Density Altitude exlpained .. and Drag Racing Calculator
 

There has to be a “Standard Reference” (SAE J2723) when determining the Rated HP of a given vehicle. And this is done by operating the vehicle in a “Standard Environmental Condition”.

Ideally, a Standard Environmental Condition (“Standard Day”), and would be:
1. Sea level
2. A barometer of 29.92” Hg.
3. An ambient air temperature of 59 degrees F (+15 C)
4. A relative humidity of 0%

This meteorlogical condition is the basis of determining rated SAE HP. It is also the standard condition for determining 1/4 mile acceleration performance.

Just because a track is at an elevation close to sea level, the Density Altitude (DA) of a given track may be real high due to a local environmental condition of low barometric pressure, high temperature and/or high humidity at the time … So, we can say that; Density Altitude (DA) is the actual altitude of the track adjusted for the barometric pressure and relative humidity at the time.

Then combine a high PA with a high ambient air temperature, and you get a very high Density Altitude (DA) … Density Altitude is the actual track altitude above sea level, adjusted for the barometric pressure, ambient air temperature and relative humidity.

And a high DA is what kills horsepower and results in high ETs and low trap speeds.

On the other hand, operating a vehicle on a track close to sea level, at very cold track temperature, a condition of high barometric pressure and very low humidity would result in faster 1/4 mile times than when operated at a Sea Level / Standard Day track, and would also not be an accurate measurment of the car’s rated HP.

Without a “standard condition reference”, there is no way to determine whether or not a certain modification has made an improvement or not in regard to power output … So, you need to make an adjustment in the ET and Trap Speed for the variation in Density Altitude on that given day and time.
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Here is a great calculator: http://www.dragtimes.com/da-density-...calculator.php

This will allow you to enter:
1. Track Elevation (feet above sea level)
2. Air Temperature
3. Barometric Pressure
4. Relative Humidity

This info that you input will give you the Density Altitude (DA) at the time.
Then enter your 1/4 mile ET and Trap Speed, and it will tell you what your “Corrected” ET and Trap speed would be had the car been run at a Standard Day condition.

NOTE: This nice thing about this calculator is that if you didn’t know the environmental conditions at the track on a certain day you raced in the past, you can first enter the name of the track and the date, and it will give you the actual elevation of the track along with the temp, humidity and baro pressure for various times of the day that you were racing.

Hope this may be of help to someone here.

Ron,

.

BTW ...

How fast is your car?
How fast is your friends car on the other side of the country?


The NHRA uses a correction factor to compensate for altitude for stock/super stock classes.

Many automotive tests also use this factor when giving 1/4 miles stats. This way you could run a car at sea level and have an identical car running at 6000 feet. The car running at 6000 feet will be considerably slower but by using the conversion factor you would see they run the same. Just because someone says they have a car that run's low 13's doesn't mean much against yours unless they are on an equal playing field.

If you really want an accurate conversion, then do your calculations with density altitude. A car running on a high density altitude at sea level will be slower than than same car running on a low density altitude at 3000 feet elevation.

http://www.hardtail.com/techtips/altitude.html

When I was operating aircraft we had to figure the local density altitude to determine the length of runway needed and the ability of the airfoil to lift.

The average altitude in south Florida compensated for density atitude was around 5,000' ASL, as you mentioned temperature and humidity were the primary factors. The air pressure was used to set the altimeter.

Nice post! Agree 100% with your conclusions.

I would like make one minor point...Density Altitude is not ACTUAL altitude (that doesn't change...at least not without earthquakes, volcanoes, or plate techtonics! ;). Rather, it is an apparent altitude, adjusted for those parameters you reference (temp, pressure, humidity). For example, a tracks actual altitude may be 200 ft above sea level, but because of the temp, hum, and pres, the Density Altitude (ie...the apparent altitude adjusted for those conditions) might be +1500, or -500, or on rare occasions, the same as the actual altitude.

I apologize for being picky on such minor semantics....but I'm a Meteorologist and a drag racer, so this is a just a bit of a pet peeve of mine. :)

Also, noticed you referenced "PA". For anyone interested, PA is Pressure Altitude, which is an apparent altitude adjusted only for pressure.

Yup ... "Pressure Altitude" is the field altitude adjusted for baro pressure.

"Density Altitude" is Pressure Altitude adjusted for temp and humidity (although humidity plays a very insignificant role) ... So, to get Density Altitude, baro pressure was already figured in with the Pressure Altitude calculation.

Your right ... I should not have used the term Actual Altitude.

Take-off Density Altitude is the altitude above sea level of a certain track or runway adjusted for current baro press, ambient temp and humidity at the time ... Or basically Pressure Altitude adjusted for temp and humidity

Now I fly closer to the ground :D

:thumbs:

Love that calculator, BTW - thanks for the link! It fixes the most common error made when figuring DA - folks using the wrong pressure reading (be it ALSTG, SLP or actual station pressure).

I don't fly at all anymore ... well, sitting in the back don't count :)

Great Info Thanks

Great explanation for those not familiar with the terms and the results that different DA numbers can make on performance.

The biggest problem of any calculator is the accuracy of the input values. The values supplied by DragTimes are only as good as the closeness of the track to the nearest airport reporting station and the elevation provided by the track. It's very common to have several hundred feet difference in DA.

Ideally, every track would install a weather station between the lanes at mid-track. Then print the calculated DA and the adjusted ET/MPH on the timeslip. I doubt that'll happen soon, but we've come a long way from the flagman and a hand scribbled timeslip.

Agreed. However, I have seen tracks that do record weather (though not at mid-track) have instruments that were WAY out of calibration, or setup incorrectly, giving very bad results. Example...putting the thermometer out in the sun. While the nearest airport may indeed have a very different set of weather readings, at least those readings can generally be depended on to be accurate.

Personally...I have a Kestral 4000 that I carry with me. Those same folks now have a 4250 that is specifically for racers. Pretty neat gadget!

It kills a fair number of pilots, too, who get too complacent. It's the triple-compounding whammy of less engine power, less thrust from the prop and less aerodynamic lift.

It seems we lose about a plane a year at Big Bear (field elevation is about 6750'). On a really hot day, the DA can easily exceed 10,000', and it's right there in front of you on a big electronic sign that displays real-time DA. Poor saps load the plane to or past allowable weight limits and are simply unable to climb, and put it into either the lake or mountainside.

Density altitude is pressure altitude corrected for temperature. At least in aviation, humidity is not considered into D.A. although it does affect performance.

They have an app out now that locates you and brings up the current DA...

http://itunes.apple.com/us/app/dragt...7263?ls=1&mt=8