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Thread: BOV Musings

  1. #1

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    BOV Musings

    I know - dead horse. But, I just wanted to understand how the whole SC and clutch thing works both with and without the BOV, so I went to a supercharger / turbocharger engineer to get his thoughts. He instantly said "Use a BOV!", but after I explained the system (which he had never seen before), he brought out the whiteboard and started to draw things up.

    First point: The clutch takes up the 'slack' between engine RPM and SC RPM and tries to maintain a match. (ratio is 5:2, but it's easier to think of them as equal)

    Loaded Acceleration with and without BOV (ski stays hooked in the water) - clutches slip upon acceleration as the SC will 'lag' behind engine RPM until SC catches up.
    NO DIFFERENCE WITH OR WITHOUT BOV

    Loaded deceleration (ski stays hooked in the water) - BOV will open, SC RPM will stay above engine RPM as it freewheels and clutches will work harder to match RPM. No BOV, pressure in system will help slow SC wheel and clutches will work less hard. Not chopping the throttle would help in both situations, but...
    ADVANTAGE NO BOV

    Uneven acceleration or static RPM (variable loading and unloading) - This is racing in choppy water. Ski comes unhooked and the rev limiter kicks in. BOV will open, which is good because the engine does not need all that boost. SC freewheels, and the engine stays at peak RPM. Clutch works to match RPM even as SC freewheels. Once reloaded, BOV closes and boost is re-aquired. Clutch works again when boost kicks back in as the SC RPM will drop momentarily as it picks up boost again. With no BOV, engine will stay at peak RPM but pressure in system will slow SC RPM (engine wil not be using the boost, so it will 'stack up'). Clutch works against slowing SC wheel to try to maintain RPM match. When reloaded, clutch will work harder as SC RPM is still below engine RPM. Even when not hittting the limiter, the engine will naturally rev up and the clutch will work to maintain the RPM match (but how often do you come unhooked and not hit the limiter?). Only difference - BOV makes clutch work when reloaded as SC has to regain boost. No BOV - clutch should maintain RPM match throughout.
    ADVANTAGE BOV

    Unloaded deceleration - You just launched off a wave and you chop the throttle. With BOV, SC freewheels and engine RPMs drop. This works the clutch extra hard to try to match RPMs as the 2 systems are going in opposite directions. The clutch will work to drag down the SC RPMs. Once you hit the water and grab the throttle again, the SC will produce more boost initially until it drops to match the engine RPM. No BOV, the pressure in the system will work to slow the SC, helping the clutch to match RPMs. Once you hit the water, you will have boost already in the system to aid in acceleration.
    ADVANTAGE NO BOV

    So what did I learn? That both sides of the argument are right. If you run flat water, it's better not to have a BOV as loaded acceleration and deceleration is easier on the clutch without a BOV. If you run in choppy conditions, a BOV will help keep RPMs matched and will make the clutch happy (but you will be slower as you will lose all accumulated boost when out of the water). If you jump waves, you probably don't want to use a BOV if you are going to chop the throttle in the air (which you will do unless you want to ride the rev limiter all the way down).

    My experience - I ride mostly ocean, and I do a lot of wave jumping. In flat or choppy water, the ski just felt smoother with a BOV. Having the backpressure work to slow the SC feels and sounds very rough, although it is helping the clutch in certain situations. Running hard and coming unloaded is smoother with the BOV, but you definitely lose power when you get reloaded as all of the boost is gone. Jumping a wave and chopping the throttle is very smooth with a BOV, even though the clutch is working overtime.

    Eric


  2. #2
    Boost addict hotbird's Avatar
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    From what I understand most people that run a BOV (in a sea doo) eventually find it leaking boost and ditch it.

  3. #3
    DallasGV's Avatar
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    good post but shouldn't the synopsis under the "Uneven acceleration or static RPM" paragraph be Advantage NO BOV?

  4. #4

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    I was thinking in terms of the BOV helping the clutch longevity when I was assessing advantage or disadvantage. For speed in chop, no BOV has the advantage.

    Eric

  5. #5
    SplishSplash's Avatar
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    Do we need a BOV, no, that been proven, and Sea Doo does not offer one, I'm sure they tested if it was needed.

    Having said that, the BOV got a bad wrap because they came out at exactly the same time that SC clutches started exploding. So naturally, lots of people blamed the BOV. Sadly, Sea doo just made a crap clutch that exploded, in no way due to the BOV. It was just the scape goat.
    But Clutches were exploding on skis without BOV's.
    I have never had a BOV leak, or fail, but I don't run one anymore, cause it jus doesn't net any performance or provide any benefit on a SC ski

    So is a BOV necessary on a SC ski, no, a turbo, Yes, probably a good idea.

  6. #6
    A centrifugal blower eats up the most torque from the driving shaft when it is moving air. A BOV allows air to exit the system between the blower and the throttle. So blowing off air would slow the blower faster than allowing the blower to spin in non moving air. It is this relative "non moving air" that leads to air moving backwards thru the fins and breaking turbine blades on turbos.

    On a turbo install, if you want to keep a turbine spinning fast during shifts, kill the air flow thru it. The fear is the potential air reversion and resulting damage is not worth the gain so people install a BOV on cars.

    Now the question is does this reversion and resulting damage ever happen on a ski? Is the allowing the air to flow help the RPM of the blower better keep up with the change in RPM of the crankshaft? I personally have no idea how you would document the loads on the clutches.

    PS: all of the turbo installs I am involve with have a BOV.

    Andy

  7. #7
    R88ory RXP's Avatar
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    To the OP great post, thanks for sharing, and the info is very good.

    I run in what seems to be similar conditions to you, I used to run a BOV, turbosmart unit, not a crap one. I agree that in chop it did feel smoother, IMO tho this was because the power did not come in as hard, I have tested GPS speed in the chop with and without and no BOV was without question, faster. I do have to question what the engine is doing with the boost when you hit the limiter, OK no it is not using it for power but its still spinning very fast and moving the same amount of air, think of the engine as an air pump, most cases the prop speed is only a couple hundred RPM away from the limiter, that equates to maybe 1000 SC rpm, which is not alot of change in the big picture for the SC, I dont feel the clutch does alot as the engine goes into the limiter, the clutch will have to work as the boat re-hooks and the rpm dips for a split second, again tho by bleeding the boost the wheel will just be free wheeling rather then hitting boost...

    I saw no difference between BOV or NO BOV as far as clutch wear was concerned, stock X SCer @ 8150, but i did see a performance loss with the BOV fitted, so it came out.

    IMO you cant really compare a turbo to our setup, the RPMs are much higher, so is the boost in most cases and there is nothing to slow the wheel so the dynamics of what is going on are very different. I would most likely run a BOV with a turbo on any setup...

    I would love to see a wheel speed sensor added to a SC and then log the SC wheel speed against engine RPM to see when the clutch is really working, ie differences between to two, outside of the gear ratio.

    R88

  8. #8
    shrabber's Avatar
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    On turbo in the Water Mandatory Run a Bov or you will be blowing off hoses and eventually buying a turbo sooner than later

  9. #9

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    My synopsis only applied to our clutched SC system. A BOV for a turbo is necessary, I don't think anybody is disputing that. A BOV on a roots type SC is not necessary (but an overpressure valve is a good idea), and a BOV on a centrifugal belt-driven SC helps with the belt life.

    The important thing to remember is the supercharger and engine are linked, so the engine will always try to drive the SC (through the clutch) whether there is boost in the pipe or not. When you close the throttle the airflow essentially stops and the engine will drag down the SC as RPMs fall at the same time the stagnant air will push back against the SC wheel. It's like a paddle boat - throttle open, you are paddling with the flow, and it's easy to move. Throttle closed, the flow stops and you are paddling against more resistance. It may be that the SC even slows faster than the engine and helps the engine slow down through the clutch linkage.

    R88 is correct in that we can only be sure of what is going on if we measure engine RPM versus SC RPM both with and without a BOV. All of this is just an educated guess based on experience (the tech, not mine).

    Eric

  10. #10
    If you happen to own one of these 2 stroke lawn blowers, try a little experiment. Pull it to full rpm and then block the outlet. Listen to the RPM. Now block the air inlet with something like a piece of cardboard. Note what the RPM does when the air flow for any reason gets decreased.

    At one point in my past-life I raced a turbocharged sucker car and had to rethink everything I had read over the years.

    http://www.ncs-stl.com/racecar/SuckerCar.jpg

    Andy

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