Question about DA
Thread Starter
Burning Brakes
Joined: Apr 2002
Posts: 1,081
Likes: 0
From: Mississauga ONTARIO
Question about DA
how do u go about calculating it
AND
how much of a difference does it make on your ET and Traps?
for example, DA = - 1000 vs DA= 750
on the C4 goard, a guy ran a STOCK LT1 in the high 12s at Etown, in cool weather, sticky track and he was also a good driver. The DA was -1000. What would he of gotten if the DA was 750, for instance?
you constantly hear people doing well at ATCO, ETown, and HRP..... wish we had good tracks like that, or prepped well enough.
[Modified by johnnyC5, 11:36 PM 11/27/2002]
AND
how much of a difference does it make on your ET and Traps?
for example, DA = - 1000 vs DA= 750
on the C4 goard, a guy ran a STOCK LT1 in the high 12s at Etown, in cool weather, sticky track and he was also a good driver. The DA was -1000. What would he of gotten if the DA was 750, for instance?
you constantly hear people doing well at ATCO, ETown, and HRP..... wish we had good tracks like that, or prepped well enough.
[Modified by johnnyC5, 11:36 PM 11/27/2002]
Le Mans Master
Joined: Dec 1999
Posts: 5,169
Likes: 6
From: Greenville SC
Re: Question about DA (johnnyC5)
Weather Tech
Chapter 3: The "Problem" with Density Altitude
In Chapter 1 of Weather Tech, the definition of the "Motorsports Standard Atmosphere" (MSA) was presented in terms of both standard pressure and temperature versus elevation. Chapter 2 spelled out in great detail the mathematical definition of Air Density Index and how to determine the Density Altitude for any set of ambient weather conditions. This chapter of Weather Tech explains the "Problem" with Density Altitude.
However, there really is nothing mathematically incorrect about the definition of Density Altitude . . . it's all about what you're going to use it for. To help illustrate this point, please consider a common tool in our racer toolbox - a standard 6 inch caliper. A 6 inch caliper is a pretty complicated device, having several components, all put together in a way that allows very accurate measurements of length - actually to the nearest thousandths of an inch. Calipers are manufactured by several different companies, and all will give basically the same answer when used properly. However, a caliper is not the proper tool for measuring the weight of the pistons in your engine.
The same idea or concept is true for Density Altitude. It is an excellent tool for determining the MSA elevation having the same air (or oxygen) density as the local, measured weather conditions . . . but it won't give you the correct answer for predicting the performance of your race car.
Why not? First, we must go back and examine the history of term "Density Altitude". Density Altitude was originally invented decades ago for use in aircraft flight manuals. Pilots are critically concerned that they will be able to safely liftoff the ground during the takeoff roll within a given runway length. The lift generated by the wings (at a critical speed) must be greater than the aircraft weight in order to actually get off the ground. Lift is directly proportional to the local air density.
Circular slide rule calculators were created so pilots could quickly determine the Density Altitude from temperature, runway elevation and barometric pressure. Aircraft flight manuals contain several charts that enable the pilot to figure out if he can safely get the aircraft off the ground under various weather conditions. The primary issue of physics for the aviation use of Density Altitude is to ensure that the lift provided by the wings is greater than the weight of the aircraft. Since lift is directly proportional to air density, Density Altitude is the proper tool for this job.
Motorsports initially used Density Altitude for jetting Holley carbs, and for this purpose, it does a really good job. Air (or oxygen) and fuel must be mixed in the proper ratio for best engine performance. The "Problem" with Density Altitude started with the advent of bracket and super class drag racing where the need to accurately predict ET under varying weather conditions became critical to winning races. The erroneous assumption of physics used by many at the time was that drag racing ET is proportional or otherwise clearly related to Density Altitude.
Unlike aircraft takeoff characteristics which are directly proportional to local air density, drag race car performance is critically dependent on engine power, and engine power does not precisely follow air density. Thus, in the same way that a caliper is not a good tool to use for measuring the weight of a piston, Density Altitude is not a good tool to use for predicting race car performance. In fact, we really don't need Density Altitude at all for motorsports use, as the Air Density Index will tell us everything we need to know for carb jetting using a much simpler, percentage scale.
The way that an engine's power varies with changing weather conditions is thoroughly documented in the Society of Automotive Engineers (SAE) standards. These are the same procedures used for correcting observed engine dyno data to the MSA standard day conditions of sea level, 60 deg F and dry air. The mathematical definitions of Density Altitude and Air Density Index get close to the SAE standard methods, but don't correctly account for the effect of ambient temperature on engine performance. A method called the "HP Correction Factor" does the proper job.
http://www.quarterjr.com/members/
Chapter 4: "HP Correction Factor"
The HP Correction Factor depends on the fuel system of the engine, that is, the type of fuel being burned and if the engine is naturally aspirated or not. The mechanical efficiency (i.e. friction) of the engine also comes into play for this calculation. The effect of very high water vapor content (above 1.0 in Hg vapor pressure) must also be addressed. PCs are absolutely required to properly handle all the math for the HP Correction Factor. The general discussion below applies to naturally aspirated gasoline burning engines.
These engines do not respond to changes in temperature as strongly as Density Altitude or Air Density Index would predict. Instead, engine power and torque vary with the inverse of the square root of absolute temperature. Racers have incorrectly reasoned that the "volumetric efficiency" (VE) of an engine was not affected by changing weather conditions, but in reality, VE increases significantly with increasing ambient air temperature. This increase in VE is a direct result of the increase in the speed of sound with increasing air temperature. Peak Mach numbers within the intake runners remain relatively constant for a given RPM. The speed of sound in air increases with the square root of absolute temperature as shown in the chart below.
However, Density Altitude has been used for years with pretty good results. That's because the math used for the pressure and water vapor content components of Air Density Index are very close to those used for the HP Correction Factor. The largest error is in the way temperature is handled. And this error is generally modest since we only race over a relatively narrow temperature range, typically between 40 and 110 deg F. This range corresponds to absolute temperatures between 500 and 570 deg R. The chart below illustrates the difference between using a correction based on the inverse of absolute temperature vs the inverse of the square root of absolute temperature.
In summary, use the right tool for the right job. The HP Correction Factor is the right tool for correcting and predicting engine power and race car ET, and for determining throttle stop settings. To further illustrate the errors that can result from using the wrong tool, try the following example using the QUARTERjr.com Home Page Weather Station and Dragstrip Dyno programs.
Assume we just raced at Columbus where it was hot and humid - 400 ft on the altimeter, 85 deg F and 43% humidity. The Weather Station calculates 2690 ft Density Altitude and a 1.079 HP Correction Factor. The next race is a Friday night in Las Vegas, where it's cool and dry - 2100 ft altimeter, 60 deg F and 16% humidity. The calculated Density Altitude is again 2690 ft, but the HP Correction Factor is now 1.096. What do we do?
Since the Density Altitude is the same, the carb jet remains the same. But the engine will be making less HP in Las Vegas. The HP Correction Factor is designed to be used to correct the observed torque from the dyno to standard day conditions by multiplying by this factor. Thus, whenever the HP Correction Factor is larger, the observed (or actual) engine HP will be smaller. With less HP we will run slower (everything else being the same).
If you're throttle stop racing, be prepared to take out some timer. If you're running Pro Stock Truck, relax . . . your engine's not hurt, everybody will be running a little slower during this qualifying session than they did in Columbus. The Dragstrip Dyno program predicts that a 950 HP, 2300 lb Pro Stock Truck will run .04 seconds and 0.9 MPH slower under this scenario.
http://www.quarterjr.com/members/
Chapter 3: The "Problem" with Density Altitude
In Chapter 1 of Weather Tech, the definition of the "Motorsports Standard Atmosphere" (MSA) was presented in terms of both standard pressure and temperature versus elevation. Chapter 2 spelled out in great detail the mathematical definition of Air Density Index and how to determine the Density Altitude for any set of ambient weather conditions. This chapter of Weather Tech explains the "Problem" with Density Altitude.
However, there really is nothing mathematically incorrect about the definition of Density Altitude . . . it's all about what you're going to use it for. To help illustrate this point, please consider a common tool in our racer toolbox - a standard 6 inch caliper. A 6 inch caliper is a pretty complicated device, having several components, all put together in a way that allows very accurate measurements of length - actually to the nearest thousandths of an inch. Calipers are manufactured by several different companies, and all will give basically the same answer when used properly. However, a caliper is not the proper tool for measuring the weight of the pistons in your engine.
The same idea or concept is true for Density Altitude. It is an excellent tool for determining the MSA elevation having the same air (or oxygen) density as the local, measured weather conditions . . . but it won't give you the correct answer for predicting the performance of your race car.
Why not? First, we must go back and examine the history of term "Density Altitude". Density Altitude was originally invented decades ago for use in aircraft flight manuals. Pilots are critically concerned that they will be able to safely liftoff the ground during the takeoff roll within a given runway length. The lift generated by the wings (at a critical speed) must be greater than the aircraft weight in order to actually get off the ground. Lift is directly proportional to the local air density.
Circular slide rule calculators were created so pilots could quickly determine the Density Altitude from temperature, runway elevation and barometric pressure. Aircraft flight manuals contain several charts that enable the pilot to figure out if he can safely get the aircraft off the ground under various weather conditions. The primary issue of physics for the aviation use of Density Altitude is to ensure that the lift provided by the wings is greater than the weight of the aircraft. Since lift is directly proportional to air density, Density Altitude is the proper tool for this job.
Motorsports initially used Density Altitude for jetting Holley carbs, and for this purpose, it does a really good job. Air (or oxygen) and fuel must be mixed in the proper ratio for best engine performance. The "Problem" with Density Altitude started with the advent of bracket and super class drag racing where the need to accurately predict ET under varying weather conditions became critical to winning races. The erroneous assumption of physics used by many at the time was that drag racing ET is proportional or otherwise clearly related to Density Altitude.
Unlike aircraft takeoff characteristics which are directly proportional to local air density, drag race car performance is critically dependent on engine power, and engine power does not precisely follow air density. Thus, in the same way that a caliper is not a good tool to use for measuring the weight of a piston, Density Altitude is not a good tool to use for predicting race car performance. In fact, we really don't need Density Altitude at all for motorsports use, as the Air Density Index will tell us everything we need to know for carb jetting using a much simpler, percentage scale.
The way that an engine's power varies with changing weather conditions is thoroughly documented in the Society of Automotive Engineers (SAE) standards. These are the same procedures used for correcting observed engine dyno data to the MSA standard day conditions of sea level, 60 deg F and dry air. The mathematical definitions of Density Altitude and Air Density Index get close to the SAE standard methods, but don't correctly account for the effect of ambient temperature on engine performance. A method called the "HP Correction Factor" does the proper job.
http://www.quarterjr.com/members/
Chapter 4: "HP Correction Factor"
The HP Correction Factor depends on the fuel system of the engine, that is, the type of fuel being burned and if the engine is naturally aspirated or not. The mechanical efficiency (i.e. friction) of the engine also comes into play for this calculation. The effect of very high water vapor content (above 1.0 in Hg vapor pressure) must also be addressed. PCs are absolutely required to properly handle all the math for the HP Correction Factor. The general discussion below applies to naturally aspirated gasoline burning engines.
These engines do not respond to changes in temperature as strongly as Density Altitude or Air Density Index would predict. Instead, engine power and torque vary with the inverse of the square root of absolute temperature. Racers have incorrectly reasoned that the "volumetric efficiency" (VE) of an engine was not affected by changing weather conditions, but in reality, VE increases significantly with increasing ambient air temperature. This increase in VE is a direct result of the increase in the speed of sound with increasing air temperature. Peak Mach numbers within the intake runners remain relatively constant for a given RPM. The speed of sound in air increases with the square root of absolute temperature as shown in the chart below.
However, Density Altitude has been used for years with pretty good results. That's because the math used for the pressure and water vapor content components of Air Density Index are very close to those used for the HP Correction Factor. The largest error is in the way temperature is handled. And this error is generally modest since we only race over a relatively narrow temperature range, typically between 40 and 110 deg F. This range corresponds to absolute temperatures between 500 and 570 deg R. The chart below illustrates the difference between using a correction based on the inverse of absolute temperature vs the inverse of the square root of absolute temperature.
In summary, use the right tool for the right job. The HP Correction Factor is the right tool for correcting and predicting engine power and race car ET, and for determining throttle stop settings. To further illustrate the errors that can result from using the wrong tool, try the following example using the QUARTERjr.com Home Page Weather Station and Dragstrip Dyno programs.
Assume we just raced at Columbus where it was hot and humid - 400 ft on the altimeter, 85 deg F and 43% humidity. The Weather Station calculates 2690 ft Density Altitude and a 1.079 HP Correction Factor. The next race is a Friday night in Las Vegas, where it's cool and dry - 2100 ft altimeter, 60 deg F and 16% humidity. The calculated Density Altitude is again 2690 ft, but the HP Correction Factor is now 1.096. What do we do?
Since the Density Altitude is the same, the carb jet remains the same. But the engine will be making less HP in Las Vegas. The HP Correction Factor is designed to be used to correct the observed torque from the dyno to standard day conditions by multiplying by this factor. Thus, whenever the HP Correction Factor is larger, the observed (or actual) engine HP will be smaller. With less HP we will run slower (everything else being the same).
If you're throttle stop racing, be prepared to take out some timer. If you're running Pro Stock Truck, relax . . . your engine's not hurt, everybody will be running a little slower during this qualifying session than they did in Columbus. The Dragstrip Dyno program predicts that a 950 HP, 2300 lb Pro Stock Truck will run .04 seconds and 0.9 MPH slower under this scenario.
http://www.quarterjr.com/members/
Melting Slicks
Joined: Dec 2001
Posts: 2,040
Likes: 0
From: Collin County Texas
Re: Question about DA (johnnyC5)
Here is a bunch of info on DA, etc. I have collected in the past:
http://www.nwlink.com/~koryg/airdensity/airdensity.html http://www.quarterjr.com/ http://www.appdatsci.com/webpages/weatherlog.htm http://www.prestage.com/carmath/ http://usatoday.com/weather/wworks0.htm http://www.altronicsinc.com/ http://www.biondoracing.com/WeatherStations.shtml#WSC-2 http://www.crew-chief.com/dragstuf/index.html http://www.altalabinstrument.com/portable.html
During the past summer, with DA's over 2500 feet here in the DFW area, I was running 12.6's. Two weekends ago, with DA's of -700 ft. and less, I ran a 12.29! The temp was 53 deg, 25 % humidity, baro. at 30.18 in. giving a DA = -700 feet! Later that night around midnight, the DA was below -1000 ft!!!!! That is what they call mineshaft conditions.
I use the Density Program that I bought from the site I listed above (http://www.quarterjr.com/). It is real accurate in determining what you may run given inputs of temp, humidity and pressure. It predicted that I would run 12.265 @ 110.66 based on my runs earlier in 2500 ft. conditions. As you can see from my signature, it was real close close!
http://www.nwlink.com/~koryg/airdensity/airdensity.html http://www.quarterjr.com/ http://www.appdatsci.com/webpages/weatherlog.htm http://www.prestage.com/carmath/ http://usatoday.com/weather/wworks0.htm http://www.altronicsinc.com/ http://www.biondoracing.com/WeatherStations.shtml#WSC-2 http://www.crew-chief.com/dragstuf/index.html http://www.altalabinstrument.com/portable.html
During the past summer, with DA's over 2500 feet here in the DFW area, I was running 12.6's. Two weekends ago, with DA's of -700 ft. and less, I ran a 12.29! The temp was 53 deg, 25 % humidity, baro. at 30.18 in. giving a DA = -700 feet! Later that night around midnight, the DA was below -1000 ft!!!!! That is what they call mineshaft conditions.
I use the Density Program that I bought from the site I listed above (http://www.quarterjr.com/). It is real accurate in determining what you may run given inputs of temp, humidity and pressure. It predicted that I would run 12.265 @ 110.66 based on my runs earlier in 2500 ft. conditions. As you can see from my signature, it was real close close!
