Track Tac

The myth about humidity and its effects on horsepower.

alvin l nunley

alvin l nunley

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http://wahiduddin.net/calc/calc_hp.htm
this will take you to a calculator where you can put in the air temperature, the altitude, the barometric pressure and the relative humidity and get a HP correction number. Making large changes in the percentage of relative humidity, versus air temperature, might surprise you.

Whenever you run a dyno test, a good piece of software will have correction factors to correct the ambient conditions to a known constant. If the barometric pressure and the temperature are different from one test to another, you have to use a correction factor to bring the numbers to a known constant. If your software doesn't correct for the ambient conditions, it's worthless.

If it's important in the dyno room, what makes you think it wouldn't be just as important at the track?

This bit of software will show you how small an effect relative humidity has on corrected horsepower.
 
It appears few are interested in this subject. LOL. I will include 2 pictures of the calculator. One with 50% relative humidity and the other with 80% relative humidity. Notice that there is only .9% difference in the horsepower calculation number. And this is without changing any of the parameters

And the fact that the air density will probably be be down with high humidity, the difference is even less
 
Al, I think there's a lot of tongue-biting going on in the background. This topic has come up before and yes, grains of water in the air are still the most critical aspect in tuning, methanol more so than gasoline. The internet widget calculator doesn't change that one wit. The pro's are going to continue to watch their weather stations and monitor the amount of moisture in the air.It's as much a myth as physics.
 
alvin l nunley said:
I know one thing, long-held beliefs are not easily changed, even with overwhelming proof of their invalid nature.
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With all your expert advise you should write a book on dirt oval tires, clone engines, Stagger, ride heights, ect. Please show us the sales volumes when you do write the book!
 
Al, that's about a tenth of an hp on a 10 hp motor.
Do you realize how much work it takes to get a tenth hp advantage on a comparably prepared industrial 4 stroke at that power level?
 
Bob Evans said:
Al, that's about a tenth of an hp on a 10 hp motor.
Do you realize how much work it takes to get a tenth hp advantage on a comparably prepared industrial 4 stroke at that power level?
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And that's huge in this business...........and does make the difference between winning and losing.
 
Good luck seeing the temp & barometer the same with only the RH different. Heck, right now, this time of year we see a 20 degree swing from morning to afternoon or afternoon to early evening and a 30 point change in humidity during the same time (65% to 95% today). If you punch that into the calculator it's a 6% difference. I use my air density meter more than a weather station.
 
Bob Evans said:
Al, that's about a tenth of an hp on a 10 hp motor.
Do you realize how much work it takes to get a tenth hp advantage on a comparably prepared industrial 4 stroke at that power level?
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and you're right, .1 horsepower difference when the relative humidity goes from 50% to 80% (not a really likely occurrence) at 76.9 ambient temperature. And that's assuming that the temperature and the barometric pressure stay the same, while they mostly don't.

In San Jose California, where I had my dyno, I've been in my dyno room at 8 AM and the relative humidity read 80%, while the temperature was around 65 degrees. As the temperature went up towards noon, to about 85°, the relative humidity went down. While the vapor content of the air remained constant, the change in temperature changed the relative humidity number. Relative humidity is the amount of water vapor in the air compared to how much it could hold at that temperature. If the water vapor content of the air doesn't change, as the air gets warmer, the relative humidity number goes down.

I wish somebody had an air density gauge and could tell us how much it changes from the late afternoon to when the sun sets and it starts getting cooler. I've seen it go from 96% to 106% at a track in Southern California.
 
SKHMotorsports71 said:
Just don't see it...Tires are what will out run that 1/10th!!!
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For a 1 mile long sprint track and running 15 laps it is huge!
That's why so much effort is going into trying to get every bit of power out of the LO206 program for sprint racing now....but of course this always was important...lol
 
Jamie Webb said:
Good luck seeing the temp & barometer the same with only the RH different. Heck, right now, this time of year we see a 20 degree swing from morning to afternoon or afternoon to early evening and a 30 point change in humidity during the same time (65% to 95% today). If you punch that into the calculator it's a 6% difference. I use my air density meter more than a weather station.
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not questioning your numbers, but to reach 95% humidity, it's got to get real hot, and when it gets hot, in almost all cases, the air density goes down. HP is much more dependent on air density than it is on humidity.

I can do the same calculations, not changing the temperature or barometric pressure, just the humidity, and get a much smaller percentage change.
 
One thing it overlooks which might be relevant to two strokes is the cooling effect due to additional humidity.
I think what throws people off is that the effect of relative humidity is not linear or direct in the manner that air density is.

I tell you what though, let's say for simplicity sake the % in the screenshots is HP. I'd rather be making 101.6HP over 100.7 anyday.
 
alvin l nunley said:
I wish somebody had an air density gauge and could tell us how much it changes from the late afternoon to when the sun sets and it starts getting cooler. I've seen it go from 96% to 106% at a track in Southern California.
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Pick from any number of aviation weather stations.
 
My last year of high school I raced a quarter-midget type vehicle for one season. I was on an inferior home-built chassis, but had a good motor, usually finishing top-5, but rarely won. My one win came on a night where we started at about 80 degrees and 90% humidity -- sticky day. We started the race at sunset, and there was a red flag for a flip on lap 15 of 25 for the feature. As we sat there the temp dropped about 20 degrees and a fog rolled in. I was the only one that had a remote carb tuner. I made a little adjustment and pulled them a whole straight on the start. (Mind you this is a 1/10 mi. asphalt oval...) They were just getting back to my rear bumper at the checkered flag...a dramatic win (for me), and all because I had a remote carb tuner... As said above, you don't get to cherry pick your variables -- God still controls the weather. I was glad I could make a change...
 
RadialFin said:
One thing it overlooks which might be relevant to two strokes is the cooling effect due to additional humidity.
I think what throws people off is that the effect of relative humidity is not linear or direct in the manner that air density is.


I tell you what though, let's say for simplicity sake the % in the screenshots is HP. I'd rather be making 101.6HP over 100.7 anyday.
Click to expand...
those numbers are percentages, reread.
 
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