School Me On Air Density and Density Altitude

[Scoobum]

Came damn close to losing a pair of head gaskets in super clean air tonite. School me on air density and density altitude. I gotta get a hand held weather station. Tuning relying on temp/humidity isn't good enuf.

[good2win22]

There are four types of altitudes.


Absolute- the actual height above ground referred to as AGL (above ground level)


True - the actual height above sea level referred to as MSL


Pressure- the actual altitude corrected for the pressure of the air mass. This is more localized and changes from areas to area. You've seen a weather forecast map with a bunch of H's and L's. Well when you're using a barometric altimeter as in an aircraft, you have to keep adjusting it to keep your MSL altitude constant based on the density of the air mass that you're flying through. Also, a standard pressure is 29.92 inches of Mercury when the temperature is 15' C at seal level. This is known as a standard day. All aircraft performance is based off a standard day. Engine performance can be closely related. If the number is higher than 29.92, say 30.01 and the temp is 15'C, the air is more dense and the engine may perform better. For every .01 on the barometric pressure setting equals 10 feet. 


Density- the altitude that has been corrected for pressure and for temperature. This, along with humidity percentages will really tell the tale on how to set up fuel settings for repeatability.


Check this link out


https://www.faasafety.gov/files/gslac/library/documents/2011/Aug/56396/FAA%20P-8740-02%20DensityAltitude[hi-res]%20branded.pdf

[Steve Wood]

I think that is Jason's field! I have a degree in meteorology but that was long long ago.

Basically, we have a standard theoretical temperature and a standard theoretical pressure at sea level which is a mandatory set of numbers.  I think standard pressure is 1013 mb or 29.92 inches of mercury.  I think the temperature is about 60 degs F.   I guess these numbers were selected as being near averages of what we observe over a period of time?

The problem in the real world is that these numbers vary considerably as the weather changes.  We know that as we go up in altitude, pressure drops.  We know that if temperature drops, the pressure increases.

So when computing density altitude, we may be at an altitude of sea level but if the temps drop and the barometric pressure increases, then we might have a theoretical altitude that is below sea level even tho the actual elevation is zero for sea level.

From an aeronautical aspect, if we have an airplane sitting on the run way at 5000' of altitude and the temperature is 95 degs and the barometric pressure is lower than expected for that altitude, the aircraft may need the power to get off the ground that it might normally require at 7500' (made up those numbers-did not look for a formula).  Further, depending upon the total weight of the plane and cargo, it might need a one mile long runway to get off the ground instead of 4000' in normal circumstances.  I have been on planes where they started throwing cargo off because they were afraid they would not clear the mountains off the end of the runway.

In cars, we know that colder temperatures increase the air density which means we have more air molecules in a cf of air than we do on a hot day.  We know that higher pressures pack more air molecules into that cf as well.  The engine and the turbo are air pumps.  How much air that gets pumped with a given combination depends on air density.

I have seen strips that the density altitudes were below sea level when calculated instead of the true surface altitude of several hundred feet.

Humidity is also a factor.  The more moisture that is in the air, the less air molecules we find in the cf of air.  We want cold, high pressure, dry air for racing.

A few years ago, I went to the strip that used to be about 30 miles down the road and it was 105 when we go there.  I was helping a guy with a pro-mode vette.  It ran an 8.80 about 3 in the afternoon.  When I winged the throttle, I could hear it blubber and I told the guy that we needed to rejet the carbs but he was insistent that the car was tuned well and had run well a few months before.  At 8;30 that night, he made a pass and ran somewhere in the 8.40s.  It was about 75 degs then.  Sounded like a totally different car.

That was density altitude at work.

So to compute it, we need a weather station to give us air temp, pressure, and relative humidity plus a calculator to grind out the theoretical altitude.  I am pretty sure pilots have a programmable calculator that makes it easy, but Jason can tell us more about that.  I am also sure, you can find an app on the computer to plug the numbers in as well

edited to add, I should have waited for Jason!  LOL

[Pyro6]

Is there going to be a quiz tomorrow? I better study

[Scoobum]

Thanks so much for the write-ups guys. Temp/humidity was the same as last week...so I added 5 percent high gear fuel to play it safe. Wasn't enuf in that air last nite...hence why I need to learn. Knock gauge turned red out of nowhere with about 50-100 feet before the traps. Let out of the throttle and yelled an expletive that almost blew the windows outta the car. No milkshake and the BLM's remained 118 at idle...so I think I dodged a bullet. I was eager to get out on the track and practice launching the car for next weeks bracket races. If I had of waited I would have witnessed the carb'd cars popping...and I woulda been tipped off with the air and added the full 20 percent of high gear fuel. Once again...thanks for the write-ups guys.

Jason...thanks for taking my call last nite. Had to take a walk and calm down before calling. I was pissed at myself for fucking up. I should know better.

[good2win22]

Been thinking how explain this a little better. 


Cooler and dry air equals more power.  Warmer and higher humidity equals less power. That being said, if there is a high pressure system in the area, you can still make power on those warmer days if the humidity is low. One thing not mentioned before was the term dew point. This is a set of temperatures. The top number is the current ambient temperature. The bottom number is the current temperature it would need to be to have 100% humidity. The closer the numbers are the more water vapor is in the air. The further the numbers are apart the less water vapor is in the air.


As Steve mentioned, which he did a really good job explaining, low pressure and higher temps has the ability to retain more water vapor because the air molecules are less dense.  Cooler temps and high pressure retains less water vapor because the the air is more dense.


Let's say we are on the ground located in Death Valley which is below sea level by let's say 100 feet because I don't know what the actual elevation is but it's 105 F and there is a low pressure system over head. There is 10% humidity. Barometer reading is 29.86. Our performance would be equal to running at 1500 feet above sea level.


Now we're back at Death Valley, temp is 65 F, high pressure system in the area, barometer reads 30.02. Still 10 % humidity. Now our performance would be like running 500 feet below sea level.


Hop that helps understand a little better


Here's a link to a density altitude calculator. https://wahiduddin.net/calc/calc_da.htm

[Scoobum]

Jason/Steve. Spose to rain tomorrow so I'm gonna go over all this info tomorrow and digest it. Once again...thanks .

[reality]

Brad said;  I was pissed at myself for fucking up. I should know better. 
[/size]
[/size]AHH Why do you think you are different than EVERYBODY else?
[/size]
[/size]Murphy treats everybody equally.
[/size]
[/size]And it's only going to get worse as you age more.

[TexasT]

This is why running a car at the ragged edge might produce the quickest times but when things aren't the same as last time or in the spring/fall last year, catastrophes can happen.
Good you got to retain the hgs this time . I like to keep it on the fat side as here the temps dip at night and you can find your self on the short end of having enough fuel going in real quick.

[nocooler]

Cool air = more oxygen = more fuel needed. Turn down the boost and take a couple degrees of timing out and then sneak up on the tune. It's going to need a few numbers put back in high gear fuel. Keeping a log book is a good idea, after while you'll have a few different tune ups for different conditions.

[Scoobum]

I record every run with the temp and humidity with it. I've never run into air that good until October. Last nite caught me off guard. Gonna do a search tomorrow on hand held weather stations...and read up on what the boys posted.

[Steve Wood]

If you are bracket racing, you need a weather station and a log book for sure!  Small portable weather stations are affordable.  the ones with the calculator built in are probably in the $200 range down here depending if you want the coffee maker option...I guess that means  $900 up there?  Things have really advanced since I bought one 20 years ago...

[Scoobum]

Bracket racing is likely coming to an end...as we found out last nite the sponsor is walking away. Thanksgiving weekend may be the last time the car goes down the track...except for the Armdrop events.

[Steve Wood]

sucks!

[nocooler]

I believe the dragtimes app will pull local weather and calculate d/a.

Maybe someone will sponsor a heads up class. We had a 10.0 heads up class that was always fun to watch, but lack of participation killed it. We have 3 classes at our track street, pro (no box) and super pro (box). My 2-step is counted as a box which would force me to run super pro.