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  1. #11
    Mothman's Avatar
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    The sc is mechanically driven and will not spin faster at higher elevations to increase boost though a turbo can be adjusted to compensate. A machine feels the lack of O2/ft3 at high elev exactly like your body, as one 'breath' contains the same % of O2 as sealevel but there are much fewer total atoms/molecules. If sealevel has 1000molecules of 'air' per ft3 (obviously more just an example) at sealevel, 21% of O2 gives 210 parts of O2. At higher elevation, such as at 3000ft, a cubic foot of 'air' only has 900molecules total meaning the 21% of O2 at 3000ft is significantly less at 189 parts. Humans and engines must 'breathe' more and work harder to consume the same amount of O2 while above sealevel. No amount of ecu tuning on a sc ski will account for this loss, which is why the most stout na or sc V8 in the mountains runs like crap compared to its sealevel performance.


  2. #12

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    Quote Originally Posted by Mothman View Post
    The sc is mechanically driven and will not spin faster at higher elevations to increase boost though a turbo can be adjusted to compensate. A machine feels the lack of O2/ft3 at high elev exactly like your body, as one 'breath' contains the same % of O2 as sealevel but there are much fewer total atoms/molecules. If sealevel has 1000molecules of 'air' per ft3 (obviously more just an example) at sealevel, 21% of O2 gives 210 parts of O2. At higher elevation, such as at 3000ft, a cubic foot of 'air' only has 900molecules total meaning the 21% of O2 at 3000ft is significantly less at 189 parts. Humans and engines must 'breathe' more and work harder to consume the same amount of O2 while above sealevel. No amount of ecu tuning on a sc ski will account for this loss, which is why the most stout na or sc V8 in the mountains runs like crap compared to its sealevel performance.

    Thank you.

  3. #13
    CLASSIC MUSCLE's Avatar
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    Quote Originally Posted by Sanfish View Post
    About time this was brought up, hopefully all these mod expectations will be compared with equal apples.

    When I make hits, I will always log the A/D (air density/psi) because without this information, you can't fairly compare run after run. Here in Florida, I've seen massive changes as a storm comes within several miles of test spot. This happened to me one time while I was tuning and wasn't paying attention to the A/D (pressure), screwed up the whole day tunes because the next week, everything was off (AFR).

    The attached picture is probably as good as it gets in South Florida (anything near 30.2 pressure is pretty rare = around 1020hPa). Notice the density altitude!!!. This screenshot was taken January during a Florida winter. When you have a day like that, you make test hits because that's as good as its going to get (bragging rights)
    Is that from a phone App?

  4. #14
    CLASSIC MUSCLE's Avatar
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    Never mind, I found it! It's an iPhone App.

  5. #15

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    I read this threat the other day, and out of curiosity checked the density altitude as I left the lake last night. It was just under 6,500'. If I'm running the calcs right, I'm down about 50hp on an RXT 260.

  6. #16
    JT jpt7779's Avatar
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    Jim, I am very sure at that RAD you are really down on rpm's. Are you aware that BRP has special impeller pitching for High Altitude users? These high altitude impellers can be ordered right here in the GreenHulk OEM store by entering the part number

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  8. #17

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    I didn't realize there were high altitude specs - I appreciate the info! I'm going to look into that.

    I'm just out west of you in Lubbock. We're only 3200' elevation but the RAD is high pretty much all the time. Both of my skis were originally sold around Abilene and were at a lake house on PK. They ran much better there!

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  10. #18
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    Quote Originally Posted by JimPTx View Post
    I didn't realize there were high altitude specs
    <snip>
    We're only 3200' elevation but the RAD is high pretty much all the time.
    Since a motor is nothing more then an air pump, less oxygen (rpm loss) requires a different setup than a place at sea-level. Motors run richer the higher the elevation is (if same given setup). Since your 260 has a SC (RPM=boost), you can easily compensate RPM loss with a smaller impeller. If you are running a VTECH tune, you have 4 fuel maps you can adjust (2 normal and 2 high-altitude).

    Once you start to think "high altitude" in terms of atmosphere it makes it easier to digest (less oxygen). The advantage of a "power add'er" (SC, turbo, NO2) is it really increases V/E (over 100%) and in simple terms, generates atmosphere.

    Each year when the big boys go to Bandimere drag strip (thunder mountain), everyone runs very different setups. For some of the top teams, its a completely different car practically. It's the N/A "pro-stock" guys that have it down to a science (but boring to watch) since they can't generate air

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  12. #19
    JT jpt7779's Avatar
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    Ever notice when flying on a commercial airliner at high altitude such as Denver, Lubbock, Albuquerque, Guatemala, Bogota, etc. the take off roll takes forever to get airborne? The same weather parameters greatly effect performance in a turbo jet also.......BIG TIME!

    We crunch the numbers for every single flight for density altitude, pressure, temperature, dew point, humidity, wet, dry, icing conditions and gross weight. The end result is what speed to abort, rotate, and lift off at any given power setting.

    The good news is everyone is effected by this equally. The ones that take it into consideration and adapt to the changes are the ones who will out perform.

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  14. #20
    Mothman's Avatar
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    Quote Originally Posted by jpt7779 View Post
    Ever notice when flying on a commercial airliner at high altitude such as Denver, Lubbock, Albuquerque, Guatemala, Bogota, etc. the take off roll takes forever to get airborne? The same weather parameters greatly effect performance in a turbo jet also.......BIG TIME!

    We crunch the numbers for every single flight for density altitude, pressure, temperature, dew point, humidity, wet, dry, icing conditions and gross weight. The end result is what speed to abort, rotate, and lift off at any given power setting.

    The good news is everyone is effected by this equally. The ones that take it into consideration and adapt to the changes are the ones who will out perform.
    Power loss accompanied by less air to 'hold up' the plane. The Bernoulli principle, which is why helicopters have a limit as their rotors literally don't have enough air to move through to support lift.
    im sitting at the lake house, 2000ft below Lubbock, and even though I'm still 1000ft above sealevel my ride absolutely barks, even with 100% humidity. Makes me giggle. Lol I can't imagine it at sealevel. <whistle>

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