Best conditions for Top Speed
Discussion
Right then,
when the air is cold, it's denser, so your engine can get a larger mass of air & fuel into the engine, therefore it produces more power.
However, that same helpful dense air which gives you more power, is also slowing you down more due to increased resistance when you're going for a top speed run.
Obviously the 1 offsets the other, but which has the greater effect?
I'm guessing that you get a better top speed when it's hot, based on my observations.
Anyone care to comment?
Assuming gearing is not a limiting factor
when the air is cold, it's denser, so your engine can get a larger mass of air & fuel into the engine, therefore it produces more power.
However, that same helpful dense air which gives you more power, is also slowing you down more due to increased resistance when you're going for a top speed run.
Obviously the 1 offsets the other, but which has the greater effect?
I'm guessing that you get a better top speed when it's hot, based on my observations.
Anyone care to comment?
Assuming gearing is not a limiting factor
I'd agree with Apache that it depends on how fast you want to go.
Based on a certain aerodynamic drag, you need a certain amount of power to reach a given speed. Just pulling numbers out of the air, let's say that at 80F (given air density) you need 300hp to reach 180mph. At 40F (denser air) you might need 302hp to reach that same 180mph. Does that denser air give you the extra 2 hp? Keep in mind that the hp required to reach a given speed increases by the third power (I think) of the increase in drag. So a small increase in drag will require a disproportially large increase in hp.
All from (cloudy) memory...
Based on a certain aerodynamic drag, you need a certain amount of power to reach a given speed. Just pulling numbers out of the air, let's say that at 80F (given air density) you need 300hp to reach 180mph. At 40F (denser air) you might need 302hp to reach that same 180mph. Does that denser air give you the extra 2 hp? Keep in mind that the hp required to reach a given speed increases by the third power (I think) of the increase in drag. So a small increase in drag will require a disproportially large increase in hp.
All from (cloudy) memory...
I remember watching an old black and white film from the late fifties, about the land speed record, (I can't remember what it was called), anyway in the film, the boffins were waiting for the precise time to do the run, which was when the atmosphere was at it's dampest, but dry on the ground........
gt5s_1985 said:I would guess that the increase in hp from the denser air would be directly proportional to density, wherease the increased drag is, as you say proportional to the cube of density, so it would be better to run on a low air density day? Would this explain why they go to the Salt Flats for top speed runs in the summer rather than the winter? Or is it down to ground surface conditions being better?
Keep in mind that the hp required to reach a given speed increases by the third power (I think) of the increase in drag. So a small increase in drag will require a disproportially large increase in hp.
in pure motorsport terms, they set the faster laps when the air is cooler...
interesting side note...
Andy Wallace drives a Audi A3 TDi (apparantly) and he likes to give it a "run" on the 3 times he's taken it to Le Mans this year, i can't remember what he got at the test day... but he only got 144.5mph going down for the race itself as it was much warmer...
going down last weekend for the 1000km he calculated that he should hit 146mph (a new maximum) due to the cooler air, unfortuantly it was warmer than the test day and he only got c.144mph again!!!
incidently, he told all this to Audi UK Press Officer (free car i believe) and he can't believe it, supposidly the A3 TDi shouldn't go this fast...
interesting side note...
Andy Wallace drives a Audi A3 TDi (apparantly) and he likes to give it a "run" on the 3 times he's taken it to Le Mans this year, i can't remember what he got at the test day... but he only got 144.5mph going down for the race itself as it was much warmer...
going down last weekend for the 1000km he calculated that he should hit 146mph (a new maximum) due to the cooler air, unfortuantly it was warmer than the test day and he only got c.144mph again!!!
incidently, he told all this to Audi UK Press Officer (free car i believe) and he can't believe it, supposidly the A3 TDi shouldn't go this fast...
We ran our Viper at TOTB1 and 2 the first year was cool this year very very hot.Despite the less dence air thisyear the high temp really hit the Supercharged/Turbo cars and on average speeds were all down(Except a mental Skyline that managed the standing mile at 194 mph?)Cars like ours (Injected NA)seemed to fair much better in fact our terminal speed of just under 180 mph was up on last year.So i would guess it also depends on the type of induction on your car.
danhay said:
Nice motor Venom!
What is TOTB?
It's my bike I'm interested in which is normally aspirated (at the moment). It seems to be slower in the winter, but perhaps I've just been unlucky with headwinds which are probably a bigger factor?
you see thats interesting, 'cos although imo in theory your car should be faster in cooler conditions mine never really has the chance to warm up and run at its full potential in winter...
it likes the warm weather more, but it lost a bit of power in france (though proportionatly far less than my brother who was really struggling) because it was too hot
not to sure 
you, see it runs a rather pathetic 4-speed box and when i got it, it wouldn't go above 108mph
with the addition of the exhaust it hits around 110mph
now, after the cylinder head change its nearer 115-120mph but its screaming like a bitch as it needs that 5th gear
plus, it still isn't running to its full potential as it is under carbed (with the new cylinder head) so with the (hopeful) additions of 2 new 45-webers (see other thread) this should increase the top speed a bit more
i'm up for a top speed run on some french autoroutes next june though
you, see it runs a rather pathetic 4-speed box and when i got it, it wouldn't go above 108mph
with the addition of the exhaust it hits around 110mph
now, after the cylinder head change its nearer 115-120mph but its screaming like a bitch as it needs that 5th gear
plus, it still isn't running to its full potential as it is under carbed (with the new cylinder head) so with the (hopeful) additions of 2 new 45-webers (see other thread) this should increase the top speed a bit more
i'm up for a top speed run on some french autoroutes next june though
best way around this is to cool the incoming charge with gas (lots of it) but it in under pressure and it will !! cool things down fine boils of at somewere around -150 degs and the power you get is just to much.But you do have to have a big blue bottle in the boot. there is the trade off
The air temp makes a very noticable difference on my esprit. On a cold winters morning the car is performing at it's best.
I think the colder air entering the turbocharger means the air will not be so hot comming out the other side, then this air is cooled further by the intercooler (which is performing better due to the colded air cooling the chargecooler liquid better).
This all equates to much better performance.
Rgds
Dictys
I think the colder air entering the turbocharger means the air will not be so hot comming out the other side, then this air is cooled further by the intercooler (which is performing better due to the colded air cooling the chargecooler liquid better).
This all equates to much better performance.
Rgds
Dictys
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