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  1. #51
    butterbean_29512's Avatar
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    Quote Originally Posted by Sea Dood View Post
    There is a major problem with this theory. The superchargers on these skis are centrifugal pumps, not positive displacement. The potential pressure rise is self-limiting, and once it reaches a certain point, the torque demand actually begins to drop instead of rise. It all depends on the design of the blades.

    You can see this in action by using a common leaf blower. If you put your hand over the end of the blower, the engine RPM actually rises.

    Depending on the pressure curve as I mentioned, the BOV could actually cause more stress since you are removing the back pressure rapidly.

    You would have to do some testing with sophisticated data logging to see what the actual pressure curves look like without BOV and with BOV, and do the same for the SC torque demand. Not easy.

    One thing is certain. Neither Yamaha nor Sea Doo put BOVs from the factory, and nobody in 15 years since these things came out has shown any conclusive data or proof that suggest BOVs are a good idea and actually have some benefit. But plenty of people have had problems with leaky BOVs, and they cost a lot of money. This suggests BOVs have no real benefit.
    A supercharger is not a pump. It is a centrifugal compressor. The reason the leaf blower RPM goes up when you cover the outlet is because you remove the load from the pump. A leaf blower is basically a pump. The same thing goes on with a centrifugal water pump also. But not a supercharger...it is a COMPRESSOR, not a pump.
    Centrifugal compressors actually have two parts...the impeller, or "wheel", and the diffuser. The diffuser is the round curved volute on the inside of the SC. The diffuser is what actually "compresses" the air before it leaves the SC outlet.
    When the throttle plate closes flow does not stop because the plate is closed, flow stops because the SC can no longer mechanically compress the air. The wheel is still trying to pack the diffuser full of air, but the air can no longer exit the diffuser. That air then suddenly "surges" back through the diffuser and impeller with a large "bang", into the air intake piping.
    Surge is defined as a reversal of air flow through the compressor. When it surges it thrusts the wheel and shaft in the opposite direction of normal thrust, and also unloads the clutch suddenly. That sudden unloading is what is so hard on the clutch.
    As the throttle plate closes vacuum increases in the intake. That vacuum is what activates the BOV to open. So....before the throttle plate opens the BOV should be open, preventing surge through the compressor. The larger the BOV the better. Remember...as long as flow in the CORRECT direction is maintained the SC and clutch remain loaded up...and wear on the clutch is minimized.
    Lastly...when the pump unhooks from the water the throttle plate does not close. That is what the rev limiter is for.
    This concludes today's lesson in centrifugal compressor operation!

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  3. #52
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by butterbean_29512 View Post
    ...
    Surge is defined as a reversal of air flow through the compressor. When it surges it thrusts the wheel and shaft in the opposite direction of normal thrust, and also unloads the clutch suddenly. That sudden unloading is what is so hard on the clutch. ...
    When the air is forced ‘backwards’ through the supercharger wheel, does that increase or decrease the torque on the already engaged clutch?

    Seems like (not speaking from certainty on this) the spike of reversed/stalled air flow would increase the torque load on the already engaged clutch, and the increased load could stress the clutch into ‘overloaded’ condition, hence slippage and wear on the sprag surfaces.

  4. #53
    MSX 150 guy lives on Mr. GP1800's Avatar
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    Quote Originally Posted by cjk6119 View Post
    That makes sense is that how we are maintaining set speed when using the cruise control???

    C J

    The ecu is moving the throttle plate constantly no matter what the throttle input method is. It may not move much but it’s still moving independent of your throttle input

  5. #54
    butterbean_29512's Avatar
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    You have a complete unloading of the impeller and clutch...it loses its "bite" on the airflow since it is going in the opposite direction. And on the throttle plate moving constantly....while riding in cruise mode what happens if you make sharp turns? Speed is greatly reduced due to the additional drag on the hull. Therefore...the throttle plate is locked in the cruise control position. So...while in cruise mode the throttle plate is locked in position and doesn't move. Not sure about rev limiter function though. That's a question for Dean or Jesus.

  6. #55
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    The super charger can not back spin... there is no compressor surge. If it does spin backwards the clutch has failed.

    the one way sprag/clutch spins in one direction.... and that's not in the backwards direction.. it allows the engine to Un- rev while free wheeing at a faster rate then the engine. It does not alow for it to spin the other way... other wise it would never make boost.

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  8. #56
    JFizzleJR's Avatar
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    I think it's important to point out here that there's a difference between back pressure and spinning backwards. It's like hitting the brakes as opposed to shifting into reverse.

    Quote Originally Posted by mittens View Post
    The super charger can not back spin... there is no compressor surge. If it does spin backwards the clutch has failed.

    the one way sprag/clutch spins in one direction.... and that's not in the backwards direction.. it allows the engine to Un- rev while free wheeing at a faster rate then the engine. It does not alow for it to spin the other way... other wise it would never make boost.

  9. #57
    mittens's Avatar
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    Right but the charger does not surge like a free wheeling turbo.

    the slip the cluch allows is in the directiong of still making boost. for example your running 8k, and chop throttle, the clutch allosws the charger to slip forward and slow down on its own speed, vs, directly related to RPM... the bluff off simple allows this free wheeling with less restriction.


    how it could possibly help clutch life is that you are running 8k, chop throttle/ come unhooked, and then back to 8k.

    if directly connected the blower has to slow, then rev back with the crank speed.

    If clutch set up and no Blow off, rpms dro, blower free wheels but has some "Braking" pressure from the pressure in charge pipes against a throttle plate. so say the charger slows down1/2 speed by them time your back on throttle... now the clutch is traying to catch/lock up on a bigger delta

    Or with a blow off, and clutch, RPM drop, blower free wheels as all the pressure can escape, and only slows the charger to 75% of the 8k speed, and thus making it some what less harsh on the clutch.


    I do not see how reving one on the trailer hurts, the clutch on a single rev the free wheeling action is just the sprag with no tension on its coasting.

  10. #58

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    Quote Originally Posted by Mr. GP1800 View Post
    The ecu is moving the throttle plate constantly no matter what the throttle input method is. It may not move much but it’s still moving independent of your throttle input
    Do you have absolute proof of this? Rev limit has never been controlled by the throttle plate on anything ever. I'm think you guys think the plate can move way faster than it is actually capable of.


  11. #59
    butterbean_29512's Avatar
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    Quote Originally Posted by mittens View Post
    The super charger can not back spin... there is no compressor surge. If it does spin backwards the clutch has failed.

    the one way sprag/clutch spins in one direction.... and that's not in the backwards direction.. it allows the engine to Un- rev while free wheeing at a faster rate then the engine. It does not alow for it to spin the other way... other wise it would never make boost.
    Google "centrifugal compressor surge" and read about it. It is a phenomenon unique to centrifugal compressors. The "surge" is a reversal of flow through the compressor, it is not a back spin of the impeller. All centrifugal compressors have a surge index, and ALL superchargers will surge if the throttle plate is snapped shut without a BOV installed.

  12. #60
    mittens's Avatar
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    Yes the air can stop. Don't dissagree my blown 427 has a blow off/bypass valve as well. But this threat was if one was needed for the clutch so saying the blow back will spin it backwards is what I was arguing.

    the air can't go forward and the valve helps Bleed it off.

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