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  1. #1
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    World Finals Weather

    The weather was pleasant, in the low 90's, But what was the altitude that
    particular day?

    I mention this because Racers will sometimes have engine failures at the
    World finals that never occurred where they ride, or they believed their
    previous problems were behind them.

    These failures are often blamed on the same old demons, like running lean,
    fuel pumps, timing, blown head gaskets and sometimes the race fuel used, etc.
    All the normal suspects for detonation failures.


    While I was at Havasu, I asked three World Finals racers what the altitude of Lake
    Havasu (LHC) was, and I received three different answers that ranged from 300' to 800'.
    Google lists the altitude at LHC as 735'.

    But the actual altitude was ~3500' feet that day! That is the density altitude
    that is controlled by the weather conditions. The barometric was much higher than
    I expected @ ~29.0" hg. Saturday's weather was pleasant temps with rain that
    evening. Not a normal weather pattern, I expected altitudes of 4000' to 4500'.

    When a TV weather-man reports high pressure over the 4 corners, that is where
    the states of Arizona, New Mexico, Colorado, and Utah intersect, it creates low
    pressure at ground level due to the venturi effect. I have personally recorded low
    barometric pressure in California @ 27.35 inches of hg, and I am certain LHC would
    be even lower. That day, the density altitude was ~5500 feet. 5500 feet is common
    for hot days in California and Arizona with blue skies, with high clouds or thunderheads.

    Whenever I go to the races, I bring along an Altronics inc. hand held weather
    station
    .

    I use the PerormAire PA2, that is commonly used by Drag Racers to estimate
    their ET times due to lost HP. With the knowledge gained by using a PerormAire,
    a racer can compensate for lost compression due to air density (I use an approximation,
    but the actual loss can be easily calculated by using the geometric compression
    ratio and the Gas Laws) by changing heads or domes. Fuel ratio is also richer with
    elevation increases, gear ratio need to be increased for altitude (for watercraft that
    means impeller pitch needs to be reduced), and horsepower is lost as altitude increases.

    My point is your setup for the World Finals should be closer to a ~4500' mountain lake
    in order to be competitive and reliable. This is a reasonable precaution because you
    can compensate for your compression loss, you can adapt your jetting for altitude
    changes, but you will still loose HP, and since your impeller size is dependent on your
    peak HP, you may also need to reduce your impeller size, or decrease the load on the
    pump by changing exit nozzle diameter, etc. in order for your engine to remain at a
    reliable rpm.

    Engine reliability decreases with HP losses, increased engine loads, and decreased peak
    rpm. The goal is always an engine that easily accelerates to peak HP rpm, and slightly
    exceeds that rpm by ~ 200 to 400rpm.



    Bill M.

    IME, the trapped compression ratio is not necessary for calculating cranking compression.
    The geometric (4-stroke) compression ratio x absolute atmospheric pressure converted from
    inches of hg. to psi (1 inch of Mercury = 0.491154 psi).

    For example @ Sea level: 14.7 psi x 12:1 CR = 176.4psi. Absolute atmospheric pressure (in psi)
    multiplied times volume is more accurate than the majority of compression testers available on
    the market. BM
    Last edited by wmazz; 10-08-2018 at 02:57 PM.
    Horsepower == Speed, RPM != Speed



  2. #2
    PWCToday.com Is My Home Away From Home SxiPro's Avatar
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    Re: World Finals Weather on Saturday Oct.6, 2018

    Bill Nye has nothing on you, brutha!

  3. #3
    PWCToday.com Is My Home Away From Home Grumpy Old Guy's Avatar
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    Re: World Finals Weather

    As usual....way over my head but I did gather some good info !!

  4. #4
    PWCToday Guru kotarbi's Avatar
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    Re: World Finals Weather

    Very interesting
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  5. #5
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    Re: World Finals Weather

    hello Bill,
    do you have a rule based on, for example, an increase of 25 degrees of temperature or a decrease (25 ° F around 1 size jet as can be seen on the famous "mikuni pocket tuner")?
    how do you combine a variation of pressure and temperature at the same time with a basic sea level setting? i looked at the description of your weather station, actually it looks good, but my English is ..... basic!

  6. #6
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: World Finals Weather

    Oops, this is why I tried to practice in a different section.
    Last edited by wmazz; 11-07-2018 at 03:54 AM.
    Horsepower == Speed, RPM != Speed



  7. #7
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: World Finals Weather

    #!$
    Last edited by wmazz; 11-07-2018 at 04:12 AM.

  8. #8
    PWCToday.com Is My Home Away From Home Waxhead's Avatar
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    Re: World Finals Weather

    My engine ran awesome at the worlds. Not so the rider
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  9. #9
    PWCToday Guru mrwhipper's Avatar
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    Re: World Finals Weather

    Great info! I am a pilot and understand air density and density altitude. I do have a question though. This is not to challenge but to get clarity related to PWCs vs aircraft. When we are at altitude in an aircraft, we have to lean the engine out because there is less air. If not they will run too rich. We use Exhaust Temp to manage (ETG). I noticed that you said with PWCs you have to run richer with density altitude.

    Thank you for any time you can spend educating me!!!!

  10. #10
    Top Dog orangefinger's Avatar
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    Re: World Finals Weather

    Havasu World finals watching tuning..... one day so...mmh... other day so mmh. same or worse unsure ? ... next day maybe ...??
    next day back to so so .....mmmh , following day im sure it was better when we arrived ? ....next day panic ...lets change everything up....
    ....and so on and so forth....I feel sorry for all these guys....
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