SOLVED Air density / atmospheric pressure / altitude

Discussion in 'Tracks' started by realdealgod, Jul 17, 2023.

  1. realdealgod

    realdealgod New Member

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    Hi guys,

    My first thread here. The question itself is in the last topic / paragraph, but I'll give an overview before it, so if not interested to the context just skip to the question.

    Context:

    I've been working in a track mod to the improve a conversion of the Curitiba track here in Brazil for a while, about a year and a half.

    I've been learning through this Forum a lot, since the original track conversion (probably from Race07 WTCC game) was very raw material and had a lot of work to do with all sorts of things, including realigning the surfaces, replacing materials, bad textures, misplaced objects, missing objects, AI, colliders, Pits objects, moving objects, spectators, etc.

    The problem I'm facing know is that I'm running a correlation test to make the grip surface as accurate as possible, and since this track received most of international races here in Brazil (except F1 and WEC), I have data. I'm using one I collected from the oficial time sheet from WTCC 2007, and since there is a very good mod for that a BMW 320si 2006 I had a starting point.

    I've made a lot of practice and came to the best time I could set with the original setup and optimal track conditions, and the car was a lot faster then in real life. My first thought was to simple dial friction parameter in the surface to get a slower car, so I did it, just to figure out that below 0.90 the AC doesn't play the skid sound anymore, and beyond that the grip seems to get really bad, slippery and unrealistic.

    Then came to my mind that Curitiba is around 3000ft (~900m) above sea level and the air density should play a big role in car performance with more speed in the straights but less aerodynamic grip, which, perhaps, should be the point missing, so I could have better mechanical grip, but with slower lap times if it is properly set.

    What I've Found:

    I've searched around here and the official Kunos Forum, but the only thing I found is that since version 1.6.0 released in May, 18, 2016 the game support air density engine performance dependency. (https://www.assettocorsa.net/forum/index.php?threads/assetto-corsa-v1-6-available.32819/)

    I also found that the car physics then included a key to make the car sensible to air density as follows:
    [ENGINE_DATA]
    ALTITUDE_SENSITIVITY=0.1 ; sensitivity to altitude

    But I could not find where (a part of the ui.json that is not related to physics) to inform the altitude of the track or it's mean see level air pressure / density.

    The Question:

    Here is my problem, I would like to properly setup the air density of the track to have a real correlation with car performance x aero grip x mechanical grip x lap time.

    If anyone could help I would be very grateful.
     
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  2. Johnr777

    Johnr777 Moderator

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    If you are using CSP... add this to the track's surfaces.ini

    [ALTITUDE]
    BASE=880
     
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  3. fughettaboutit

    fughettaboutit aka leBluem

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    afair you will probably only see a differences in turbo cars
     
  4. Johnr777

    Johnr777 Moderator

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    The opposite, NA cars lose about 3% horsepower for every 1000 feet. Turbo cars on the other hand can compress more air to make up that difference and still maintain the same horsepower as if it was at sea level.
     
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  5. realdealgod

    realdealgod New Member

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    Thanks mate! It's in meters right?
     
  6. Johnr777

    Johnr777 Moderator

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    Correct, Autódromo de Curitiba averages around 880 meters in altitude
     
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  7. luchian

    luchian Administrator Staff Member

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    Interesting thread. What's the new laptime? :nerd:
     
  8. realdealgod

    realdealgod New Member

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    Hi there,

    I ran some tests.

    I put the AI to race with BMW 320si 2006 WTCC naturally aspirated, in a 10 minutes qualify session with same conditions:
    Time: 10:40
    Weather: Clear Sky
    Air temp: 23º C
    Track temp: 31º c
    Grip: 100%
    Tires: SS

    I did two runs both track configs (without altitude adjust / with altitude adjusted to 893m got from local data).

    I took the best time from each altitude config.

    The best times set by the AI was:
    . Without altitude adjust: 1:26.363 (230 km/h) / 1:26.315 (230 km/h)
    . With altitude adjust: 1:27.558 (226 km/h) / 1:27.549 (226 km/h)

    So, in fact, the adjust worked. 1,23s (1,43%) slower using the fastest times.

    As said by @Johnr777, it's expected to loose 3% of power each 1000 feet, almost 9% for Curitiba, but it's a touring car that rely in mechanical rather than aerodynamic grip, so I wouldn't expect it to loose that much in lap times.

    So now I can dial the surface friction up again until I reach a time equivalent with the real world. In 2006 the best BMW clocked 1:26.006 and in 2007 1:24.769 for the qualifying sessions.

    Later I'll make some tests with turbos and high aerodynamic dependent cars too.

    Thanks for the help guys. I'm very content with the result!

    Cheers!

    20230724_233811.jpg 20230724_234942.jpg 20230725_000104.jpg 20230725_001210.jpg
     
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  9. Christoph Leipold

    Christoph Leipold Member

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    I did not find any information about this. Where did you find it? And is there more information what it does?
    That would help me a lot for my Pikes track. As for now I just cut the engine power to 65% as an average - but this is really not convincing
    So any help would be much appreciated
     
    Last edited: Jun 26, 2024
  10. fughettaboutit

    fughettaboutit aka leBluem

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    Not documented, but thats only a feature when running CustomShadersPatch.

    In vanilla AC this has no impact whatsoever.

    The origin (0,0,0) of the track is at that altitude you insert there.

    Edit: some say reference is 1st pitbox

    Edit2: Oh and extended physics must be on, that is mostly in practice or hotlap, or on servers configured for that
     
  11. Christoph Leipold

    Christoph Leipold Member

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    Thank you a lot! That was really helpful.
    I will give it a try - and report if I got success :)
     
  12. fughettaboutit

    fughettaboutit aka leBluem

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    luchian and Christoph Leipold like this.
  13. Christoph Leipold

    Christoph Leipold Member

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    So this would reduce the engine power by 3% for every 1000feet.
    Same for Turbos and natural engines? Or is there a parameter to influence that. As turbos lose far less than naturals engines.
    Would be nice to have some more deep information - but is a start and a much better way than my first try .
     
  14. fughettaboutit

    fughettaboutit aka leBluem

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    no other params, thats handled by CSP itself, it knows about turbos
     
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  15. Christoph Leipold

    Christoph Leipold Member

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    So here I am back after a testing session:
    It seems to work quite well: This was on a BMW Pro car natural aspired engine with 420HP: Altitude-Sensitivity-natural aspired-0-9999m.jpg
    My over the thumb measurements fit quite well to the real stuff.
    upload_2024-6-30_19-7-29.png
    For real the drag would be = "C*rho/2*v²" and the engine torque would also be directly linked to the intake air density and thus the mass flow.
    Only the turbo engines are hard to tell: It all depends on the turbo design, so you could completely avoid any power loss (as it is done here: Turbo Manta had no power loss at all but very reduced drag of course)
    I would prefer to give some power loss to the turbos also as they run into cooling problems in the thin air.
    But as I said: It is hard to tell.
    Congrats to this "physics extension"
     
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  16. Christoph Leipold

    Christoph Leipold Member

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    I forgot to test the effect on the downforce though.
    I will do some aditional testing
     
  17. Christoph Leipold

    Christoph Leipold Member

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    It is OK also on the aero effects Altitude-Sensitivity-turbo charged-0---4999-downforce.jpg

    Great stuff! :)
     
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