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  1. #1
    Click avatar for tech links/info, donation request K447's Avatar
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    Water pressures in Polaris Domestic 2-stroke cooling systems

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    Either I knew this and forgot or I never noticed before

    Pictured here are the springs from the pressure bypass valves inside the thermostat housings of two different Polaris engines.

    The longer spring is found in my 2004 MSX 140. The shorter spring is from a two cylinder 700 engine, which I think came from a 1997 Hurricane. Parts look up says the shorter spring is 7041310, used on both 2 and 3 cylinder engines across multiple years.

    I measured the spring forces with the spring compressed to the length it would have installed with the engine off and calculated the following;

    MSX 140 spring almost 3.0Kg or 6.61 pounds force against valve seat

    '1997' Hurricane twin spring 1.5 Kg or 3.31 pounds force against valve seat

    Bypass valve hole internal diameter is 0.6 inches. This is the hole in which the bypass plunger rests when it is seated.
    0.283 inches surface area of the opening

    If my numbers are correct it would require about 24 PSI in the MSX cooling system to begin opening the pressure bypass valve. In contrast the shorter spring in the 700 housing would only need 11 PSI to open.
    MSX 24 PSI
    Hurricane 11 PSI

    Plunger face outside diameter is 0.8 inches. This is the area upon which water pressure would be applied once the plunger is pushed away from sealing against the seat.
    The plunger face had 0.503 inches surface area

    Once water is flowing the entire face of the plunger is exposed to water pressure. Ignoring any back pressure on the reverse face of the plunger, it would require about 13 PSI to hold the valve open against spring pressure in the MSX 140 and about 7 PSI in the 700 motor.
    MSX 13 PSI
    Hurricane 6.7 PSI


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Here you can see that the housings are also different.

    If you look closely you will see that not only is the bypass spring chamber much larger on the MSX 140 thermostat housing, but that the spring has worn grooves deeply into the cast metal 'guides' of the housing. In effect there are now teeth worn into the casting, which I expect are not helping the spring move smoothly!

    In contrast with the thermostat housing from the MSX engine the gap between the thermostat support section and the water exit section is much narrower on the 700 housing (5131008, 5631098 as substitute). I suspect this allowed the original housing design to omit the separate metal support ring (7556174) for the thermostat itself (7051008 ) that can be seen in the MSX 140 housing 5631136 used on multiple (all 2-stroke?) engine versions from 2001-2004.

  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
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    I was surprised at the amount of wear the spring caused inside the MSX 140 thermostat housing. I now consider this part worn beyond repair and will replace it with the 700 housing.

    The 700 housing seems a more robust design with the spring restrained directly by the cylindrical housing rather than narrow cast rails.

    I have no worries about reduced water flow with this housing as Polaris took other steps to reduce water flow through the stock fuel injected 1200 engines. For example, there is a white plastic flow restrictor (5133977) located inside the end of the water cooling tube at the jet pump.

    On some Ficht engines there is also a white (plastic/ceramic?) spacer (for which I have never found a part number) installed below the bypass spring to increase spring tension and further boost the water pressure needed to get that bypass valve to open.

    Trivia point; On the MSX 140 engine the thermostat housing is located at the MAG end of the water bar!

    It does appear that water flow at low engine RPM can indeed be safely limited to very low flow rates. As engine RPM rises the water pressure coming fom the jet pump into the cooling system also rises and at some RPM the pressure is enough to open the bypass valve and increase flow.

    The thermostat does of course open at roughly the specified 143F temperature and allow some water flow to maintain the engine temperature.

    It would be interesting to measure the actual water pressures at various RPM and see when the bypass valve opens.

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  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    Did you notice that the darker bypass plunger is badly worn on one side?

    There is a hook shape formed near the tip where the plastic ridge has been rubbing against the upper housing. This is why Polaris recommends regular inspection and replacing these plastic parts when wear occurs.

  5. #5
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    Interesting comparison. I think your right on water pressure. Direct relationship with the 1200 vs 700 expected water pressure and speed.

    But also could be based on the prop installed at factory. The higher pitch would produce more force at same RPM.

    Of course my MSX doesnt have that problem. No thermostat. My hand tells me when its ready to let it rip.

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    That was one of my reasons for posting. So many guys think that these engines need huge water flows just to maintain proper engine cooling.

    The stock MSX engine is rated for roughly the same power output as the carburetor 1200 engines. That the MSX engine receives sufficient cooling despite the high bypass pressure, standard 143F thermostat, and even a flow restrictor in the cooling supply tube tells me that it is very easy to overdo the cooling when making modifications.

    If the highly restricted stock cooling system can keep the 1192cc MSX 140 engine at optimal temperature across a wide range of operating conditions, how much MORE cooling do the carburetor engines really need?

    One side effect of removing the thermostat or pressure bypass valve is that the engine will flow much more water during low to mid throttle operation. I suspect what happens is that every time the no-thermostat engine throttles down the engine temperature drops due to over cooling, then when throttle is reapplied the engine temperature goes back up. The engine case temperature is not steady but is cycling hot and cool, hot and cool.

    And the water pressure inside the engine cooling jacket would be much lower across the RPM range without the thermostat and pressure valve. I have no idea what difference constantly lower (than stock) water pressures would cause, if any.

    In contrast a carburetor engine with properly operating thermostat and bypass valve in place may be able to maintain a more stable temperature range despite varying throttle levels.

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