How to correct for Density Altitude (DA). Step inside.

[Dave B]

Have you ever been to the track and heard a gearhead say, "Damn the DA is crappy today!" and you think to yourself what the hell is DA? DA stands for Density Altitude and it's the way drag racers correct thier times to compensate for weather conditions and altitude. Changes in weather and altitude can play a huge role in how your car performs. Keep in mind that some tracks are just plain faster/slower than others thanks to differences in timing equipment, the grade of the track, track surface, and the way MPH is calculated.

Here's how to start:

1) Find out the elevation of your track. Use a good search engine and you should be able to find it.

2) Go to www.wunderground.com and find the city where your track is. Once you've located your area, you can dig up historic weather conditions for any day in the past 10 years. You'll want to get the temp, baro pressure, and dew point of the time and date of your run.

3) Go to http://wahiduddin.net/calc/calc_hp_dp.htm and input your numbers. Hit the calculate button and locate the calculated Density Altitude. The DA number generated is the calculated theoretical altitude during you run.

4) Take your DA number and apply to the NHRA correction factors table http://www.gnttype.org/techarea/misc/altitude.html

5) Calculate your numbers and that is what your car should have run under ideal conditions. Sometimes your DA actually shows NEGATIVE altitude which means the car is performing far better than normal and there's nothing to correct and compensate for. Usually only coastal areas in the South, East Coast, and Northeast will experience these rare conditions thanks to low altitude and strong high pressure areas due to the proximity of the ocean (~400 miles). It's also a primary reason most of the "freak" cars reside in these regions.

Here's my favorite corrected DA:

August 20, 2003
96 degrees
32% humidity
29.89 baro
62 degree dew point
950 altitude
DA 3800'

Actual ET/MPH: 14.65@94 2.23 60'
Corrected ET/MPH: 13.98@98mph

On average, my DAs show my car should technically be in the 13.9-14.0@99mph+ range with 2.2 60 foots. However, until I run 13.9-14.0, my car will remain a 14.4 second car.

Calculating for DA is great in that it lets you gauge your cars performance during the season and between the tracks, but for the love of God do not use it was justification that your car is quicker than some other guys. You run what you run. If anything just state that the DA you ran in was really poor and people understand. Correcting your slips and putting it your sig is as bad as using G-tech numbers.

[I30tMikeD]

Good stuff Dave

[I30tMikeD]

My DA on the day of my PB was -1129
The DA on Wed when I ran was 383

[Nealoc187]

Originally Posted by I30tMikeD

My DA on the day of my PB was -1129
The DA on Wed when I ran was 383

-1129 feet lol dead sea dragway

[Aaron92SE]

Crap! Those are some good conditions.

My personal best was run on a day that had a 1200 ft above sea level air density.

I once ran in -490 ft when I ran years ago in NC. I am capable of running 14.2s, if I can just get an DA of around -500 ft.

[NmexMAX]

I'm glad this was brought up. Angers me when there are people asking why they ran a 16.xx when they're 5000'+ above sea level and have a reasonable amount of mods.

[bgates1654]

The only other minor thing to consider is the situation in which a time with a positive DA is converted to the lower time at sea level. Would the driver have been a good enough driver to deal with the extra power (traction) if they actually were at sea level? Correcting times = fun

[Nealoc187]

Corrected times are more useful in determining whether or not a car is making the power it should from the trapspeeds. Cars are much more consistent than drivers in most cases.

Dealing with the power shouldn't really be an issue I don't think as were only talking about fluctuations on the order of 5%, maybe 10% in very extreme cases.

[Aaron92SE]

Originally Posted by bgates1654

extra power (traction)

You mean traction problems? What's that? I never hear my tires chirp off the line.

[Mad-MAX_SE]

wow...i inputed all my info and calculated the DA for my best time recently which was a paltry 10.128 at 71.63mph at Dunn-Benson Dragway, NC. I was shocked that i went up almost .7 from my last outing.

Temp: 75.2
Baro: 30.07
Dew Point: 69.8
Altitude: 5800ft

The DA was an astounding 8338ft!! Using the calculator from the nhra website, which by the way didn't measure that high, lol, i extrapolated the numbers for 8300 ft...times go down by .0013 per 100ft, traps go up .0014 per 100ft...

9.065 at 79.67mph...time seems a little low, but since i beat a rather rich supercharged 2000SE max, it seems decently close,

[Aaron92SE]

Originally Posted by Mad-MAX_SE

wow...i inputed all my info and calculated the DA for my best time recently which was a paltry 10.128 at 71.63mph at Dunn-Benson Dragway, NC. I was shocked that i went up almost .7 from my last outing.

The DA was an astounding 8338ft!! Using the calculator from the nhra website, which by the way didn't measure that high, lol, i extrapolated the numbers for 8300 ft...

9.065 at 79.67mph...time seems a little low, but since i beat a rather rich supercharged 2000SE max, it seems decently close,

I don't see how the track in NC can be that high of DA. Crunch the numbers again and see what you come up with. It has to be over 145 degrees for you to get those kind of numbers.

[Mad-MAX_SE]

it may be that the altitude is wrong, but 5800ft is what i found searching for it....

[Aaron92SE]

Originally Posted by Mad-MAX_SE

it may be that the altitude is wrong, but 5800ft is what i found searching for it....

I don't think the mountains of NC get that high is most places. When I searched for the city of Dunn, NC; I am getting around 200 ft elevation.

[Nealoc187]

5800 foot elevation for a track in North Carolina? I think you better check that figure hehe.

[Nealoc187]

I just typed in dunn-benson dragway elevation in google and found what I think you were looking at... 5800 feet. But that was for Bandimere dragway in Colorado. It was just on the same page as dunn benson was (it was a page of grand prix 1/4 mile times.)

[Mad-MAX_SE]

lol, i guess i should have clicked the link, at 200ft it's a DA of 1447ft...now time to recalculate....

9.96 at 72.85mph....that's seems better.

i was runing consisten 9.5-9.6 at 74-75mph at rockingham in the 1/8th with my 16's and 225/50 16 sumitomo htr+'s. now i have 18's with 245/40 18 BFG Geforce TA KDW's, which are over an inch in diameter taller than the 16" wheel tire combo.

[Jason R]

i hate numbers. my DA turned out to be 598 and that isn't even close to the numbers on the NHRA correction factors table. i dunno maybe its just becuase i'm tired? :dunno:

[Mad-MAX_SE]

your et correction factor is .9952 and you mph correction factor is 1.0045

[Jason R]

so my corrected time is 14.19 @ 96.44 mph. I'll have to see if I can't better this corrected time next time out.

[BEJAY1]

Good info Dave. Makes me feel better. My PB was a DA of 2367 plus a headwind of 20mph. Like Mike said, I need to get out again.

[JSutter]

nice thread

[Dave B]

Originally Posted by BEJAY1

Good info Dave. Makes me feel better. My PB was a DA of 2367 plus a headwind of 20mph. Like Mike said, I need to get out again.

Headwinds are strange things. I've run in some pretty strong wind 10-25mph and you'll agree anything over 15mph is downright gusty. My timeslips show a very slight decrease in ET, but my MPH goes to crap. The day I ran in 20mph+ headwinds, my traps were down well over 3mph and my ETs were done just under .1. I guess on the last 1/8 mile is where the wind really robs your power.

I've seen it work the other way too. When I ran my 99.51mph trapspeed, I had a nice 15mph tailwind.

[SR20DEN]

Originally Posted by Nealoc187

5800 foot elevation for a track in North Carolina? I think you better check that figure hehe.

Yeah to my knowledge there are no dragways in the Appalachian Mountains.

http://americasroof.com/nc.html <- Highest point east of the Mississippi.

http://gorp.away.com/gorp/publishers...n/hik_ash2.htm

[Mad-MAX_SE]

gimme a break guys....i made a mistake, lol. even neal saw what i did...but i still had a decently high DA for my recent runs...