The easy way to calculate air density.

[alvin l nunley]

Here's a handy little bit of software I found. Put in your elevation, the current temperature, the barometric pressure and the relative humidity percentage. It will give you the correction factor for the numbers you get from your dyno.

It will also show you how little humidity effects horsepower. To make a long story short, at 70 degrees, 100 foot altitude and the barometric pressure at sea level, a change from 40% relative humidity to 80% relative humidity, the correction factor only changes .012. A change of 10 degrees in temperature would have almost double that effect on the correction factor.

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

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)

 

[Steve Vermeer]

On methanol,the amount of moisture is VERY important and while relative humidity is a rough guideline, the grains of water is the single most important factor.Changes in air density often don't reflect this correctly.

 

[alvin l nunley]

On methanol,the amount of moisture is VERY important and while relative humidity is a rough guideline, the grains of water is the single most important factor.Changes in air density often don't reflect this correctly.

you should play around with that utility. Change the humidity percentage, a lot, and you'll see how little humidity affects the horsepower correction factor.

"the grains of water is the single most important" What is that? Do you have a link?

"Humidity" is water vapor, a gas like any other.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)

 

[flattop1]

When I watch the NHRA fuelers run .
They seem very interested in the grains of water.
That's enough for me to believe it is important.

 

[DirtyDan]

My Android app will lookup all the necessary info using gps and the nearest weather information for temp and humidity. Just open the menu item and there it is! Several other useful features: stagger calculator that considers degrees of banking, gear ratios, max speed, corner weights. Kartworx.

In beta now!
DD

 

[Steve Vermeer]

Computech has an excellent weather station,not cheap but very good.Check their website.The two factors that most of the methanol racers feel are critical are grains of water and OBSERVED barometric pressure.

 

[CarlsonMotorsports]

That's the one we used when we travelled with the sprint car, Steve. Very accurate and I thought it was reasonably priced - especially considering the cost of melting down a $50,000 850 HP sprint car engine. Much more accurate than simply going off of an air density gauge alone. In it's simplest format, it gives you Density Altitude, considering a number of variables, as a single number to work off of and makes safely fueling the car pretty simple. Used with a good notebook, it's really helpful. I still use it in conjunction with our dyno here in the shop even though I have other equipment that does basically the same thing -- the duplicity is a good confirmation that what I am reading is correct.
Al's link is a neat little resource and free for the those who don't have access to dynos and air fuel ratio meters, etc...or even to those who do.

Thanks,
Brian Carlson

 

[alvin l nunley]

Computech has an excellent weather station,not cheap but very good.Check their website.The two factors that most of the methanol racers feel are critical are grains of water and OBSERVED barometric pressure.

I can't believe that's true! If you look at the correction factors, you will see that they include barometric pressure, temperature and humidity. Humidity has the least effect on horsepower of the three. If you look at an air density gauge, which many use, you see that it does not include humidity as a factor. The thing about humidity is, if the water vapor content of the air doesn't change, say from 50 degrees to 80 degrees, the relative humidity number gets smaller. The amount of water vapor, (a gas like any other) the air can hold, (absolute humidity) goes up in a bell curve depending on the temperature of the air. 50 degree air, even at 80 percent relative humidity, is holding very little water vapor. Relative humidity is the amount of water vapor in the air, as a percentage, of the amount of water vapor the air could hold at that temperature.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)

 

[Steve Vermeer]

Engines don't give a hoot about what we think or believe,they want what they want.Once you clear away the other "zamma zamma" technology,those are the two things that the engines are MOST responsive to.

 

[tom rickard]

I think this article describes humidity and tuning well: http://racecarbook.com/articles/humidity.shtml

This is what the author has to say about water grains:

WATER GRAINS: This is a recent value that is circulated in conversations around many of the Pros. It is a measure of the amount of water in the air. One pound of water is 7,000 grains. At 90 deg. F and 50% relative humidity, there are approximately 107 grains of water per pound of humid air. A value of 107 grains in a pound of air is basically 107 / 7,000 grains per pound = 0.015 pounds of water in a pound of humid air.

Sparing the details, that is a little over 8 grains of water per cubic foot of air. A 400 cubic inch race engine at 100% volumetric efficiency is passing 926 cubic feet per minute of moist air at 8,000 RPM. That would then include over 7,500 grains of water per minute. That is over one pound of water per minute or over one pint per minute. The water vapor is in place of air with 23% power making oxygen for combustion. That represents the air density change of over 2% from humidity that is an influence on performance.