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  1. #1
    Piston? WHAT Piston?!? JGringo's Avatar
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    Polaris SL780 Engine Operating Temperature??

    Hey,
    I have looked through several tech threads and sensor threads,
    but have yet to discover this very important detail out.
    What is the operating temperature of this 780 engine?
    I am converting the engine to liquid cooling for an engineering
    design project at University of Iowa, but need to know to choose
    the correct Heater Valve for this application.

    Any help would be greatly appreciated!
    -Jgringo


  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    2 strokes operate most efficiently at 143*. That's why the thermostat is set to open at about 145*

  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by JGringo View Post
    ...I am converting the engine to liquid cooling...
    The engine is liquid cooled in stock form

    Are you converting it to a closed loop cooling system, possibly with a radiator?

    If so, keep in mind that the incoming lake water in the stock open loop system is always much less than 140F, and the thermostat is located at the exit from the engine. Components closest to the water inlet will be cooled by water colder than 140F, even when the exit temp at the thermostat is 140F.

  4. #4
    Piston? WHAT Piston?!? JGringo's Avatar
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    Yes,
    I want to convert the engine to a closed-loop Liquid cooling system with a radiator -- Sorry for the confusion.

    What do you mean when you speak of the drastic water temp difference compared with the closed-loop'd liquid?

    I cannot use the stock thermostat with a closed loop cooling system, can I? I actually think my thermostat has
    been removed because the rail on top of the cylinders doesn't have a valve or a temp sensor installed...

    -Jgringo

  5. #5
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by JGringo View Post
    Yes,
    I want to convert the engine to a closed-loop Liquid cooling system with a radiator...

    What do you mean when you speak of the drastic water temp difference compared with the closed-loop'd liquid?

    I cannot use the stock thermostat with a closed loop cooling system, can I? I actually think my thermostat has
    been removed because the rail on top of the cylinders doesn't have a valve or a temp sensor installed...

    -Jgringo
    Well, I didn't say drastic temperature difference. I was simply pointing out that there are factors that may not be immediately obvious when you change the design of something.

    Be sure you fully understand how the OEM water cooling system really works before you make your modifications.

    You MAY be able to use the stock thermostat, but you also need to think about how the pressure bypass valve works in the stock configuration.

    What happens in the stock open loop system when the engine RPM and cooling system pressure rises above the release pressure for the bypass valve? Does the engine run cooler than at low RPM when that valve is closed? Does the extra water flow balance out the higher heat output from the engine? At what RPM would the engine run hottest, in stock form?

    A PWC is a somewhat unusual application for an engine. The jet pump RPM and output pressure are directly related to the engine RPM and engine power output. There are no gears, so the designers can directly map engine RPM to cooling system supply pressure, and to engine cooling requirements.

    In some Polaris models the plastic water tube running forward from the jet pump intake screen contains a restriction insert. It is there specifically to reduce water flow into the cooling system to better match the actual engine cooling needs.

  6. #6
    Piston? WHAT Piston?!? JGringo's Avatar
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    Quote Originally Posted by K447 View Post
    A PWC is a somewhat unusual application for an engine. The jet pump RPM and output pressure are directly related to the engine RPM and engine power output. There are no gears, so the designers can directly map engine RPM to cooling system supply pressure, and to engine cooling requirements.
    Exactly, that is why I assumed I would have to modify the cooling system to have a non-pressure-oriented "heater valve" to maintain a constant and consistent temperature across the cylinders. But this is what poses a problem. I don't know enough about "heater valves" or thermostats to be able to just look up a part number and know that it will work.

    And so, the research journey begins...

  7. #7
    Piston? WHAT Piston?!? JGringo's Avatar
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    Also, my application involves the use of snowmobile style clutch system (primary and secondary clutches). Due to this, I cannot simply belt up a water pump either (because the jet ski engine clearly does not have any sort of water pump). So I have come up with a solution that may or may not work (depending on how the thermostat and "heater valve" end up) by using a CSR remote electric water pump (pushes 37 GPM). A link to it is here: www.csr-performance.com

    I am thinking this will resolve my water pump issues, but I still need to figure out the valving and routing issues...

  8. #8
    Click avatar for tech links/info, donation request K447's Avatar
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    Are you putting this engine into a snow mobile, or a land based machine?

    The sled guys have lots of info and experience mixing and matching watercraft and snowmobile engine parts. One big difference is that the watercraft have fully water jacketed exhaust systems which the snow and land machines don't need.

  9. #9
    Piston? WHAT Piston?!? JGringo's Avatar
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    It might be intended for a land based vehicle, a track machine sittin 2'' off the ground. But this is the last project on my mind. I have a conversion jet ski I need to finish which will run me at least 3 Grand before I can really "invest" into this idea... I just wanna have a super fast go-kart that I can make road-legal haha...

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