Air density from temp is calculated in GT6
- Thread starter KinLM
- 76 comments
- 5,982 views
- 4,101
- California
- Sk8er913
How does air density affect hp though?
2x the air = 2x the hp?
So would ALL cars have a higher top speed with higher temps?
I decided to use Google to see if I could find information helpful to us. Unfortunately instead of bringing to a university study Google brought me back to GTPlanet.
But it was a different GTPlanet, one that I was never a part of. It was 2006 GTPlanet. 
https://www.gtplanet.net/forum/threads/temperature-and-the-top-speed-of-cars.76756/
Here's a quote that I think was the most helpful in that thread.
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- 1,598
- Vienna
- bmw123_
Which is why Pikes Peak cars start of with 1000+ hp and cross the line with about 800 (I think don't quote me on that.
And now you guys will laugh at me, but I don't care. Here is an input from real life.
I drive an older 90 hp Turbo Diesel and this engine has significant turbo lag (under 2000 rpm it feels like a tractor. No joke)
When it was really hot here in Austria, about two weeks ago, I felt that the Turbo-'kick' was way weaker and I had to rev the engine higher when accelerating up to highway speed.
- 4,101
- California
- Sk8er913
Acceleration is slower because of less power, but top speed is higher, because of less drag. Its like a big car with a lot of power than a small sized car with a medium engine. The small car will accelerate faster off the line, but the heavier car will eventually pass it.
- 7,527
- Exorcet
- OE Exorcet
Car engines use the Otto Cycle to generate power. The air is used to convert heat into work. Air holds a certain amount of heat for a given mass, so the amount of work you can generate is proportional to mass. Work is [horse]power * time.
There is an upper temperature limit though because you need to worry about pre-ignition and disassociation (break up) of the air/fuel mix. You can't add any more heat than the amount that causes one of those things to happen. Lower temperature allows more heat to be added to the air/fuel mix before those limits are hit. I'm rusty on my thermodynamics though.
On Mach effects, it's actually the faster cars that will be slowed more than the slower ones.
Drag goes up with V^2 generally, but the drag coefficient (CD) is roughly constant at low Mach. At high Mach the CD will start to increase and it can increase greatly for something not properly shaped for high speed. Low temperature shifts Mach effects toward lower speed. You can think of the Mach effect as feeling like the deployment of a parachute. In cold temperature the parachute deploys earlier.
Mach effects tend to be seen around Mach .7 +, which is about 532+ mph. Keep in mind that the speed of the car is only V_freestream which is the speed of the airflow far from the car. Closer to the car the speed is multiplied by numbers in the 1.3-1.6 range because of flow acceleration, so you can see this while you are going slower than Mach .7.
- 3,411
- the bottom of my heart.
- kolio123
- 4,180
- Finland
- GTP_MadMax
If this is true then I would need to retest my Tomahawk X. I got up to 730 km/h (456 mph) at Route X going downhill, drafting and with perfect gearbox tune, but I did it in the wee morning hours. Let's see if I can squeeze a few more km/hs.
Anyone knows what time gives the hottest temp at Route X?
Yeah , you should re-tune and re-run your SRT X.
This thread caught my interest and I ran a few tests using the SRT X
SRT X (no oil change) at Route X (AT , DS3 no driving aids) (grip real)
Max throttle before the track has loaded , no steering inputs. (black screen launch)
No DRS used.
Start time (progression 1) Air Temperature
6:45 Temp 24 C
T1 31.675 , 603km/h in downhill
T2 1:01.699 597km/h on back straith
6:45 Temp 21 C
T1 31.666 , 603km/h in downhill
T2 1:01.673 598km/h on back straith
7:40 Temp 22 C
T1 31.635 , 604km/h in downhill
T2 1:01.596 599km/h on back straith
10:45 Temp 26 C
T1 31.558 , 606km/h in downhill
T2 1:01.401 601km/h on back straith
14:00 Temp 27 C
T1 31.532 , 607km/h in downhill
T2 1:01.337 602km/h on back straith
14:45 Temp 27 C
T1 31.521 , 608km/h in downhill
T2 1:01.309 602km/h on back straith
15:30 Temp 27
T1 31.530 , 607km/h in downhill
T2 1:01.335 602km/h on back straith
16:45 Temp 27 C
T1 31.533 , 607km/h in downhill
T2 1:01.339 602km/h on back straith
20:45 Temp 25 C
T1 31.566 , 606km/h in downhill
T2 1:01.421 601km/h on back straith
23:50 Temp 23 C
T1 31.612 , 605km/h in downhill
T2 1:01.537 599km/h on back straith
0:00 Temp 27 C
T1 31.525 , 607km/h in downhill
T2 1:01.321 602km/h on back straith
1:10 Temp 24 C
T1 31.601 , 605km/h in downhill
T2 1:01.511 600km/h on back straith
3:00 Temp 22 C
T1 31.646 , 604km/h in downhill
T2 1:01.624 598km/h on back straith
Max throttle before the track has loaded , no steering inputs. (black screen launch)
No DRS used.
Start time (progression 1) Air Temperature
6:45 Temp 24 C
T1 31.675 , 603km/h in downhill
T2 1:01.699 597km/h on back straith
6:45 Temp 21 C
T1 31.666 , 603km/h in downhill
T2 1:01.673 598km/h on back straith
7:40 Temp 22 C
T1 31.635 , 604km/h in downhill
T2 1:01.596 599km/h on back straith
10:45 Temp 26 C
T1 31.558 , 606km/h in downhill
T2 1:01.401 601km/h on back straith
14:00 Temp 27 C
T1 31.532 , 607km/h in downhill
T2 1:01.337 602km/h on back straith
14:45 Temp 27 C
T1 31.521 , 608km/h in downhill
T2 1:01.309 602km/h on back straith
15:30 Temp 27
T1 31.530 , 607km/h in downhill
T2 1:01.335 602km/h on back straith
16:45 Temp 27 C
T1 31.533 , 607km/h in downhill
T2 1:01.339 602km/h on back straith
20:45 Temp 25 C
T1 31.566 , 606km/h in downhill
T2 1:01.421 601km/h on back straith
23:50 Temp 23 C
T1 31.612 , 605km/h in downhill
T2 1:01.537 599km/h on back straith
0:00 Temp 27 C
T1 31.525 , 607km/h in downhill
T2 1:01.321 602km/h on back straith
1:10 Temp 24 C
T1 31.601 , 605km/h in downhill
T2 1:01.511 600km/h on back straith
3:00 Temp 22 C
T1 31.646 , 604km/h in downhill
T2 1:01.624 598km/h on back straith
And after running these I oil changed the car and tuned it for the obvious.. max speed. 724 km/h (unassisted) ,or 725 as my GT6 status page claims.
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- 28,471
- Windsor, Ontario, Canada
- Johnnypenso
- 4,180
- Finland
- GTP_MadMax
Good catch. 👍
- 2,228
- Bielefeld
- tarnheld
Great find @KinLM!
Aren't we comparing terminal velocities here? A quick rearangement of the relevant formulas shows that:
At terminal velocity v_t the accelerating force F and the drag force cancel each other:
1/2 rho v² Cd A = F
=>
v_t = sqrt( 2 F /(rho Cd A) )
The density of air rho depends on temperature T like this:
rho(T) = p / (R T)
p is pressure, R is the specific gas constant
Plugging it all together we get a sublinear dependance of terminal velocity on temperature:
v_t = C sqrt(F T)
with C = sqrt((2 p R)/(Cd A))
Maybe i'm ignoring something important, i'm not an aerodynamics expert.
I cannot make this out in the plot of @MadMax's data, maybe we need more data:
EDIT: one way to test the formula might be to 'extend' the temperature range by using a car with the same aerodynamics but a different engine like the Opel Speedster/Turbo and pretend they generate the same force, but at a different temperature. This might show the square root dependence...
Aren't we comparing terminal velocities here? A quick rearangement of the relevant formulas shows that:
At terminal velocity v_t the accelerating force F and the drag force cancel each other:
1/2 rho v² Cd A = F
=>
v_t = sqrt( 2 F /(rho Cd A) )
The density of air rho depends on temperature T like this:
rho(T) = p / (R T)
p is pressure, R is the specific gas constant
Plugging it all together we get a sublinear dependance of terminal velocity on temperature:
v_t = C sqrt(F T)
with C = sqrt((2 p R)/(Cd A))
Maybe i'm ignoring something important, i'm not an aerodynamics expert.
I cannot make this out in the plot of @MadMax's data, maybe we need more data:
EDIT: one way to test the formula might be to 'extend' the temperature range by using a car with the same aerodynamics but a different engine like the Opel Speedster/Turbo and pretend they generate the same force, but at a different temperature. This might show the square root dependence...
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- 3,603
- London, Ontario
- voodoovaj
The question was asked but never answered, does this affect the handling? I believe it does but i haven't tested it yet.
Also, I wonder if this has something to do with the out of whack top speeds in the game. The difference is speed isn't significant, so I wonder if they adjusted the game to have higher top speeds so that we'd notice.
Also, I wonder if this has something to do with the out of whack top speeds in the game. The difference is speed isn't significant, so I wonder if they adjusted the game to have higher top speeds so that we'd notice.
- 725
- Southfield, Detroit
- BroncoFLHX
You took my statement waaaaaay out of context. This is like the fuel consumption system in the game. Be at high revs and you consume a set amount and be at low revs and you consume less at a constant rate. What I was saying is that what this air density physics are simulated by having the track temp determine top speeds. When the track is cold you go slower and when the track is hot you go faster. Air density is then simulated without having to actually program it.Great find @KinLM!
Aren't we comparing terminal velocities here? A quick rearangement of the relevant formulas shows that:
At terminal velocity v_t the accelerating force F and the drag force cancel each other:
1/2 rho v² Cd A = F
=>
v_t = sqrt( 2 F /(rho Cd A) )
The density of air rho depends on temperature T like this:
rho(T) = p / (R T)
p is pressure, R is the specific gas constant
Plugging it all together we get a sublinear dependance of terminal velocity on temperature:
v_t = C sqrt(F T)
with C = sqrt((2 p R)/(Cd A))
Maybe i'm ignoring something important, i'm not an aerodynamics expert.
I cannot make this out in the plot of @MadMax's data, maybe we need more data:
View attachment 423874
EDIT: one way to test the formula might be to 'extend' the temperature range by using a car with the same aerodynamics but a different engine like the Opel Speedster/Turbo and pretend they generate the same force, but at a different temperature. This might show the square root dependence...
I was thinking from a game developers perspective.
- 2,228
- Bielefeld
- tarnheld
No offense intended.
I'm just guessing too, and i don't think just like you that PD does a simulation of the actual air, they will likely model the drag force and add that to the forces acting on the vehicle at any moment. I'm just pointing out that this force seems to imply a sublinear dependence of the terminal velocity (we can measure that, but not the actual drag force) on temperature -- something we can test to see if PD is serious about the REAL driving simulator.
- 28,471
- Windsor, Ontario, Canada
- Johnnypenso
You could also take elevation into account when determining air density which would also be simple since all the tracks have known elevations. You could take it to the next step and introduce weather as a variable since most tracks have weather and throw in some volumetric throttle response while you're at it.
LeGeNd-1
Premium
- 7,253
- Australia
- GTP_LeGeNd-1
Alright peeps, I finally retested the X with the hottest temperature. I was expecting a few km/hs improvements, but ended up with 30 km/h so my top speed is now 767 km/h (480 mph) 
Settings
SSRX arcade mode race
Professional difficulty
Track time 15:00 (should give you 27C / 80.6F)
Grip reduction LOW
All driving aids OFF
Racing Soft tyres
Don't forget to oil change and wash (every little bit helps)
Suspension
Ride height max min
Spring rate min min
Damper (C&E) 1 1
ARB 7 7
Toe 0 0
Gearbox
Max speed 700 (437 if you are still using imperial)
4.608
2.642
1.730
1.195
0.871
0.670
0.580
Final gear 4.650
LSD
Initial 5 5
Accel 5 60
Decel 5 5
Ballast
200kg max front (-50)
The Race
Press the nitro button to activate low drag mode (even before you control the car).
Don't press the gas until the lap timer hits 3 seconds (as close as you can to 3.000, any sooner or later will impact the draft timing greatly).
Full throttle and shift at redline (slightly lower for 6th and 7th gear as it tends to bog down a bit).
You should draft and overtake the first car going uphill.
Right after the crest you should draft and overtake a the first group.
Halfway down the hill draft and overtake second group.
Near the end of the hill you should just catch and overtake the last group.
Most times you'll end up hitting the back of the last group, in my 767 km/h run I got lucky and the AI parted in exactly the right moment so I didn't hit anything and got an extra 2-3 km/h.
Offline I think this is the limit, online with a group of drivers in organized drafting and getting everything perfect I reckon you could nudge 800 km/h
So there you have it folks! Try to beat my record and enjoy chasing jumbo jets in the process
EDIT: Updated the settings with the wheelie glitch. Managed to squeeze an extra 7 km/h. Not that anyone cares by this point lol.
Settings
SSRX arcade mode race
Professional difficulty
Track time 15:00 (should give you 27C / 80.6F)
Grip reduction LOW
All driving aids OFF
Racing Soft tyres
Don't forget to oil change and wash (every little bit helps)
Suspension
Ride height max min
Spring rate min min
Damper (C&E) 1 1
ARB 7 7
Toe 0 0
Gearbox
Max speed 700 (437 if you are still using imperial
)4.608
2.642
1.730
1.195
0.871
0.670
0.580
Final gear 4.650
LSD
Initial 5 5
Accel 5 60
Decel 5 5
Ballast
200kg max front (-50)
The Race
Press the nitro button to activate low drag mode (even before you control the car).
Don't press the gas until the lap timer hits 3 seconds (as close as you can to 3.000, any sooner or later will impact the draft timing greatly).
Full throttle and shift at redline (slightly lower for 6th and 7th gear as it tends to bog down a bit).
You should draft and overtake the first car going uphill.
Right after the crest you should draft and overtake a the first group.
Halfway down the hill draft and overtake second group.
Near the end of the hill you should just catch and overtake the last group.
Most times you'll end up hitting the back of the last group, in my 767 km/h run I got lucky and the AI parted in exactly the right moment so I didn't hit anything and got an extra 2-3 km/h.
Offline I think this is the limit, online with a group of drivers in organized drafting and getting everything perfect I reckon you could nudge 800 km/h
So there you have it folks! Try to beat my record and enjoy chasing jumbo jets in the process
EDIT: Updated the settings with the wheelie glitch. Managed to squeeze an extra 7 km/h. Not that anyone cares by this point lol.
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Lain
Premium
- 7,067
- Pasadena, TX
- Yaywalter
I'm gonna have to agree with iName, track temp probably just has a very basic relationship with speed rather than there being any sort of consideration of air density.
If air density were being taken into consideration, then in theory cars should slow down faster at lower temps. But everbody seems to be overlooking this post:
Maybe testing with a particularly un-aerodynamic vehicle could provide more definitive results, but those results sound pretty conclusive to me.
If air density were being taken into consideration, then in theory cars should slow down faster at lower temps. But everbody seems to be overlooking this post:
Maybe testing with a particularly un-aerodynamic vehicle could provide more definitive results, but those results sound pretty conclusive to me.
- 2,228
- Bielefeld
- tarnheld
The fact is that @KinLM discovered that with higher temperatures cars have faster top speed. At top speed the main influence is the drag force/air resistance going against the vehicle engine force. If you do the math, you'll see that drag force depends (linear) on air density, and air density depends on temperature, so top speed is a function of temperature like this:
top speed = C * sqrt(engine force * temperature)
So it's perfectly reasonable to attribute the top speed change to the temperature change, all other things being equal.
@MatskiMonk's coasting experiment doesn't contradict the top speed issue, as it was inconclusive. It tests a different thing as the drag force quickly diminishes with less speed, so the effect might not be measurable.
- 725
- Southfield, Detroit
- BroncoFLHX
Those blocks are able to slip through the cracks of space time and continuum. They decide whether they are to be in GT5 or GT6, and nobody knows why they do this...Maybe PD uses quantum mechanics to do their simulation, or how else can you explain this?
Maybe it's a sign. Sometimes you can hear them whisper "Soon.."
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- 2,228
- Bielefeld
- tarnheld
