5degreeC-outside air temp.change=unimpressive acceleration
06-17-2004, 04:10 PM
#17
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Originally posted by Garth S
Yeah, confusing! Go back to intro Physics and the Gas Laws: The world of temperature starts at Absolute Zero, -273K: Consquently, small changes in ambient temperature are referenced to absolute zero .... so the relative change in density for a 10 C change is 'peanuts', for example!
Yeah, confusing! Go back to intro Physics and the Gas Laws: The world of temperature starts at Absolute Zero, -273K: Consquently, small changes in ambient temperature are referenced to absolute zero .... so the relative change in density for a 10 C change is 'peanuts', for example!
Glenn
06-17-2004, 06:45 PM
#18
Rennlist Member
Its really based on the temperature's effect on the air molecules. as the temp goes up, they move around more and need more space, hence they create a less dense mass per unit of volume.
I dont have my charts with me, but from memory, if you look at air density at sea level vs at 6000ft, there is a 20% drop in density. this effects hp almost directly. now, temperature, to a lesser effect, changes density as well as humidity. on a hot humid day, at altitude, you see the biggest losses in density and subsequently power produced by our engines vs at sea level.
to have the same effect with temperature for say a 6% drop in density, you need to drop the temp about 40degrees F. so, to answer your question , we are talking about a % drop in density (slugs per cubic/ft) vs % drop in temp. they are related and the best example of this is on the dyno SAE conversion table .
so, anyone that sees a noticable drop in HP (this has to be in the 20hp range i would guess) would have to see a substantial change in temp, not just 10 degrees. Now, if there is a substantial drop, it would be due to other factors, and not just the temp.
Mk
I dont have my charts with me, but from memory, if you look at air density at sea level vs at 6000ft, there is a 20% drop in density. this effects hp almost directly. now, temperature, to a lesser effect, changes density as well as humidity. on a hot humid day, at altitude, you see the biggest losses in density and subsequently power produced by our engines vs at sea level.
to have the same effect with temperature for say a 6% drop in density, you need to drop the temp about 40degrees F. so, to answer your question , we are talking about a % drop in density (slugs per cubic/ft) vs % drop in temp. they are related and the best example of this is on the dyno SAE conversion table .
so, anyone that sees a noticable drop in HP (this has to be in the 20hp range i would guess) would have to see a substantial change in temp, not just 10 degrees. Now, if there is a substantial drop, it would be due to other factors, and not just the temp.
Mk
Originally posted by sharkmeister85
I'm no mathematician (I'm not even sure I can spell it
), but you are pretty good at it so indulge me please. I don't follow how "10 degrees is less than 1% change". If 10 degrees were 1% does that make 1000 degrees 100%. Sounds stupid, so maybe I am. Just looking for answers, not trying to be a smartass.
Thanks,
Glenn
I'm no mathematician (I'm not even sure I can spell it
), but you are pretty good at it so indulge me please. I don't follow how "10 degrees is less than 1% change". If 10 degrees were 1% does that make 1000 degrees 100%. Sounds stupid, so maybe I am. Just looking for answers, not trying to be a smartass.Thanks,
Glenn
06-17-2004, 06:53 PM
#19
Rennlist Member
it may feel like it, but there should be little difference. I have 1000s of laps at laguna, and from turn 11 to the start finish, it takes me about 6.88 seconds. This is regardless of the type of day. Hot, cold etc. a range of 50 degrees to 100degrees. my lap times are also with in second over/under in a 1:40-1:42 range.
I think generally, there are trade offs. as the track gets hot, sometimes the tires work better, but engine looses a little. as the temp goes up, the air is thinner ,and there is less wind resistance, but then less downforce too.
temperature alone cannot effect hp that much. who better to know this than folks that have been on the dyno . Devek has lots of dyno runs at the same dyno on different days. I, myself, tend to go to the dyno on a cool day in winter and a hot day in summer to check the actual HP. usually, its pretty darn close, although the correction factors are all over the map.
my car has felt really slow the last month or so. However, im sure the dyno will prove my seat-accelerometer wrong as well.
Mk
I think generally, there are trade offs. as the track gets hot, sometimes the tires work better, but engine looses a little. as the temp goes up, the air is thinner ,and there is less wind resistance, but then less downforce too.
temperature alone cannot effect hp that much. who better to know this than folks that have been on the dyno . Devek has lots of dyno runs at the same dyno on different days. I, myself, tend to go to the dyno on a cool day in winter and a hot day in summer to check the actual HP. usually, its pretty darn close, although the correction factors are all over the map.
my car has felt really slow the last month or so. However, im sure the dyno will prove my seat-accelerometer wrong as well.
Mk
Originally posted by GRGGOODSPEED
Mark,
Good point but,
It's more like 30 degrees is a BIG change.
The car runs much stonger at 45 than 70+
and hates 80+
Mark,
Good point but,
It's more like 30 degrees is a BIG change.
The car runs much stonger at 45 than 70+
and hates 80+
06-17-2004, 11:34 PM
#20
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Hi there guess what? I STILL HAVE THE PROBLEM! I doubt If Einstein or Hawking can help me here. Slow and Steady does not-win the race!I do not want to have to wait for the fall or winter to regain my confidence/pride in this car. Not to many accelerated runs can be done in an airconditioned underground parking lot. Please give generously your intellect on this topic- b.s is O.K too. Your 928 representative-Mike.
06-18-2004, 12:14 AM
#21
Nordschleife Master
Start looking into sensors.
Odds are it's NOT a fuction of charge density, but sometime else.
Lets see, both your ignition timeing, and mixture are a fuction of tempreture, both the air temp, and the engine temp. If the sensors report back wrong, you could easily get much less power. IIRC, the MAF is sensitive to tempreture fluxs, and has a correction sensor for that too, right?
Enjoy.
Odds are it's NOT a fuction of charge density, but sometime else.
Lets see, both your ignition timeing, and mixture are a fuction of tempreture, both the air temp, and the engine temp. If the sensors report back wrong, you could easily get much less power. IIRC, the MAF is sensitive to tempreture fluxs, and has a correction sensor for that too, right?
Enjoy.