Many people hear the term "Density Altitude" or "DA" mentioned in racing discussions but don't know what it means, or how to calculate it.

What is Density Altitude?

The definition of Density Altitude according to the FAA is "Pressure Altitude corrected for non-standard temperature variations." Since most people racing cars aren't pilots - here is my simplified definition as it pertains to drag racing, adapted from another definition I read once: Density Altitude is the altitude which your engine thinks it is at, and performs in accordance with.

Basically a lower DA is better. Lower DA means more dense air, more dense air means more air molecules entering the engine, and the amount of air an engine can injest is what determines how much horsepower the engine can make. So more oxygen is good, and since air gets less dense as you go up in altitude, lower Density Altitude is good. Any engine will make more HP at a lower DA than at a higher DA, all else being equal. Turbo and supercharged engines are not effected AS MUCH by changes in density altitude because they create their own artificial atmospheric pressure in the intake manifold, but they are affected somewhat and still adhere to the 'lower DA is better' rule, they just don't suffer AS MUCH when the DA goes up.



How do you calculate DA?

The calculations are rather complicated, but luckily there are DA calculators out there on the web for drag racers to use to calculate their DA.

Here's what I do to calculate DA. The first website I visit is www.weatherunderground.com or www.wunderground.com (same site). I put the zip code of the track in and bring up the weather page for that zip code.

First thing to find is the station elevation, it's on the left side right underneath the current conditions. Obviously you have to use common sense here - if the airport or station your reading is taken at is significantly higher or lower than where your track is, you may need to adjust this in your calcuations. If your station reading is taken in a valley but your track is up in the mountainous area, you have to take this into account and adjust accordingly.




Next thing you need to do is find out what the atmospheric conditions were at the time you ran. For this you scroll down some, and on the left side you'll find "Detailed History and Climate." Obviously you need to put the date you were at the track in the dropdown menu and then click "Go"




Final thing you have to do on the weather website is find the conditions at the time you ran, after you clicked "detailed history and climate" for the date you were at the track and the station closest to your track. Scroll on down to the time closest to the run or runs you want to find the DA for, and note the Time, Temperature, Dewpoint OR Relative Humidity (whichever your calculator wants, the one I am going to post can use either) and the barometric pressure which is labeled as "Sea Level Pressure". Write these down or whatever and then it's time to calculate your DA.




Now you go to your DA calculator of choice, I the one below because it was the first one that came up on google, and you can select your units and whether or not you want to use relative humidity or dewpoint.

http://wahiduddin.net/calc/calc_da_em.htm

Input your data: elevation, temperature, dewpoint (or relative humidity if you click the link on the left to use RH%), barometric pressure, make sure you have your units set correctly, and press calculate. That's the DA at the time you ran. Lower is better (more horsepower.)



Why should I care about DA?

Because a lower DA makes your car faster, simple as that. For reference, my old black '96 went 101-102 all day when the DA was about 400 feet (52 degrees, 30.13 baro, 21deg DP). I went to the same track a week later when the DA was more like 2400 feet (80 deg, 30.06baro, 59deg DP), same setup on the car of course, and my highest trap of the day was 98.x mph. That is how much a difference a good DA can make as opposed to a crappy DA - 2000 feet sucked 4mph out of my car (and everyones car, everyone at the track that day was running very slowly.)

 

That's exactly what I do to calculate DA. I couldn't have said it better myself!

DA is VERY important if you are interested in setting new records. It takes the best conditions possible for me to squeak the times I run.

When the DA is 2500' compared to something like -601' (when I ran 13.7), I would run 14.2 -14.4.

When I bracket raced the most (at The Strip in Vegas), I found that for every 750 feet in DA, I got 1 tenth slower. I knew my car VERY well in Vegas (higher elevation) and won many bracket races the couple years I raced.

But, in other tracks around the country (SoCal, SoFLA, East NC), I've found that for every 500 feet in DA, I get 1 tenth slower.

It's just a rule of thumb that must to applied only to give you a ballpark figure of what you would with a particular DA.

It's a fun equation to calculate. For instance, Krismax said he ran 13.13 in 2763' DA. If he ran in conditions that I know NY can get, he would easily run 12.7s if he was able to get the same amount of traction he did when he ran 13.13. And in order for him to run 12.7s, all it would take is a 50 degree day with decent barometric pressure and normal humidity.

 

sad thing is i'm dieing to race my car with the 3.5 IM and good weather it feels so much better ... but the 29 of oct is the last day to race and sat was rained out. my last hope is tomarrow but it will prob be closed to .

its been raining around me alot all year the day i ran the 13.13 the tracked got rained out 30 min later. it was 99 % humidity that day.

its killing me, ive driven my car on a cool 50 degree day no humidity with the 3.5 IM and it pulls so hard feels much faster than that day with 00 vi

 

I follow how you figure out your DA. Mine is at 5106. Now where do I input that figure to find out my corrected run would be?

 

wow where i go race its 205 elevation.

 

You need the atmospheric conditions for the time of your run from the link. Nealoc provided --- > http://wahiduddin.net/calc/calc_da_em.htm Are you absoluteyl sure the actual elevation is that high?

Lac Cruces is ~ 4100', and El Paso is lower than that. What track is it?

 

Here is the software I use at the track for Density Altitude and how I predict my ET using a Davis wireless weather station connected to my laptop in the car.

The software is called RaceLog Pro and was created by Bob Kodadek of http://www.ifamilysoftware.com/. Bob is a racer of some fame and knows what racers need.

The software is designed to connect directly to a variety of weather stations including my Davis which allows me to be connected wireless up to 600 feet away which is plenty to reach the staging lanes.


The first 3 pics show how the temperature and humidity affect the DA. The first pic the DA is -148. The second I have changed the temp from 55 to 75 which has increased the DA to 1490. (huge difference) The 3rd pic I have increased the humidity from 67 to 100 which has increased the DA from 1490 to 1821.

The next 3 pics show how this will affect your ET. First one using that days conditions was 12.867. The second one I have increased the temp again from 55 to 75 resulting in an ET prediction of 13.011. The third is increasing the humidity again from 67 to 100 resulting in an ET prediction of 13.065.

As you notice the humidity does not have the same dramatic effect that the temp change does.

The next two pics are showing how wind affect the ET. The base was 13.011 at -6 MPH. At -25 MPH the ET is predicted to be 12.897 in the first pic. At 25 MPH the ET is predicted to be 13.197 in the last pic.

The software also calculates each run to a standard weather condition so all runs can be compared accurately to each other using the STP Run Segment Box. (This can been seen in the first 3 pics.)

The software will also calculate how much weight you must add or remove to run the same et. (great for the index races I do).

You can download a fully functional evaluation copy from the website above if you are interested. This is what many if not most of the V8 guys use that I have observed.

 

 

My elevation is 3957. We dont have a track anymore. We run at a crappy private airport that just gives et and mph. When I said my da was 5106, I already had entered the figures into that table. Now how do I get my corrected time. My best time is 15.6 at 90. And no I dont know my 60ft or 1/8 or anything else. I wish we had a real track!!!!!!!!

 

I wish more people would factor in DA when noting gains from their mods because often times it was the better DA that was responsible for the gains, not mod itself. Incorrectly reporting gains from mods leads to false beliefs that intakes and catback exhausts shed .3 seconds and gain 3mph.

My track sits at around 900' and depending on the type of season, the DA range anywhere from 5,000' (upper 90s, sub 30.0 baro, high dew point) to -200' (mid 40s, 30.2+ baro, low dew point). For my cars, that can mean the difference between running 14.7s@94mph to running 14.3s@98mph.

 

Very good read, I've wanted to figure out how to tell this. I thought the conditions were worse for me for my best run, but it seems that for my P/B, the DA I calculated was -338 ft.

What does "relative density" mean?

 

I think the NHRA website has a correction utility. Just google for it I'm sure there are a thousand of them out there. I don't know where really because I don't like corrected numbers so I never bothered to look for one.

 

Taken straight from the same exact page that Neal linked you to with the calculator on it...

So the higher the percentage, the MORE dense the air is compare to standard conditions at sea level. The lower the percentage, the crappier it is compared to standard conditions at sea level.

 

haha I've been . Thanks for the knowledge