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  1. #1

    A theory on the Pop-off valve

    I might have a theory on the pop-off valve and why its needed. Has many of you already know I have been using a digital boost gauge with memory for the last month or so. This gauge uses a map sensor which is very accurate and very fast reacting. Normally when riding around I don't pay to much attention to it, and if I am making a speed run I'll just let the memory do it's job and check it after the run was to see what max boost I am getting. Well today I am hauling ass across the lake,not wide open,but real close,just a hair from WOT, like maybe 7/8 throttle and I happen to glance down at the boost gauge and its reading 12.8. WTF I'm thinking,were did all this come from. So I pin the throttle to see what she will do with all this new found boost and soon has I pin the throttle it goes down to 11.6-7. About were it usually shows when wide open. So then I back off a little and sure enough the boost goes back up to 12.7. After playing with this for a while I can get 12.9 -13.0 lbs at will just by manipulating the throttle while keeping the RPM's in the 7500-7600 range. My guess is that the supercharger is spinning enough rpm to push all it can,but with the throttle slightly closed it creates enough of a restriction to make more pressure. This could create a lean spike when the throttle is opened and possibly detonating momentarily. Over time this could be a problem damaging the pistons and rings lands and other undesirable effects of detonation. The Kawasaki engineers may have recognized this has a potential problem,especially over the long term and added the pop-off valve to prevent these spikes in pressure. Perhaps Skip, Rudy or any of you other guys using LM-1's and such could share some of your datalogging results and tell us if theres a lean spike in the A/F under these conditions.
    In my quest earlier for more boost I adjusted my valve a little at a time and checking the results each time, so now its cranked down has far has it will go. Tomorrow I will take it to the shop and put it on the wastegate machine and set it back to factory specs and sees whats happens next week.

  2. #2
    Silence! I Keel You! earthshaker's Avatar
    Join Date
    May 2007
    Northern CA Coast
    Right On!!!!

  3. #3
    Beagleman62's Avatar
    Join Date
    Mar 2007
    Richmond, Virginia
    You are the man......
    That is some great info you posted. Intake,exhaust and ride plate info is one thing but stuff like this is invaluable.

  4. #4
    Nice vary nice.

    Thats why in my entry I stated that the long term effects is unknown.

    I dont have enough testing under my belt to give a good debate. I do have several questions about this mod though.

    What is now showing oddly is the amount of gas I am now getting in the oil. Since I have put in this mod I have noticed a faster amount of oil level rising. This is gas getting into the oil making the reservoir levels higher.

    At first it wasn't a big deal but now I wonder if the back pressure in the throttle body verses lower RPM's in hard turns is forcing past the rings.

    Follow me here.

    I can hit a hook turn with full throttle and maintain the full throttle (65 to 67) with a whip effect around a buoy but the RPM's fall off due to engine load even when the throttle is wide open and full boost is being forced into the cylinder. I am wondering if this excess fill is blowing into the crank case carrying the gas with it.

    If the BOV was in place and working properly would it relieve the pressure during these conditions which could lower the amount of blow by?

    Is this BOV better describe as a waste gate. Only opening to waste off excess build up during heavy loads and excess stacking ac cures???????? Maybe we dont see its value during rec riding and I'm missing it during race conditions. It would make sense to me as before the block off my turns had a week feel to them. As if I lost a bit of power now with the block off it has a strong response and full power feel. Maybe the excess boost is blowing into my crank case instead of the BOV??????

    My Datalogger shows no lean spots. It will show spikes in the Boost pressure but it seems to have enough fuel to cover the extra. Remember that at idle is running around 14.5 AFR with WOT being close to 11.5. Surly in the safe zone at WOT. This 11.5 is what is killing our gas mileage but saving the motor.

    I'm still learning about this one.

  5. #5

    Join Date
    Jun 2005
    rome ga
    The bov also helps keep the engine from running too high of rpms when coming unhooked and starting to free rev.

  6. #6
    Quote Originally Posted by rxpstan View Post
    The bov also helps keep the engine from running too high of rpms when coming unhooked and starting to free rev.
    Centrifugal verses roots, different world.

    More Engine RPM's dont give us more boost. Unlike the SD when coming unhooked our BOV wont open because the boost matches the the engine consumption. SD when coming unhooked will spool the SC higher making more boost then the engine can safely consume.

    I could be wrong on this though.....

  7. #7

    Join Date
    Apr 2007
    Melbourne, FL
    Quote Originally Posted by rxpstan View Post
    The bov also helps keep the engine from running too high of rpms when coming unhooked and starting to free rev.
    I would agree. More RPM gives more volume of air. It is a pump afterall. If there is no place for the higher volume to go it will spike the pressure. That is what the BOV is for. Any absolute pressure above the set limit. The wastegate functions whether or not the absolute limit is reached by vacuum pressure in the manifold. So, if the manifold vacuum is positive and the wastegate is closed then the only safety valve is the BOV. If the engine free revs then bogs the BOV is there to prevent a boost spike since the wastegate stays closed.

  8. #8

    Join Date
    Jan 2007
    Taken right from Kawasaki's web site on the Ultra 250X

    To prevent over boosting at low and medium rpm, two different air bypass valves are used

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