and at wide open throttle. Otherwise it sits there, held shut by engine vaccume.

    The fuel it passes goes to the main boosters, not anywhere else (unless a diaphram is ruptured, in which case it leaks constantly out the vac. port in the bottom.

    I was incorrect here, I said cruising RPM, should have said IDLE vac. level. Again, straight from Holley Tech.:
    QUESTION How do I tell what size power valve I need?
    ANSWER To properly size a power valve, take a vaccum reading at idle and if it is above 12" for a standard transmission a 6.5" will be safe to use. For automatic transmissions take a vaccume reading in gear at idle and if the vaccum is below 12" divide that in half for proper size. Example 9" of vaccume in gear at idle will require a 4.5" power valve.
    Okay I explained that bass-ackwards so here we go again anal cranal intercourse. Oppps M

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    E-7 Sheepdog (ret) SmokinLowriderSS's Avatar
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    Happens to all of us ol guy. :-)
    "As democracy is perfected, the office of president represents, more and more closely, the inner soul of the people. On some great and glorious day the plain folks of the land will reach their heart's desire at last and the White House will be adorned by a downright moron."
    H.L. Mencken

    OBAMA: Some people deserve this.
    The rest of us are being dragged along kicking and screaming.

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