[Question] step up box
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step up box

  1. #1
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    Default step up box

    has anyone used a step up box between motor and jaccuzi 12yj to get pump spinning faster and what max revs can jet unit take?

    so if my motor runs at 5000rpm with 13% step up jet unit would be spinning at 5650rpm and should make boat run faster or will this not work

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    ive never understood why someone doesnt make a small transmission for a jet. if u can slap 1000hp to a pump why not turn it a little faster...

  4. #3
    Senior Member pilot940's Avatar
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    Get an overdrive unit. Might have to move the motor forward quite a bit.

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  6. #4
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    Default gear overdrive

    Quote Originally Posted by ka0tyk View Post
    ive never understood why someone doesnt make a small transmission for a jet. if u can slap 1000hp to a pump why not turn it a little faster...
    we have here in australia a company that make z drives and v drive gearboxes with overdrive up to 35% which are only about 6"-8" thick face to face which means i would need to move the motor forward that same amount which i dont think would matter

  7. #5
    Senior Member pilot940's Avatar
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    Interesting... I suppose the next question is determining impeller size. Whats gonna move the boat faster, an A turning 5500 or a C turning 6500. It seams like a C with an inducer in front of it would be the way to go. It would also be cool to have a dual tach showing impeller rpm and engine rpm.

  8. #6
    "Need For Speed" Gearhead's Avatar
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    Quote Originally Posted by bryonz View Post
    has anyone used a step up box between motor and jaccuzi 12yj to get pump spinning faster and what max revs can jet unit take?

    so if my motor runs at 5000rpm with 13% step up jet unit would be spinning at 5650rpm and should make boat run faster or will this not work
    What is going to be much more the challenge is the power to turn the overdrive unit. With your current setup, I would estimate that if you put a 13 % overdrive unit on your boat with your current power level that your engine RPM would decrease from the 5000 rpm and you would also not achieve the 5650.

    What impeller are you currently using? If you are looking for more pump efficiency, then you may need another impeller. Do you know the operating power range on your engine? This is very important to properly size your impeller.

  9. #7
    Senior Member lbhsbz's Avatar
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    If your deal spins the pump to 5000 now, without adding power or changing the impeller, the pump won't spin any faster. If you overdrive the pump, the motor will spin slower....and you'll also have losses in the gearbox. It's not like a prop where can burn it outta the hole until you get into the power band....it takes "x" amount of horsepower to spin "y" impeller to a "z" rpms....without changing x or y, z won't change.
    GN7: "If you were to have ever had two brain cells you could rub together and make heat, you be dangerous"

  10. #8
    Seriously off center slowride's Avatar
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    It'd be money better spent to upgrade to an Energizer kit. You will then be able to tailor an impeller to your combination.






  11. #9
    Senior Member jetboatperformance's Avatar
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    Quote Originally Posted by slowride View Post
    It'd be money better spent to upgrade to an Energizer kit. You will then be able to tailor an impeller to your combination.
    X's 2

  12. #10
    Senior Member tkriley13's Avatar
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    I have a question for you jet guys - none of guys spin your jets 8000 to 10,000 like prop guys right? you just change them impeller to load / grab harder to go faster right at the same rpm`s or less, right?

  13. #11
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    Quote Originally Posted by tkriley13 View Post
    I have a question for you jet guys - none of guys spin your jets 8000 to 10,000 like prop guys right? you just change them impeller to load / grab harder to go faster right at the same rpm`s or less, right?
    TK,

    Typically the jetpump is most efficient from the 6000 to 7000 RPM range and generally goes over the hill around 7500. There are a few jetboats that indeed are running the 500" Pro-Stock style engines that make peak power from the 8,000 to 10,000 range that you refer to of the prop guys, but they are doing this using underdrive units. Turning the engine in the 8,000 to 10,000 range and use a planetary style underdrive, use a large impeller and turn the pump around 7,000 RPM.

    We did this a few years ago with the Final Answer Pro Gas Jet.

    Gear

  14. #12
    AKA OhOneWS6 Last Mohican's Avatar
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    Quote Originally Posted by jetboatperformance View Post
    X's 2
    Quote Originally Posted by slowride View Post
    It'd be money better spent to upgrade to an Energizer kit. You will then be able to tailor an impeller to your combination.
    X3. What all these other guys except SlowRide and Tom missed is you said a Jacuzzi YJ pump. I'm sure Tom will correct me if I am wrong but a Jacuzzi YJ is only good for ~5K and ~60 MPH. Beyond that is starts to cavitate.

  15. #13
    Senior Member H20MOFO's Avatar
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    I asked a similar question ... on hb. On a jet...apparently its more common to underdrive a jet....rather than over drive. Most hi -perf motors make bigger hp above 5k then below. Drop the r's...and your outta the power band...so to speak.
    Edit; What gearhead said...that was a good post or 2.. as usual.
    Last edited by H20MOFO; 05-10-2011 at 07:39 PM.
    Another Hot Boat refugee

  16. #14
    Senior Member Mash on It's Avatar
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    Quote Originally Posted by Last Mohican View Post
    X3. What all these other guys except SlowRide and Tom missed is you said a Jacuzzi YJ pump. I'm sure Tom will correct me if I am wrong but a Jacuzzi YJ is only good for ~5K and ~60 MPH. Beyond that is starts to cavitate.

    YJ is a prop in a tube,
    But will pump concrete

    Daniel
    Mash on it

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