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  1. #1
    Site Admin Green Hulk's Avatar
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    Elevation and how it affects engine performance and impeller pitch

    We see all the time how one forum members ski with the same mods may run better than another ski in another part of the country or world. A big factor that causes this is ambient temperature differences and most importantly ELEVATION!

    A big mistake I see many people make is they think they can install an impeller right out the box and expect perfect results. This is not always the case, people lose speed, because they lost RPM and they quickly blame the impeller for being "junk" or the pitch being wrong right out of the box.

    One thing we've learned over the years is no one impeller pitch is going to be just right for every ski in every geographical location. If you think you will install an impeller and RPM be perfect on your ski right out the box you are setting yourself up for disappointment.

    If you want the most out of your ski you will have to fine tune your impeller for optimum rpm which will provide for best results. Afterall, RPM = speed.

    Pitching your impeller is EXTREMELY easy to do following these instructions on the forum. Yes, these instructions were written based off of a Sea Doo impeller, but the same exact principal applies to Yamaha and Kawasaki impellers:
    http://www.greenhulk.net/forums/show...light=pitching

    Elevation affects engine HP in a big way and the more HP your engine makes the more it's affected as the elevation increases. That said, the higher the elevation in your riding area the less impeller pitch you will want to run in order to keep the RPM's up higher. RPM = Speed. Reduced RPM = less speed!

    Here's a forumula for calculating your HP loss at your elevation. You may be shocked at just how much power your ski loses at your riding elevation.
    HP Loss = (elevation x 0.03 x horsepower @ sea level)/1000

    So for example, a Sea Doo 300 HP ski at 1500' elevation:

    1500' elevation x .03 x 300hp divided by 1000 = 13.5 HP loss


    To overcome any HP loss due to high altitdude the simple fix is to depitch the trailing edge of your impeller slightly to get the RPM up and you will benefit both from higher speed and improve acceleration.

    Here is an online calculator that will easily calculate HP losses at altitude
    http://www.wallaceracing.com/braking-hp.php



    Hope this helps you guys!
    Jerry


  2. #2
    JFizzleJR's Avatar
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    I have noticed over the years that many folks experience approximately 100 RPM drop per 1000 ft elevation. (or 10 RPM per 100 feet, how ever you want to look at it)

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  4. #3
    Steve STSPERFORMANCE's Avatar
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    Hell factor in the whole good air/bad air effect on rpm and Hp gains and losses are even more. I am at 880 feet in Ohio and when I go to drags on Tavares I usually pick up almost or right at 1000 rpm and and undisclosed number of mph. Motec allows us to see these changes. What I do is tune everything for Florida elevations and conditions and see if we can hit the numbers through hull setup and pitches here keeping in mind the 1000 rpm we gain when we go

  5. #4
    Super Newb Captain Insaino's Avatar
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    I am currently going through this tweaking process. I had never pulled an impeller before and was dreading the whole process. I installed a new Skat and lost some mph, so its coming off this morning and getting shipped off to R&D for a retweak per Glen and Jerrys recommendation. My problem was as Jerry pointed out, my particular situation, altitude, temperature, etc. don't expect a one-size-fits-all. The thing is, once you have pulled an impeller off, you realize the whole process is not that bad. I am certain I could have this thing off within an hour easily now. Depending on where you send it, turnaround time can be, including shipping both ways, the same week @ under $100.00 if you don't want to do it yourself. I am a muscle car guy. I have pulled countless motors, transmissions rear ends, tested, tuned and tweaked for minimal gains happily. If you're trying to gain every last mile per hour out of your ski, just expect to do it more than once. You might score right out of the box, but I doubt it. The upside is, there are a lot of people on this forum that are willing to help. I got a taste at what this impeller is capable of. Now I have zero cavitation and the mid range is badass. I am now excited about the whole process. Get an accurate tach($50.00), note your peak RPMs and go from there. And if after getting retweaked, there is still some gains left on the table, I will do it again. I've made peace with it, and realized it is really not that big of a deal and is part of the fun.

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  7. #5
    VdeKock's Avatar
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    Can't say I gain any speed or rpms compared to you guy's...
    and I am below sea level

    Is there any zero point that you notice the difference. Like between 0/1000 and 1000/2000?

    or i must buy a new tach....

  8. #6
    bullett55's Avatar
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    And for those OCD folks that want to cover
    all the bases, this takes into acct temp, dew pt,
    and barometric pressure in addition to elevation.
    Really cool, especially if you can't do all your testing
    on the same day or at the same location.
    Posted I think by Gus or Jarrett some time back.

    http://wahiduddin.net/calc/calc_hp_dp.htm

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  10. #7
    bullett55's Avatar
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    I forgot...
    Here is the easy access link to that extra
    atmospheric data... any location, any day,
    any hour.

    https://www.wunderground.com/history/

    After submitting your location and date just
    scroll way down to “Hourly Weather History &
    Observations’ table and there ya go.

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