Power Valve Tuning a Jet Boat
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Power Valve Tuning a Jet Boat

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    Senior Member Rivernut's Avatar
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    Default Power Valve Tuning a Jet Boat

    Is there a modified formula for power valve tuning a jet boat as compared to a car? My original Holley came with a 2.5". It makes sense that it would be lower than a car application because jet boats are run at full throttle much of the time. My Marine Demon came stock with a 6.5". It is a pain on the ass to remove and drain the magnum fuel bowls for tuning purposes, so I'd like to take out as much of the guess work as possible. The engine is a 320 HP Vortec 5.7 with a "Ramjet" cam. Its duration is 288/308; duration at .050" lift (intake/exhaust) is 196/206 and maximum lift is (intake/exhaust) is 431/451. Lobe centerline is 109 degrees. I'm running an American Turbine SD-309 pump with an "A" impeller in a 20' Eagle aluminum bass boat.
    Last edited by Rivernut; 07-29-2010 at 04:19 AM.

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    The power valve is designed to add fuel at WOT. You should hook up a vacuum gauge below your carburetor and find what the vacuum is at WOT (if there is any). You want to be sure the power valve does not close at WOT or you are going to lean out your engine. I have high flow 6.5's in my carbs, but its blown and on e-85 too.
    Last edited by slowboy; 07-29-2010 at 05:46 AM.
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    As mentioned above

    a vacuum gage is the way to tell what powervalve is required.

    check the vacuum at cruise and pick a number that is a little lower than what the gage reads at cruise.,

    The number on the powervalve is the inches of vacuum it takes to keep it closed. when vacuum drops below that number it will open. just make sure WOT vacuum reading is lower than the powervalve selected
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    Vac. reading when sec. just start to open. That is the pv you need. At wot, vac. should read 0-1. If vac reading is 1+ carb is too large.

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    Quote Originally Posted by slowboy View Post
    You want to be sure the power valve does not close at WOT or you are going to lean out your engine. I have high flow 6.5's in my carbs, but its blown and on e-85 too.
    That would pretty hard to do on a N/A motor unless the carb is Waaaay small. But on a blower deal like yours it is entirely possible. But then, PVs on a blower deal in a boat are more than a little senseless. Even if they are referenced.

    Quote Originally Posted by Tombstone View Post
    Vac. reading when sec. just start to open. That is the pv you need. At wot, vac. should read 0-1. If vac reading is 1+ carb is too large.
    Vac and 1+ in the same line? Wouldn't that just be called boost and not vac? On a N/A? I'm lost.



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    Last edited by gn7; 07-29-2010 at 08:40 AM.

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    Quote Originally Posted by Infomaniac View Post
    As mentioned above

    a vacuum gage is the way to tell what powervalve is required.

    check the vacuum at cruise and pick a number that is a little lower than what the gage reads at cruise.,

    The number on the powervalve is the inches of vacuum it takes to keep it closed. when vacuum drops below that number it will open. just make sure WOT vacuum reading is lower than the powervalve selected




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    Quote Originally Posted by Tombstone View Post
    Vac. reading when sec. just start to open. That is the pv you need. At wot, vac. should read 0-1. If vac reading is 1+ carb is too large.
    You mean too small. Increasing the cross sectional area decreases velocity and resistance (vacuum).

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    You can only hope. If the carb is/was too small the vac guage would like 2" of vac. But not 1+. Your carb is sized "correctly" according to the carb comapnies when yur vac reads 1.5" at WOT. Performance use usually reads more like 1"-.5"



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    Last edited by gn7; 07-29-2010 at 08:56 AM.

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    Quote Originally Posted by gn7 View Post
    But then, PVs on a blower deal in a boat are more than a little senseless. Even if they are referenced.



    .
    Is there another way to add fuel WOT that I am not aware of?
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    You also don't want it opening at idle.

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    Quote Originally Posted by gn7 View Post
    You can only hope. If the carb is/was too small the vac guage would like 2" of vac. But not 1+. Your carb is sized "correctly" according to the carb comapnies when yur vac reads 1.5" at WOT. Performance use usually reads more like 1"-.5"
    I stand corrected. I tried to remember a conversation w/ AED Carb regarding carb size and vac readings. Those boys know there chit.

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    Quote Originally Posted by Burnt Sticks View Post
    You also don't want it opening at idle.
    You could take the PV out and throw it in a trash can, and the motor would idle one bit different. The PV is part of the main circuit and has zero effect on the idle. And it would take one hell of a cam to idle at 6.5" of vac. or less.



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    Quote Originally Posted by slowboy View Post
    Is there another way to add fuel WOT that I am not aware of?
    Quote Originally Posted by slowboy View Post
    I have high flow 6.5's in my carbs, but its blown and on e-85 too.




    No, the PV is the only way to enrichen the fuel mixture. But if the PV is refered to the manifold, the PV opens long before the motor goes into boost. In fact it opens when the manifold gets to the rating on the PV. Say, 4.5" And due to the load on most boat motors, you can hit 4.5" or less, pretty easy with a blower deal. Most blown boat motors run around 0" at cruise. No boost, no vac. Your PV is now open. Why have the PV. The heavier the boat, the sooner this occurs. The bigger the blower, and the higher the max boost the sooner this occurs.
    If it is refered from under the carb, the faster you run the engine the more likely it is to close the PV. Son at max boost, the PV is closed and the motor goes leaner. So when is a PV a good idea in a Blown boat.



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    Last edited by gn7; 07-29-2010 at 06:03 PM.

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    Quote Originally Posted by gn7 View Post
    You can only hope. If the carb is/was too small the vac guage would like 2" of vac. But not 1+. Your carb is sized "correctly" according to the carb comapnies when yur vac reads 1.5" at WOT. Performance use usually reads more like 1"-.5"
    High performance stuff yes less than 1" vac. Even though I'd say 1.5" is high for a boat my engine did pull 1.4" at max rpm on the dyno and yes we determined the car is holding it back.

    It sounds like he has more of a family boat as opposed to high performance.

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