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  1. #11
    Top Dog Benflynn's Avatar
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    Re: HP loss at altitude

    Temp, humidity, and pressure. Pretty sure that's all u need, humidity being the cowboy hat wearing wild card that just cut ur brakes...(obscure cultural reference anyone?)

  2. #12
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    Re: HP loss at altitude

    Yes very important to tuners and long range shooters. Some of this power loss can be mitigated with higher compression domes.

  3. #13
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: HP loss at altitude




    I am still trying to get the table code correct. I keep having unexpected results like the table will display
    correct, but the position of the table is wonky



    Altitude Remaining % of HP Jetting Changes Pump Modifications Comments
    Sea Level 100% Test and record peak
    rpm
    Use 50 to 100yrd sprint
    Not 1/4 mile WOT or power
    Tune rpm.
    1000 ft 97%
    2000 ft 94%
    3000 ft 91%
    3500 ft 89.5% -1 main jet or 1/4
    turn in HS screw.
    Adjust low circuit.
    Test peak rpm Cool World Finals
    4000 ft 88% 1) Test peak rpm
    2) possible exit nozzle
    Common World Finals
    4500 ft 86.5% 1) Test peak rpm
    2) poss reduced pitch
    impeller
    Hot World Finals
    5000 ft 85%
    5500 ft 83.5% A hot day +100' in
    CA. and AZ.
    6000 ft 82%
    6500 ft 80.5%
    7000 ft 79% -2 main jets or 1/2
    turn in HS screw
    Adjust low circuit.
    1) Test Peak rpm
    2) reduce impeller size
    3) Possible nozzle size
    7500 ft 77.5% +115' day @ lake Havasu
    8000 ft 76%
    Last edited by wmazz; 10-28-2018 at 07:04 PM.
    Horsepower == Speed, RPM != Speed



  4. #14
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: HP loss at altitude

    Quote Originally Posted by JC-SuperJet View Post
    On your Altitude Table. what are the Jet increments: 2.5, 5 or 10?
    Since we use "reverse jets," Mikuni jets sizes are in 2.5 increments. Kehin jets sizes are in 2.0
    and 3.0 increments depending on where the jet size is, in the 10 digit scale. Less than x5, the
    size increases by 2. Greater than x5 the size increases by 3.

    For example: a low jet between 70 & 75 is a 72. A low jet between 75 & 80 is a 78.

    All the jet sizes are in mm's. So a 100 main jet is 1.0mm. The exception seems to be
    Mikuni low jets, which seem to be 97% of the size on the jet.

    I encourage drilling old jets, so long as you keep good notes and storage habits.

    Quote Originally Posted by JC-SuperJet View Post
    Nice of you to create this Reference Tool. And if you get inspired, add Barometric Pressure, Temperature and Humidity to this
    Reference Chart,; that would be the ultimate Racing Day tuning chart.
    There are too many possibilities to cover in a chart.

    For an inexpensive Density Altitude calculator I recommend a Bosch BME280 combined
    with whatever micro-controller you prefer, and a display. There are several easy solutions
    for housing the uC and sensor, and we can talk more about that later.

    A Bosch BMP280 is about $5 less expensive (about $3 compared to $8) but the BMP280
    does not measure humidity.

    Quote Originally Posted by JC-SuperJet View Post
    Friendly question, why do you leave out Torque in your Speed Equations?
    I use Blair's tuned length formulae to calculate the recommended peak rpm. I use an estimated
    peak HP rpm because of the impeller choices available. Impellers can not be treated like a
    sprocket. The wrong impeller will decrease peak HP, and increase heat (load), and compromise
    an engine reliability. Carburetor size is also an important choice if the wrong impeller is used.

    Peak Torque is important because the reliability of the engine is somewhat related to it.
    I found out a long time ago that if engine rpm is less than peak torque rpm, the reliability
    of the engine will be compromised. With ported engines the critical rpm is somewhere in-
    between peak torque and peak HP rpm.

    Quote Originally Posted by JC-SuperJet View Post
    Is that because you prefer Impeller choice for high rpm over Stroker Crankshafts for increased Torque?
    I asked Ed Miller which was better to create the best top speed, rpm or hp.
    He said horsepower. Either way, speed is a combination of available HP at a given rpm,
    and the difference between land vs water is the pump limits peak rpm.

    One of my 23cc Gopeds with my pipe on it will make peak HP @12,500 and over-rev to 19,500
    with me riding it. It will over-rev further for a kid. But on a watercraft the pump controls the
    peak rpm, and depending on the pump set-up there may be no over rev at all.



    Bill M.

    The best advice I can give a racer is to develop a reliable platform with over-rev (over rev accelerates
    significantly slower than power before peak hp), then re-evaluate the pump setup and try to increase
    start line speed, acceleration, and or top speed within the limits of your estimated peak hp rpm.
    Last edited by wmazz; 10-28-2018 at 09:04 PM.
    Horsepower == Speed, RPM != Speed



  5. #15
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: HP loss at altitude

    A correction to my last statement.

    Quote Originally Posted by wmazz View Post
    The best advice I can give a racer is to develop a reliable platform with over-rev (over rev acceleration
    can be
    significantly slower than power before peak hp), then re-evaluate the pump setup and try to increase
    start line speed, acceleration, and or top speed within the limits of your estimated peak hp rpm.
    A few factors that affect how fast over-rev acceleration decreases (when thee HP curve falls off quickly)
    depends on how much water enters the pipe, and the pipe design and exhaust manifold.

    Keep in mind that drying the pipe may increase over-rev, but it can have unintended side effects too. It can
    slow down re-acceleration and increase the engine compartments air temp. BM
    Last edited by wmazz; 10-28-2018 at 09:22 PM.
    Horsepower == Speed, RPM != Speed



  6. #16
    PWCToday.com Is My Home Away From Home
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    Re: HP loss at altitude

    Mikuni already made a chart for this.

    6$ shipped on amazon:
    triceps are the biceps of the back of your arm.

  7. #17
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: HP loss at altitude

    I found a better source for the HP lost. It is now titled "Relative HP" which is the
    inverse of the SAE dyno correction factor. I used the Engine Tuner's Calculator
    from this source.


    Other source data came from the National Weather Service and Air Density Online
    Air Density Online is an excellent source for weather conditions at nearby race tracks.

    Altitude Relative HP% Jetting Changes Pump Modifications Comments
    Sea Level 100% Modified 44, 46, 48, and 50mm
    carbs are more sensitive to
    density altitude changes.
    Test and record peak
    rpm
    Use 50 to 100yrd sprint
    Not 1/4 mile WOT or power
    tune rpm.
    2000 ft 98.9%
    3000 ft 96.4%
    3500 ft 94.6% -1 main jet or 1/4
    turn in HS screw.
    Adjust low speed circuit.
    Test peak rpm Cool World Finals
    4000 ft 93% 1) Test peak rpm
    2) possible exit nozzle
    Common World Finals
    4500 ft 92.1% 1) Test peak rpm
    2) poss reduced pitch
    impeller
    Hot World Finals
    5000 ft 90.4%
    5500 ft 88.9% A hot day +100' in
    CA. and AZ.
    6000 ft 88.2%
    6500 ft 87%
    7000 ft 85.9% -2 main jets or 1/2
    turn in HS screw
    Adjust low speed circuit.
    1) Test Peak rpm
    2) reduce impeller size
    3) Possible nozzle size
    7500 ft 82.9% +115' day @ lake Havasu
    8000 ft 81.5%
    Last edited by wmazz; 11-01-2018 at 03:41 AM.
    Horsepower == Speed, RPM != Speed



  8. #18
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: HP loss at altitude

    The post is about the World Finals held in October and the (possibly) surprising differences in altitude compared to
    where the watercraft was originally tuned for. But the chart is a decent reference for relative HP anywhere, and for
    any motorized competition. For example, it is Nov. 1st, and there is still really low barometric pressure ~27.1 @
    El Mirage dry lake bed where they hold speed runs (time trials).



    Altitude Relative HP% Jetting Changes Pump Modifications Comments
    Sea Level 100% Modified 44, 46, 48, and 50mm
    carbs are more sensitive to
    density altitude changes.
    Test and record peak
    rpm
    Use 50 to 100yrd sprint
    Not 1/4 mile WOT or power
    tune rpm.
    2000 ft 98.9%
    3000 ft 96.4%
    3500 ft 94.6% -1 main jet or 1/4
    turn in HS screw.
    Adjust low speed circuit.
    Test peak rpm Cool World Finals
    .
    1st week in October
    4000 ft 93% 1) Test peak rpm
    2) possible exit nozzle
    Common World Finals
    .
    1st week in October
    4500 ft 92.1% 1) Test peak rpm
    2) poss reduced pitch
    impeller
    Hot World Finals
    .
    1st week in October
    5000 ft 90.4%
    5500 ft 88.9% A hot day +100' in
    CA. and AZ.
    6000 ft 88.2%
    6500 ft 87%
    7000 ft 85.9% -2 main jets or 1/2
    turn in HS screw
    Adjust low speed circuit.
    1) Test Peak rpm
    2) reduce impeller size
    3) Possible nozzle size
    7500 ft 82.9% +115' day @ lake Havasu
    8000 ft 81.5%
    Last edited by wmazz; 11-01-2018 at 03:29 PM.
    Horsepower == Speed, RPM != Speed



  9. #19
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: HP loss at altitude


    Altitude Relative HP% Jetting Changes Pump Modifications Comments
    Sea Level 100% Modified 44, 46, 48, and 50mm
    carbs are more sensitive to
    density altitude changes, and
    may require to be readjusted
    as the temps change at a race.
    Test and record peak
    rpm
    Use 50 to 100yrd sprint to test peak rpm.
    Not 1/4 mile WOT or power-tune on trailer rpm.
    2000 ft 98.9%
    3000 ft 96.4%
    3500 ft 94.6% -1 main jet or 1/4
    turn in HS screw.
    Adjust low speed circuit.
    Test peak rpm Cool World Finals
    .
    1st week in October
    4000 ft 93% 1) Test peak rpm
    2) possible exit nozzle
    Common World Finals
    .
    1st week in October
    4500 ft 92.1% 1) Test peak rpm
    2) poss reduced pitch
    impeller
    Hot World Finals
    .
    1st week in October
    5000 ft 90.4%
    5500 ft 88.9% A hot day +100' in
    CA. and AZ.
    6000 ft 88.2%
    6500 ft 87%
    7000 ft 85.9% -2 main jets or 1/2
    turn in HS screw
    Adjust low speed circuit.
    1) Test Peak rpm
    2) reduce impeller size
    3) Possible nozzle size
    7500 ft 82.9% +115' day @ lake Havasu
    8000 ft 81.5%
    Last edited by wmazz; 11-07-2018 at 03:57 AM.
    Horsepower == Speed, RPM != Speed



  10. #20
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: HP loss at altitude

    It works here

    Altitude Relative HP% Jetting Changes Pump Modifications Comments
    Sea Level 100% Modified 44, 46, 48, and 50mm
    carbs are more sensitive to
    density altitude changes, and
    may require to be readjusted
    as the temps change at a race.
    Test and record peak
    rpm
    Use 50 to 100yrd sprint to test peak rpm.
    Not 1/4 mile WOT or power-tune on trailer rpm.
    2000 ft 98.9%
    3000 ft 96.4%
    3500 ft 94.6% -1 main jet or 1/4
    turn in HS screw.
    Adjust low speed circuit.
    Test peak rpm Cool World Finals
    .
    1st week in October
    4000 ft 93% 1) Test peak rpm
    2) possible exit nozzle
    Common World Finals
    .
    1st week in October
    4500 ft 92.1% 1) Test peak rpm
    2) poss reduced pitch
    impeller
    Hot World Finals
    .
    1st week in October
    5000 ft 90.4%
    5500 ft 88.9% A hot day +100' in
    CA. and AZ.
    6000 ft 88.2%
    6500 ft 87%
    7000 ft 85.9% -2 main jets or 1/2
    turn in HS screw
    Adjust low speed circuit.
    1) Test Peak rpm
    2) reduce impeller size
    3) Possible nozzle size
    7500 ft 82.9% +115' day @ lake Havasu
    8000 ft 81.5%
    Last edited by wmazz; 11-07-2018 at 04:17 AM.
    Horsepower == Speed, RPM != Speed



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