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Thread: Extended Pump

  1. #1
    rob2337's Avatar
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    Extended Pump

    I have a SL750. I need to pull the pump to reseal the shoe as it has started to cavitate badly.

    While the pump is out I am planning on rebuilding it. I am also considering adding a home made pump extension. I have two questions:

    #1) I have an OP Ocean plate already on, will the 4" extension be beneficial or not really worth the effort on a 750?

    #2) Would anybody be able to give me the dimensions for the extension, as I would like to make it myself... or if anybody has an extension lying around that they want to get rid of for cheap... Also what size and length are the bolts?

    I know about the steering, water inlet tube and trim extensions and will make them myself.


  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Yes it would be beneficial for a 750.

    I'd just buy the parts VS. making your own. Much easier and fits perfrect.

    You'll also need the longer drive shaft, and steering cable extension too.

  3. #3
    rob2337's Avatar
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    I was talking about making the extension that goes after the impeller. I know it is not as good, but it would be easier and cheaper. Any thoughts?

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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Oh, the pump wedge.....

    They come in different degrees. 2,3,4 (stock),5,6 and even 8 degrees.

    A 2 seater would work best with a 2,3,or 4. Unless you ride on super smooth water.

  5. #5
    rob2337's Avatar
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    nope, I'm talking about making a 4 " extension that fits in between the nozzle and impeller. I have some spare time and a full NC shop at my disposal. Even a 5 axis machine

    I thought the idea of the extended pump was to get the point of exit of the thrust 4" further back.

    Also wouldn't an extension after the impeller be more efficient? By putting it before the impeller, aren't you just increasing the low pressure distance that the water has to travel, thus increasing the pumping loss?

    If this is a bad idea just let me know

  6. #6
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    Quote Originally Posted by rob2337 View Post
    If this is a bad idea just let me know
    It's not a bad idea. It works on the pro785 just fine.


  7. #7
    rob2337's Avatar
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    ok, so why did Polaris engineering decide to put the extension in front of the impeller on all models except the pro? After the impeller makes perfect sense to me.

    Before the impeller makes me think higher pumping losses.

  8. #8
    Moderator beerdart's Avatar
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    The Pros were twisting the longer driveshafts so they moved the extension and went with the shorter driveshaft.

  9. #9
    rob2337's Avatar
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    So is there any advantage / disadvantage to a spacer after the pump?

    In polaris's eyes there must have been some advantage to have it before the impeller, otherwise they wouldn't have designed longer drive shafts and placed the impeller further back for nothing?

    To me, I think that it would be less efficient in front of the impeller....but what do I know

  10. #10
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by rob2337 View Post
    So is there any advantage / disadvantage to a spacer after the pump?

    In Polaris's eyes there must have been some advantage to have it before the impeller, otherwise they wouldn't have designed longer drive shafts and placed the impeller further back for nothing?

    To me, I think that it would be less efficient in front of the impeller....but what do I know
    When the jet pump re-loads after jumping a wave or choppy water and ingesting air, the pump must re-fill and pressurize the entire cavity behind the impeller before you get any thrust. The smaller the cavity behind the impeller, the sooner you regain thrust after water re-entry.

    That is why the extension was position before the impeller. With the Pro 785, the engine RPM is so high (8000+ RPM) that the longer drive shaft would vibrate and oscillate, causing drive line wear and other problems.

    Polaris shortened the drive shaft and moved the extension behind the impeller. This works fine, as long as the hull doesn't leave the water very often. No air into the jet pump, no problem with the extension being set back behind the pump.

    The Pro hull is designed to stick to the water, even in rough conditions and hard turns. Apparently the combination works well, since the Pro 785 used to win lots of races back in the day.

    There is also a hardened version of some drive shafts, which also reduces the drive line problem at high RPM.

    WetWolf developed the AAT variable volume tail cone, which in the Pro models fills more of the empty space behind the stator, which improves pump hook-up in rough water, and when tuned properly helps both acceleration and top end.

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