rev limiter

[JimEli]

Thank you, this software has shown me that my dyno spreadsheet arrives at the same correction factor when using the same barometric pressure and temperature. My spreadsheet uses the same SAE 1990 standards. This is the first time that I have been able to verify the accuracy of my spreadsheet. Do you have any idea where I can find the formula for calculating the humidity correction factor?

The molecular weight of water is considerably less dense than dry air, and the humidity transforms moist air into a slightly different type of gas with a lower molecular weight.

The comparison of density altitude between completely dry air and 100% saturated air on an ISA sea level day is 0 vs. 221’. However, consider the same comparison done at 4000’ and 35 degrees, results in 7121 vs. 7923’ (about a 2.5% decrease in density ratio).

There is an additional factor to consider for engine performance, due to moist air displacing some O2 content. By my calculations, using the above examples causes the engine to experience (or “see”) equivalent densities of 582’ and 9268’.

The above comparisons are for academic purposes.

See “Performance of Light Aircraft,” John T. Lowry, and if interested I could give you the mathematical derivations of the above.

 

[alvin l nunley]

The molecular weight of water is considerably less dense than dry air, and the humidity transforms moist air into a slightly different type of gas with a lower molecular weight.

Tell me if I'm wrong; is water vapor, a gas like any other, heavier than oxygen or nitrogen? It's my understanding that it is, and it's the reason it displaces, near equally, oxygen and nitrogen. The amount of water vapor in the air, (humidity) is very precisely related to the temperature of the air. At 100° F, the air we breathe can hold a max. 4% water vapor. As it gets cooler, the air can hold less water vapor, and the curve is a bell curve, getting steeper as the temperature goes up.

Liquid water, or gasoline, is something near 700 times denser than dry air.

When I had my dyno, a friend gave me a correction chart for humidity. The correction chart for humidity shows a number so small, over such a wide range, that you can pretty much ignore it and still get very very close HP readings.

I haven't seen any dyno readings where the humidity correction factor was used. Barometric pressure/temperature have so much more of an effect on HP readings, and the correction factor for humidity is so small over such a wide range, it's pretty much ignored. At least in our world of Karting.

 

[Flash-o-matic]

Tell me if I'm wrong; is water vapor, a gas like any other, heavier than oxygen or nitrogen? It's my understanding that it is, and it's the reason it displaces, near equally, oxygen and nitrogen. The amount of water vapor in the air, (humidity) is very precisely related to the temperature of the air. At 100° F, the air we breathe can hold a max. 4% water vapor. As it gets cooler, the air can hold less water vapor, and the curve is a bell curve, getting steeper as the temperature goes up.

Liquid water, or gasoline, is something near 700 times denser than dry air.

When I had my dyno, a friend gave me a correction chart for humidity. The correction chart for humidity shows a number so small, over such a wide range, that you can pretty much ignore it and still get very very close HP readings.

I haven't seen any dyno readings where the humidity correction factor was used. Barometric pressure/temperature have so much more of an effect on HP readings, and the correction factor for humidity is so small over such a wide range, it's pretty much ignored. At least in our world of Karting.

You are correct sir !

 

[alvin l nunley]

When you're at the track, and your hot, and the humidity is high, and you're going slower, you would like to attribute that slowness to the humidity, but it's not the humidity, it's the low air density that's making you go slower. As the temperature goes up, and the barometric pressure goes down, the air density goes down. If you had an air density gauge, it would tell you why you're going slower.

 

[CarlsonMotorsports]

Great discussion on air density and it's value. Thanks to all involved; but I am still convinced that the fact that the OP's engine not hitting the rev limiter after half of a season of racing has absolutely nothing to do with the air density. If it had fallen off during the course of one night, I could possibly see it, or even on a rare night of extremely poor air; but we're talking months of racing later. Unless you could accurately predict the air density to be consistently lower throughout the season, (I don't see any particular reason for that other than a warming trend of summer months,) and how would you account for the "good air" summer days that he still isn't hitting the rev limiter?

 

[Jimbo]

rewilson64
When you find out what's wrong with it please let us know.
I agree with Brian. It ain't the air density!

 

[JimEli]

Tell me if I'm wrong; is water vapor, a gas like any other, heavier than oxygen or nitrogen? It's my understanding that it is, and it's the reason it displaces, near equally, oxygen and nitrogen. The amount of water vapor in the air, (humidity) is very precisely related to the temperature of the air. At 100° F, the air we breathe can hold a max. 4% water vapor. As it gets cooler, the air can hold less water vapor, and the curve is a bell curve, getting steeper as the temperature goes up.

Maybe this will help your understanding:
http://www.theweatherprediction.com/habyhints/260/
Run your own experiments here (I believe this to be accurate):
https://wahiduddin.net/calc/calc_da.htm

The curve is logarithmic, a "bell shape" implies a completely different shape to me. My calculations show that at sea level @ 59F, the difference between dry and fully saturated air is the equivalent decrease in air density for ~4.5F degree increase in temperature. However, as temperature and altitude increase, the difference becomes greater.

 

[rewilson64]

I cleaned carb, and set slide for a richer mix, put on new tires, and all is well, hitting limiter as before

 

[Jimbo]

That's good news.
Tires, handling and driving often solve what seems to be a motor issue.

 

[Elmer Patterson]

Thought so from the start as I posted But as always as post go with thought and discussions we all learn a thing or to . If nothing else it's entertaining. Thanks all

 

[alvin l nunley]

The curve is logarithmic, a "bell shape" implies a completely different shape to me. My calculations show that at sea level @ 59F, the difference between dry and fully saturated air is the equivalent decrease in air density for ~4.5F degree increase in temperature. However, as temperature and altitude increase, the difference becomes greater.

The green line is what I I was talking about. exponential. In that the line increases exponentially.

Wiki;The inverse function is the natural logarithm ln(x); because of this, some old texts[3] refer to the exponential function as the antilogarithm.