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1/4 Mi. Weather Correction

Weather has a significant effect on how much power engines make. Changes in weather result in changes in the air density. The air density is an indication of how much oxygen content the air contains. This oxygen content relates to how much fuel is burned and therefore how much power is produced. Since the weather is always changing this adds another level of confusion to the mix in our quest to go faster and make more power.

Density Altitude (DA) - Density altitude is a term this article uses frequently. Let's start with the definition for density altitude.

Density Altitude (DA): The altitude in the international standard atmosphere at which the air density would be equal to the actual air density at the place of observation.

Density altitude (DA) is used in the aviation world. The reason is because the pilots needed to know how much power the engine produces at different altitudes and to make sure enough power was available for take off. The racing world has followed aviation because an engine is an engine and correction factors the pilots figured out can be directly applied to the racer. The weather is so important that standards have been set by SAE, Soceity of Automotive Engineers, and others. These standards are used correct power ratings to these standards so people can compare apples to apples. The bad part about standards is usually not everyone goes by the same standards and hence you get multiple standards. Table 1 has a list of these standards and the weather conditions associated to them along with the DA for these standards.

Correction Standard Air Temp (°F) Relative Humidity (%) Barometric Pressure (in-Hg) Density Altitude (feet)
SAE J607 60 0 29.92 84
SAE J1349 77 0 29.235 1962
ECE 77 0 29.235 1962
DIN 68 0 29.92 605
JIS 77 0 29.235 1962
SAE J1995 77 0 29.53 1623

Weather - The weather is constantly changing which effects how much power engines make. Since weather can have a large effect on how much power an engine makes we need to log the weather information and determine the DA from this information. How do I get this information? The best way to get this information is measure it yourself right before the run. You can purchase your own mini weather station for this. Sometimes the track itself has a weather station and can give you the DA directly.

Another way to get this information is to use SMOKEmUP's 1/4 Mi. timeslip correction auto lookup calculator. To use this calculator simply enter the date, time, select the dragstrip, ET, and trap speed. The calculator will automatically go out and retrieve the weather information from the closest airport to the dragstrip and select the reported weather conditions closest to the time of the run. The calculator will then compute the DA and correct the timeslip to sea level for you. One of the really cool things is the weather information goes back 20 years for most locations. So if you have a timeslip from three, six, or ten years ago just enter the information and the calculator will do the rest for you!

(Sample Output)
The closest weather results for 07/06/2005 at 11:25 am

Time recorded 11:55 AM
Temperature °F 69.8
Dew Point °F 68.0
Altimeter Setting 29.96 in Mercury
Density Altitude: 1073.9 feet
Track Elelvation: 90 feet

UnCorrected ET:
12.8 (sec) @ 110.5 (MPH)

Corrected ET to Sea Level:
12.656 (sec) @ 111.776 (MPH)

Predictions - With accurate log book information including weather conditions, namely DA, you can compare your timeslips removing one of the variables, weather. NHRA © has chosen the density altitude of "sea level" as a altitude standard ( 59 °F, 29.92 inches mercury, 0% humidity). Using this information SMOKEmUP.com has written some calculators to determine Density Altitude, Correct 1/4 Mile Timeslip to Sea Level, Correct 1/4 Mile Timeslip from one DA to another DA, and SMOKEmUP's 1/4 Mi. timeslip correction auto lookup calculator. Using these calculators you can accurately compare your timeslips removing one of the variables, weather.

Let's walk through a couple of examples using the calculators. First you made a run at the end of the season last year and ran 11.8 with a DA of -2500 feet. In the off season you do a cam swap and in the begin of the season you run a 12.1 with a DA of 2000 feet. On the surface last years run was faster by 0.3 seconds. Now factor in the weather for the runs and you'll see the cam swap did in fact pay off and the car is running faster since the corrected ET is approximately 0.3 seconds faster.

Another example would be a friend went 12.88 the night before you went 13.2 he had a DA of -1236 yours was -396. Who went faster? Using Correct 1/4 Mile Timeslip from one DA to another DA. Enter ET 13.2, DA of run -396 feet, and new DA of -1236 feet. The result is 13.085 corrected to -1236 feet DA, and sorry to say he went faster than you did .

Run # ET (actual) Density Altitude (DA) ET (corrected)
1 11.8 sec -2500 feet 12.11 sec
2 12.1 sec 2000 feet 11.822
3 13.2 sec -396 feet 13.085 corrected to -1236 feet DA

Summary - As you can see from Table 2 above you need to consider weather conditions to accurately determine if the changes you made helped or hurt performance.


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