What has a bigger effect on ET, Temp or Humidity?

[bitemark46]

Which do you think has a bigger impact on your times? -Mark

[Strokd85]

I would go with humidity..

[HybridSS]

I will go with temp....but need to quilify that answer.

For us on an avg day humidity will be a huge factor.

However we are almost always going to be in the 40%-100% range of humidity. Which of course will have a great affect on power from lowest to greatest.

But temp on the other hand has a much wider range of possibilities. Its possible we could see 20 degree days out on a very cold day...and as much as 110 degrees or more on a hot day. The HP difference would be HUGE between those two days.

For instance: Take a car that makes 300 hp to the tires on a 70 degree day with 60% humidity. Use that as our standard day.

Run that car at 70 degrees with 100% humidity and it might make 285-290. Run that car at 70 degrees with 40% humidity....it might make 310. Thats a 25 hp split.


Now run that car at 60% humidity on a 110 degree day...it will be way down on power. ID say in the 275 range uncorrected HP.

Again run it at 20 degree weather. It may make 325 uncorrected.

a 50 hp swing.

Again...it seems the huge spread that can be had with temp would be the bigger factor.

BTW...none of these figures are real or conclusive...just thinking out loud how It might be affected.

[Suicidal Racing]

i'm going with temp.
if its real hot out its going to make the track hard to hook up on and if u cant hook then u got problems,
just look at your 60ft times and how much of a diff it makes in your e.ts when u can hook and u can't.u spend most of your time in the first 60ft of the track then anywhere else.
and nos cars have their temp in that little old bottle.
now on the other hand humidity kills the power of a blown car.

[bitemark46]

I always took this in a metoroligist perspective. I thought when the temp is hotter there is less oxygen molecules in the air thus less air in the cylinder chamber, less power, etc. That's why I always run good(who doesn't? ) when the temp is cooler. More oxygen in the air. But the other question I had is usually when the temp is hot the majority of the time the humidity is high. Right? That's why I always try to run in the morning. Less humid and temp is cooler. Now how does barometric pressure play into this? I always see guys measuring that but I don't really know what that means or how to read it. -Mark

[HybridSS]

Well..looking at the barometer will tell you alot too. That is the actual local pressure...which fluctuates up and down quite a bit.

What you really need to look at is density altitude. That is the figure that combines the local pressure and temperature along with dew point. Relative humidity can be inferred by the dew point and actual temp.

Density altitude probably is what alot of drag racers use since it takes everything into account.

http://wahiduddin.net/calc/calc_da.htm

Here is a calculator that just enlightened me alot on which one is more important. Its a Density altitude calculator. Put in values for temps, actual altitude, barometer (altimiter), and Dew point (humidity).

here is a little something to help understand humidity VS dew point:

When the dew point temperature and air temperature are equal, the air is said to be saturated. Dew point temperature is NEVER GREATER than the air temperature. Therefore, if the air cools, moisture must be removed from the air and this is accomplished through condensation. This process results in the formation of tiny water droplets that can lead to the development of fog, frost, clouds, or even precipitation.

Relative Humidity can be inferred from dew point values. When air temperature and dew point temperatures are very close, the air has a high relative humidity.

So in the calculator...put the dew point temp at the same point as the outside temp and that should be 100% humidity if im understanding this right.


The calculations that are provided from the info you put in is very useful. Looks like one of those is relative density. I think this value would be very helpful in calculating how much power is possible on a given day. Since its relative density...Im going to assume this is based on the universal standard of a "standard day" and the differences from that standard day.

Standard day is defined as:
29.92 inches
59
0% humidity


Anyways...check out the calculator a bit and play with the numbers. Very informative. Especially the relative density...you could probably use that figure to calculate expected HP. The higher the relative density...the more the power. The lower...the less power.

[bitemark46]

Damn. I didn't think it was that in depth. Look like I got alot of reading to do. Thanks AL. -Mark

[HybridSS]

Did some calculations

maintaining 29.92 barometer, at 100 degrees with almost 0% humidity density altitude is 2598 feet and relative pressure is 92.62 %

same barometer and same temp of 100 degrees with 100% humidity will get pressure alt of 3407 feet and 90.41% relative pressure. A difference of 2.21 % from high to low conditions of humidity.

Now at the same barometer of 29.92 and 0 degrees F with 100% humidity density altitude is -4164 and relative pressure is a whopping 112.77 %...awesome racing conditions even though the humidity is 100%.

you could expect your engine to make 20% more actual HP from the first example to that last example:o

[HybridSS]

Originally posted by bitemark46
Damn. I didn't think it was that in depth. Look like I got alot of reading to do. Thanks AL. -Mark

the only reason I looked into it was because we use alot of this info during power assurance 10 point runs on the jets here at work. But we always rely on charts and tables. And we have real problems sometimes hitting power mins on a very hot day.

Now I know why.

[Strokd85]

hrm. I would've figured humidity, with water not being able to be compressed and displacing the oxygen molecules... looks like I need to read up on it more myself...

[HAVOC]

Very interesting topic! thanks for sharing the info.