Does anyone know the formula for correcting your ET for elevation? I'm not sure what it is at my local track, but I'm just curious. thanks

 

There is no simple formula. You need to take into account the barometric pressure, temperature, and humidity. These can be used to calculate the "density altitude". This is the equivalent altitude in reference to "standard ambient temperature and pressure". IOW, the "density altitude" is the altitude at which the current air density would be found if the barometric pressure were 760mm of Hg, the temperature 25 degrees C. and 75% humidity. The point is to get at air density, because this what determines the oxygen content and thus hp.

If you look at a table like this one: http://www.greatlakesdragaway.com/co...onfactors.html that normalizes ET for altitude it is accurate only if you use density altitude, not actual altitiude because it doesn't take into account temperature or humidity or variations in barometric pressure that are not related to altitude. And it still doesn't take into account the ways in which a particular combo will respond to changes in air conditions. An EFI car, for example, will adjust fuel delivery in response to IAT, MAP, etc. while a carburetor car will not.

The formula for density altitude is not simple. That's why pros use a "weather station" which has instruments to measure temp, humity, and pressure with a preprogrammed calculator to do the claculations. Look here for the formula: http://wahiduddin.net/calc/density_altitude.htm and you will see what I mean.

An Excel spreadsheet here: http://pages.globetrotter.net/equipekcm/densite.xls will automate the calculation or you can use one of on-line calculators like this one: http://www.srh.noaa.gov/elp/wxcalc/densityaltitude.html

Rich Krause

 

I do my corrections using dyno formulas. They're accurate enough to do ET predictions.

As mentioned above, the best way to do a correction is with density altitude. Knowing the air density is good when determining if a jet change is needed. If it was a simple correction factor then you car would run the same every day you raced if you didn't change anything. On a week to week basis, you could be off .250 from the week before and didn't change anything. This is just because the weather has changed so much.

Purchase a little weather station. You want to be able to accuratly record temperature, humidity and barometric pressure. From these numbers and knowing the elevation of the track, you can calculate the density altitude. Once you have that then you can use the NHRA conversion factor to get a close estimate to what the car will run at sea level.

If you want the actual formula on finding the density altitude go here. http://www.hardtail.com/techtips/weather.html It took me a long time to work out all the numbers. The formula isn't very simple but a breakdown is given so that you could input the formula into any spreadsheet program to use at the track. Testing it against the online java versions is accurate.

Coming up with a density altitude number depends on the inputs. Find 3 different racers at a track all with TAG systems and they'll tell you 3 different answers. Use your own inputs every time and keep logs of every race.

What is your local track? I should be able to tell you what the elevation is.

 

Hey, Thanks a lot fellas. I noticed on my time slips that, at the top, they have Temp:78*F, Relative humidity:68, and absolute barometer:28.20. That is what it says on my latest time in sig. I'll check out that formula in detail, and come up with a consistant time for my car. Thanks again.

Dave

 

EFI cars - especially early LT1's - are also affected by the dew point, which everyone seems to forget about and (to my knowledge) altitude density doesn't factor in.

 

Isn't the dew point derived from temperature and humidity and therefore does not need to be accounted for separately?

Rich Krause

 

Sorry, I just wrote a big long response with links and everything and the f*cking board lost it!

Do some research - a google search on 'dew point'. Look for sites that have info on 'vapor pressure' as well.

After logging 670+ runs I've learned that dew point is not necessarily derived from temp & humidity alone, and that it affects performance just as much (at least with my car).

 

Dew point is the temperature of the air when it can't hold any more water and dew will form on objects. Relative humidity is just a percentage of water in the air.

Warm air can hold more water than cold air.

Early morning,65*, you could have a dew point of 40* and a relative humidity of 70% (guessing for an example). Bu afternoon the temperature can rise to 75* dropping the humidity down to 40% but the dew point can still be 40*.

50% humidity at 75* has more water in the air than 50% humidity at 60*.

 

....which is relative to the vapor pressure of the moisture in the air. Thanks for a good basic explanation It really pissed me off when my detailed explanation with links was lost

 

As are stock NA LS1 cars. As the temps drop you'd expect the car to speed up but if your near the dew point it will actually slow down. It makes it hard to predict ETs on night races.

 

While it is certainly true that Air Density (taking temp, hum, and press into account) is the best method of "correcting" to a sea level condition (actually sea level at a "standard" atmosphere), the NHRA does have a simple altitude correction table/chart. I have a copy of it on my page.

Also, temp, humidity, and barometric pressure affects ALL engines - though forced induction engines (especially turbo's) can compensate to varying degrees.