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Old 09-06-2016, 05:28 PM   #1
dothatshht
Scooby Newbie
 
Member#: 335243
Join Date: Oct 2012
Chapter/Region: Tri-State
Location: Nyack, NY
Vehicle:
13 WRX
SWP

Default Density Altitude

Long story short, I ran 12.153 at Island Dragway in NJ this weekend. A friend of mine said if I was at Englishtown NJ (Raceway park) I would probably be in the 11's. I asked why and he said youre faster at Englishtown because its at a lower altitude.

So i looked it up and island is about 2,440 DA and raceway park is 2,030 DA.

Would 400 feet really swing .15 in either direction?

I ran across this post from juice14 on a lexus forum. Seemed like an interesting read.
_______________________________

I did this a couple of weeks ago, I just didnt have the time to post this.
Thanks to Caymandive I was able to get some information form his drag strip journal and use it to answer some questions that i had about drag racing in the IS350... So using a a statistical software i ran some tests.

the main variable taken into consideration was density altittude (DA)..... I wanted to know, not only if DA affects 1/4 mile times(because it obviously does), but by how much. and what times to expect on 1/4 mile on my IS350 according to the weather conditions.
It is hard to explain what density altittude is, but it is basically an international standard of measuring the density of the air, humidity, elevation, and altimeter, all into one number. the lower the density altittude the faster the car will be (basically). usually the lower the temperature is the lower DA is.

So I ran a regression, which is a statistical analysis that lets you forecast 1/4 mile times (in this case) and answer other questions.
From caymandive's journal, I took all the 1/4 mile times and their respective Density Altutudes at the time of the runs. I only took IS350's I did not take IS-F times. otherwise this test would be pointless.

Here are the results:

Regression Analysis: 1/4 mile time versus DA
The regression equation is
1/4 mile time = 13.4 + 0.000137 DA

Predictor Coef SE Coef T P
Constant 13.3584 0.0258 516.78 0.000
DA 0.00013677 0.00002112 6.48 0.000

R-Sq = 53.8%

Analysis of Variance

Source DF SS MS F P
Regression 1 1.0086 1.0086 41.93 0.000
Residual Error 36 0.8659 0.0241
Total 37 1.8745


So, what do these numbers mean:

• Question: Does density altitude have an effect on mile times?
Yes, Density altitude does has an effect on mile times. Density altitude has a positive effect on mile times, which means that as density altitude goes up, so do mile times. And as density altitude goes down, so do mile times.
There is only five percent chance that we have answered this question wrong, so we are 95% sure that these statements are true.

• 53.8% of the variation in mile times can be explained by density altitude. The other 46.2% is explained by other factors such as: track conditions, driver’s skill level, weight reductions, tire psi, quality of tires, gas, head wind, tail wind, etc.
So basically, 53.8% of your 1/4 mile time you can say its due to Density Altittude. the other 46.2% you can say, well this track sucks, or Im not a good driver, or my tires are old. blah blah, etc.

• What times can I expect by looking at a specific DA number?
With density altitude at 0 feet, the Lexus IS350 should run 13.4 1/4 mile E/T time.
For every 1000 feet change in DA, there is a .13 second change in mile times.
Example: You should run 13.40 @ 0 feet of DA.
You should run 13.27 @ -1000 feet of DA.
You should run 13.53 @ 1000 feet DA.

This is how you can estimate your 1/4 mile time before going to the track by looking at the density altittude.

example: if density altittude is -700......
you use the regression equation = 1/4 mile time = 13.4 + 0.000137 DA
= 1/4 mile time = 13.4 + .000137(-700) = 13.30 E/T time
So, basically with Density Altitude at -700ft. I should run 13.3 on the 1/4 mile. anything lower than that is a good time.


Important NOTE: remember most of these runs were done with intake/exhaust, and spare tire and jack removed. this will give the model some kind of error.
____________________________________
Do you drag race? Are you racing at high altitude? Notice that your times don't match other times you see on the net? Well, don't worry. There's nothing wrong with your car. It's ET is a little slower because the air at higher altitudes is less dense so your car's engine has less air during the combustion process.

The NHRA uses a correction factor to compensate for altitude for stock/super stock classes. To accurately convert your ET, find your altitude with the chart below and convert your quarter mile ET and MPH to sea level times.

Here's how you do it: Simply multiply the elapsed time factor below times your ET or MPH.

ET Example: 16.1 x .9692 = 15.60412 corrected ET
MPH Example: 84 x 1.0325 = 86.73 corrected MPH

To find the elevation of the track where you drag race, check out the Geographical Nameserver. Enter the city, state and the server will find the info for you.

and the altitude converters:
Altitude(ft) ETFactor MPHFactor
1200 .9874 1.0129
1300 .9861 1.0143
1400 .9848 1.0157
1500 .9835 1.0171
1600 .9822 1.0185
1700 .9809 1.0199
1800 .9796 1.0213
1900 .9783 1.0227
2000 .9770 1.0241
2100 .9757 1.0255
2200 .9744 1.0269
2300 .9731 1.0283
2400 .9718 1.0297
2500 .9705 1.0311
2600 .9692 1.0325
2700 .9679 1.0339
2800 .9666 1.0353
2900 .9653 1.0367
3000 .9640 1.0381
3100 .9627 1.0395
3200 .9614 1.0409
3300 .9601 1.0423
3400 .9588 1.0437
3500 .9575 1.0451
3600 .9562 1.0465
3700 .9549 1.0479
3800 .9536 1.0493
3900 .9523 1.0507
4000 .9510 1.0521
4100 .9497 1.0535
4200 .9484 1.0549
4300 .9471 1.0563
4400 .9458 1.0577
4500 .9445 1.0591
4600 .9432 1.0605
4700 .9419 1.0619
4800 .9406 1.0633
4900 .9393 1.0647
5000 .9380 1.0661
5100 .9367 1.0675
5200 .9354 1.0689
5300 .9341 1.0703
5400 .9328 1.0717
5500 .9315 1.0731
5600 .9302 1.0745
5700 .9289 1.0759
5800 .9276 1.0787
5900 .9263 1.0787
6000 .9250 1.0801

this should be useful in comparing times since it levels it all and takes away the altitude differences.
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Last edited by dothatshht; 09-06-2016 at 05:39 PM.
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