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I've been reading a lot of post about 1/4 mile times and I see a lot of you guys talking about the "DA"... What exactly is it? How does it affect my ETs? And how do I calculate it?

Thanks!!!
DA = Density Air.

DA Calculator


Example of DA.

Sealevel someone puts down a 11.1ET. Same car and weather conditions, except now the car is at the 5500' elevation. This same car will now run 12.1 - 12.3 ET.

N/A (Naturally Aspirated) cars are most effected by DA. However F/I (Forced Induction) cars are still greatly effected, but not quite like an N/A car.


Many things factor into a crappy DA. Heat, Humidity are the main contributors.

Hope this helps.
 

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To get even more basic. Engine needs air, the more air (to a point) the better. This is why turbos and superchargers make lots of power, by packing in more air (and we add fuel).

The more dense the air is, the more you air pack into the cylinders. Density is temperature/altitude dependent, the higher you go, the less dense the air is. The hotter it is, generally, the less dense it is. This is why folks in CO never see get the kinds of times sea level folks do.

And why August, in most parts of the country, is a crappy time to go drag racing. Around here, east coast, late October, early November, is optimum. Temps are in the mid-low 50s, weather cooperates, you can see density altitudes in the negative numbers, like racing below sea level :)
 

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Discussion Starter · #4 ·
Thanks for the info... I live in Monterrey, Mexico. Monterrey is about 150 miles from Laredo or Mcallen, Texas. The altitude in Monterrey is of 1765'.

Yesterday at the track my best ET was 13.51 @ 107.7 mph., with a RT ok .675 and a DA of 3851 (according to the calculator), is that any good?
 

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The closer you are to sea level the more oxygen there is in the air and the more dense it is. Which leads to better performance.
 

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Discussion Starter · #6 · (Edited)
Thanks for the info... I live in Monterrey, Mexico. Monterrey is about 150 miles from Laredo or Mcallen, Texas. The altitude in Monterrey is of 1765'.

Yesterday at the track my best ET was 13.51 @ 107.7 mph., with a RT ok .675 and a DA of 3851 (according to the calculator), is that any good?
Went to the track yesterday, ran 4 times getting in 3 of them ETs of 13.60 the DA of 4046... The car temerature and the tire pressurewas were the same. This week I ran .09 seconds slower than last week, also the DA went up 195 points... Could this be the reason why I got .09 seconds slower? What other factor should I look in to?
 

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As GLHS837 pointed out, for us at the dragstrip, think of density altitude as a measure of the air density taking pressure, temperature, and humidity into account. They use the units of "altitude" since it is something easy to which most of us can relate. We understand that the air is thinner (less dense) at Denver than it is in New Orleans. Denver is at an altitued of 5280 ft while New Orleans is at or below sea level. Increasing temperature, increasing humidity, and decreasing pressure will all cause the air density to decrease (density altitude to increase). Decreasing temperature, decreasing humidity, and increasing pressure will all cause the air density to increase (density altitude to decrease).

An engine operating at different air densities will consume a constant amount of air by volume, but not mass. Since, the fuel rate remains the same (unless you change it or the car's computer changes it for you), the air-fuel ratio (A/F, by mass) changes. Changes in A/F will cause the power output of the engine to change.

Increasing the air density (or decreasing density altitude) will increase the A/F, leaning the engine out a bit. Depending on which side of stoichiometry (A/F = 14.7:1) you are on, will depend on the engine's response. Most of us (if not all...as I don't know of too many high performance engines utilizing 'lean combustion') operate on the rich side (less than 14.7:1), so leaning the engine out will increase power, but will increase combustion temperatures. Decreasing the density (or increasing density altitude) will have the opposite effect.

Depending on how close to stoichiometry you are will determine the sensitivity. If you are closer to stoichiometry...your response may not be as much. But if you are pretty far away, a small change in A/F can cause a much more noticible response.

I'm not a really big fan of converting times at one density altitude to sea level conditions. The reason I don't think it is totally correct is that you are assuming you are operating from the same points on the A/F curve as the data that was used to create the conversion charts...most likely this is not exactly the case. Rather, I use density altitude to predict ET changes with small pertibations in air conditions. It is also a gauge for me to make sure the car is running properly from one weekend to another.

I can hear some say...but my car has a computer and should correct for air conditions. Well, at the track you are generally WFO...and at WFO the computer operates in open loop (i.e. no O2 sensor feedback). Therefore, unless you change the trim tables for WFO fueling, it will operate similar to a carburator w/o a jet change.

Anywho...I hope this helps.
Keith
 

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Whatever you do, please do not use it like so many people around here to post a "corrected" track time in your signature or say "Well I would have run xxx" because it's total BS on the street.

The cars you run at the track or around your town are at the same DA so what is the point of quoting or worrying about DA? Great - you can compare to someone in another part of the country, etc. but it's still a made up number and to put it in a sig is hilarious to me.

Sorry, but I absolutely hate when people quote corrected times because I could just as easily say "9.5 @ 150 (corrected for when I put a huge turbo on)" or something similar.

I will say this though - it appears you want to use it in a good way...looking at the DA for your area and deciding when is/isn't a good night to go to the track to try for your best time *Thumbs up
 

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I do agree with both of the above, I dont like corrected times any more than I like "estimated" crank horsepower ratings. State under what test conditions you achieved a result, let the reader figure out how that applies.
 

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Discussion Starter · #12 ·
I agree with Hemi_SRT8. The DA can help you to see if your car is actually running slower because the DA is way too high or if there is something wrong with your car... Some nights you can’t tell much difference in the weather, but this calculator can help you do it…
 

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its Density Altitude. we use it in aviation before every flight. the slang definition for it is the altitude that the engine thinks it is running at. temperature, high or low pressure systems, and your actual altitude effect it
 
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