Tell me about DA

[G8V8]

I've seen DA in a few discussions and did a little looking, very little, but found a DA calculator and entered the data for my 13.262 @ 106.77 run at Cecil county. It said the DA was 1284. What does that mean and how does it relate to my run?

I suspect these questions highlight my drag racing noob status!

[GeorgeInNePa]

It means you were running in the equivalent of air at 1284 feet above sea level.

The lower the Density Altitude, the better your runs will be. The DA can and does go negative.

Get some -1200 air and you won't believe the difference in your times!

[wreckwriter]

Most DA calculators, find the one at Drag Times, also have a calculator to enter your DA and ET/speed to get a corrected time (ie what it would have been in good air).

[wreckwriter]

Here: http://www.dragtimes.com/da-density-altitude-calculator.php

[G8V8]

Cool. My run was at 1:25 on 10/3/08. The DA calculator gets weather data at about 11 am and 3 pm. At 11 the DA was 870. At 3 it was 1284. I extrapolated to about 1-1:30 and the DA was 1081.

The effect is pretty amazing:

Using 11 am DA= 13.152 @ 107.65
Using 3 pm DA = 13.080 @ 108.27

Using the DA I extrapolated to about the time of the 13.262 2106.77 run give a run of 13.115 @ 107.97 mph.

This is cool stuff. I'm going to MIR tomorrow. It is at 80' vs 325'. Temps should be the same but humidity is going to be higher (damn!).

[Robert@KBXPerformance]

This is how DA and weather conditions affect a vehicle. The lower the DA the faster the vehicle will run, assuming proper traction.


Temperature: The higher the temperature of the air the less dense it becomes due to the molecules moving around much faster and thus expanding in volume. This means that if your engine draws in 500 cfm of air on a cold day it doesn't contain the same amount of mass as 500 cfm of air on a hotter day. Even with the same volume of air coming into the engine there will be more airmass ingested on the cold day. The less dense the aircharge the less airmass which leads to less power due to proportionally less O2 able to be burned. This is why most people run their personal best ET's in the falltime or early spring because in most locations that is when the tracks will still be open but also the coldest weather. This is why you are seeing allot of people posting new personal best ET's with their G8's lately because even with the same exact mods as back in summertime the car will run quicker and faster now. A lower Temperature will lead to lower DA.

Barometric pressure: The higher the baro the more heavy (airmass) the air is due to higher density. This is more or less in conjunction with temperature, though not a 100% direct correlation. Usually the colder the day the higher the baro will be because the "column" of air above is more dense and pressing down harder thus compressing the air underneath it. A higher baro pressure will lead to lower DA.

Relative humidity: The amount of moisture in the air. The higher the percentage of humidity the more air is displaced by it. This means that once again there is less burnable oxygen mass in a given volume of air. Lower relative humidity will lead to lower DA.

Dew Point: This is just another way to show how much humidity is present in the air. The closer the dew point gets to the actual temperature the higher the relative humidity. If I race on a 70 degree day with 65 degree dew point versus a 70 degree day and a 35 degree dew point I will turn a faster time on the day with the proportionally lower dew point because there is more airmass present and less humidity displacing it. Proportionally lower dew point will lead to lower DA.

Altitude: This is more or less self explanatory. The higher you go in the atmosphere the less air there is due to Earths gravity pulling it closer to Earth. This is why if you take a stock G8 GT and run it at sea level and you will probably hit about a 13.5-13.9 ET in average weather conditions. Take that same G8 GT and run it at 7,000' altitude in the same weather conditions as before and it will be substantially slower. Lower actual altitude leads to lower DA.


If you understand Volumetric Efficiency (VE) and how it pertains to internal combustion engines then the weather affects more or less equate to something pretty similar. Weather overall affects the amount (total mass) of burnable oxygen that the engine can draw in.




Whenever I run any of my vehicles at the track I take full logs of the exact weather conditions when the pass was made as well as the technique used for the particular run. This all goes into my computerized time slip database that I can look back in and see how the runs compare to each other. Leads to more consistency and quicker ET's due to operating the vehicle in optimal conditions more often.

[G8V8]

Thanks! Great read.

[Panther 2]

Running a car on spray , they create their own d/a

[Robert@KBXPerformance]

Running a car on spray , they create their own d/a



Yes they do but DA still affects a sprayed car, just not as much.

[g8_795]

Cool. My run was at 1:25 on 10/3/08. The DA calculator gets weather data at about 11 am and 3 pm. At 11 the DA was 870. At 3 it was 1284. I extrapolated to about 1-1:30 and the DA was 1081.

The effect is pretty amazing:

Using 11 am DA= 13.152 @ 107.65
Using 3 pm DA = 13.080 @ 108.27

Using the DA I extrapolated to about the time of the 13.262 2106.77 run give a run of 13.115 @ 107.97 mph.

This is cool stuff. I'm going to MIR tomorrow. It is at 80' vs 325'. Temps should be the same but humidity is going to be higher (damn!).

This site is great for getting your weather variables. You don't have to extrapolate just unless you want to extrapolate on the 20 minute readings ;-)

I think they store data for a year or so.

http://www.wunderground.com/cgi-bin/findweather/getForecast?query=22033&wuSelect=WEATHER


-Robert

[G8V8]

Thanks. We got rained out today at MIR. Actually it stopped raining early this morning but the track was way too wet and more rain was expected.
Bummer is the DA was 102.

[99-LS1-SS]

Yeah, we're getting a ton of rain too. Apparently it's a big front rolling through the area.

[G8V8]

Amen to that.