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  1. #1
    coldspace's Avatar
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    RPM spikes thread

    Hey Pete,

    There is some discussion about seeing over revs in our data files.
    My limiter is set to 8400 rpm.
    I usually do some fast riding out in the chop and always see rpm spikes in the Channel report section sometimes around 8900 ish.

    Are these spikes somethng too worry about or is the engine going to be OK as its not sparking above 8400. If you get what I mean.

    Some guys are reporting the same"spikes' well above their rev limiters well into the mid 9000's.

    Can you explain whats happening here mate and anyone else care to join in with their data/experiences on this topic.

    Should we set the limiters back more for surf riding mate or all is OK.?

    Cheers guys.


  2. #2
    MoTeC Pete's Avatar
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    HI
    This is actually a big subject and I have done quite a bit of research into it as I noticed the same thing happening some time ago.
    I will try to explain how it all works, hang on to your hats


    Example is FZR that runs 8300 rpm in clean calm water.
    RPM limit set to 8600

    Rider is flat out at 8300 when he encounters a wave that completely unloads the jet unit, the engine then immeadiatly accelerates due to no load on the crank.
    We have measured this acceleration rate and on average it is between 30 to 40 thousand RPM per second. yes true
    Now the ECU measures rpm in a very accurate way.
    To do this it times between teeth on the crank but it cant do it between 2 neighbouring teeth as the engine speeds up and slows down as each piston fires.
    So the only way to get super acurate rpm is to measure at the same time in the cycle for each cylinder.
    So that means that it updates its RPM calibration twice per revolution.
    Sounds fast enough but at 30 thousand RPM per second acceleration rate the rpm is usually well on its way over the rpm limit before the ECU measures that its time to turn the coils off.
    Ok, so lets assume that the ECU has now detected that the rpm is over 8600, it now has to shut off the coils. trouble with this is that if a coil has started to dwell (charge up ready to fire) the ECU has to let it charge completely before switching it off, the reason for this is that if it turns off the charging coil early it will fire its spark, the spark would then occur early and likely cause detonation (knock)
    So the result of all that is that you will almost always get 1 firing coil after the engine has exceeded the limit.
    The jet unit by this time is empty of water and you get 1 firing at close to full throttle.
    Thats at least a 70 HP bang that will whip the crank around a few more revolutions.

    Thats the simple version believe it or not and hopefully will allow you to understand why it happens.

    Now some ECU's will start cutting power before the RPM limit to prevent that happening but thats certainly no good if racing is your game.
    You can lower your RPM limit and that will help but the closer it is to your clean water rpm the worse your recovery time will be when landing in the water again.
    RPM limit tuning is very important for Jet Skis and can make a big difference to speed in the chop.

    You should have your RPM limit set to # 5 ign only and have your power recovery rate set to zero.

    Hope thats been some help

    Cheers


  3. #3
    SHOBiz's Avatar
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    Quote Originally Posted by MoTeC Pete View Post
    You should have your RPM limit set to # 5 ign only and have your power recovery rate set to zero.

    Makes perfect sense to me.

    My RPM Limit type was set to 2. That could cause a lean situation right?
    My recovery rate was set to 0

  4. #4
    It's not the destination, it's the journey!! Rip'nTear SHO's Avatar
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    Thx a ton Pete....very helpful info....salute!!

    With ur permission Pete i would like to quote ur post on my build thread...as this is where the conversation was happening. It would shed alot of light on once happening to the high-hp skis that r hammering down in the ruff stuff.

    Thanx again Coldspace for bringing the topic up in this section............

  5. #5
    MoTeC Pete's Avatar
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    HI Shobiz
    #2 is Ign Cut with Fuel cut 100 rpm above hard cut.
    This wont cause lean and is designed to stop exhaust systems loading up with fuel when there is a lot of limiting active (bang)
    That is no problem on a ski as the exhaust is too cold.
    The reason we dont want any fuel cut is to do with recovery time when landing back in the water. I will explain.

    In my previous post I explained what happens when the engine leaves the water, this time I will explain what happens when it lands in the water.

    The ski is flying through the air and the rpm has peaked at 8950, the ski then lands back in the water, the rpm falls at a huge rate of close to 70,000 rpm/second. yep
    So this time you have the opposite problem.
    The ECU is now looking for when the rpm goes below our limit setting of 8600, its calcultating in real terms at a very fast speed but the rpm is falling lightning fast.
    The reality is that the rpm is below 8600 before the ECU starts to turn every thing back on.
    At that point the ECU needs to wait for the correct time to start dwelling the coil, at that point it turns the coil on to charge it up and then once charging is complete, (approx 2 milliseconds later) the coil fires and off you go. The idea of having no fuel cut is that when the first coil fires we want pleanty of fuel there so that there is no further delay.
    Sounds like the ECU is a bit slow, well I can tell you we have one of the fastest processors in the industry, its just the the acell and decell rpm rates in the jetski world are amazingly fast.

    How can you make it better if you are at the WF and want to win.

    1. If legal, fit a CDI to the engine, CDI's pre charge their capacitor so there is virtualy no delay from when the ECU wants to fire.
    2. Lift your RPM limit further away from your clean water rpm, if you never hit the rpm limit then the coils never get turned off, when the ski lands back in the water there is no delay, this is obviously mechanically dangerous for the engine and in surf conditions where wave frequency is much slower you could get big RPM numbers, if you have good internals fitted to your engine then no problem.

    Again thats the simple version, I will happily explain the whole thing in person to anyone who wants to keep listeneing.
    Will be in Havasu next week so if your in the area track me down and say Hi

    Cheers


  6. #6
    MoTeC Pete's Avatar
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    Thx a ton Pete....very helpful info....salute!!

    With ur permission Pete i would like to quote ur post on my build thread...as this is where the conversation was happening. It would shed alot of light on once happening to the high-hp skis that r hammering down in the ruff stuff.

    Thanx again Coldspace for bringing the topic up in this section............

    HI
    No probs
    I actually find all that stuff quite interesting ( sad really) and think everyone should know why things happen the way they do.
    Of cource you can copy it over, once something is on the net, its anything goes

    Cheers


  7. #7
    SHOBiz's Avatar
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    So, how is the Stock ECU handling this? Is it not an issue due to the Stock ECU using a boost cut as well? Or, is this simply exacerbated by higher rpm's allowed by aftermarket ECU's?

    Any information on aftermarket coils in use and theiur setup?

    David

  8. #8
    MoTeC Pete's Avatar
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    Quote Originally Posted by SHOBiz View Post
    So, how is the Stock ECU handling this? Is it not an issue due to the Stock ECU using a boost cut as well? Or, is this simply exacerbated by higher rpm's allowed by aftermarket ECU's?

    Any information on aftermarket coils in use and theiur setup?

    David
    Hi David
    Cant say what happens on stock set up
    Couple of things to consider on this thou, most stock ECU ski,s are very close to the limiter, what that means is the engine doesnt get time to accelerate BEFORE hitting the limiter, this would make a big difference.
    So if you tootk all the mods off your ski except MoTeC and put the RPM limit back to the same as stock then bladed the jet unit to sit just under the rpm limit, you wouldnt see the big over revs.

    In fact what does the stock ECU over rev by ?????? ahhhh thats right, no logging


    I have never changed coils, no need unless you are going to a CDI, then you need to change to either our dedictaed CDI coils and run ign leads or find a 2 wire coil to mount in the same place as the factory.

    Cheers


  9. #9
    fx160's Avatar
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    great info Pete
    that sure explains alot
    not sure if you can answer this , but would the ecu speed be the reason some of the vipec over revs are in the 9,000 rpm range

    kev

  10. #10
    MoTeC Pete's Avatar
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    Quote Originally Posted by fx160 View Post
    great info Pete
    that sure explains alot
    not sure if you can answer this , but would the ecu speed be the reason some of the vipec over revs are in the 9,000 rpm range

    kev
    Hi Kev
    I cant really comment on other products but even if another ECU had an even faster processor its what you do with it that matters.

    Cheers


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