Humidity vs. jetting

Strick

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#1
Rich:

Can you enlighten me on the effects on jetting as humidity increases, and why? I live in the desert, but on rare occassions we do experience some humidity.

Thanks in advance!
Strick

I hope this is the correct forum. The why is the reason I posted here
 
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#2
The effects of humidty on [HASHTAG]#jetting[/HASHTAG] tend to vary with temperature. High humidity with low to moderate temperatures (say below about 50) will tend to make the engine run a bit rich, but the effect is pretty minimal. While high humidity at high temperatures will cause rich running and a loss of power.

Why a difference? It has to do with the saturation pressure associated with specific temperatures. Basically when water is a vapor in the air it exerts a pressure that becomes part of the total atmospheric pressure. So lets say it's your basic Chicago day in July it's 100 degrees (F) and it's 100% relative humidity . Because we are at 100% humidity it means we've reached the point where no more water will evaporate into the surrounding air (i.e the saturation point. That water vapor in the air will exert pressure (it's a gas now) of 1.93" hg. So if we have a barometric pressure reading of 29.00, only 27.07" hg will be exerted as AIR pressure. The lower density air would require less fuel to hold our air/fuel ratio at best power 12.5:1

If we left every thing else the same but dropped the temperature to 60 degrees, the water vapor would only exert a pressure of about 0.5" hg, so our actual air pressure would be 28.5" hg. The available air density would be MUCH higher at this temp. Higher density air means we need more fuel to hold our air/fuel ratio at best power 12.5:1

For a given temperature, as the relative humidty goes up the air pressure we can use to make horsepower goes down.
Basic rules HOT and HUMID lean it out a lot, Cool and humid MAYBE lean it a little. But keep in mind as the air temp goes up the ignition delay period is reduced and engines become more knock prone, so tread lightly.

Keep in mind I generalized the hell out of this for the sake of clarity, but hopefully you see the basic connection. I really think this is easier to understand than it is to explain :)
There are standard charts for relating relative humidity and temperature to pressure for those of you who want to play around with this stuff. A lot of good drag and road race tuners use baro, temp, & humidity to form a relative altitude (i.e pressure) value to tune from.

The ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) is a good starting point if you want to really learn the science of this stuff. http://www.ashrae.org/ . Aviation sources should have similar info.
 
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Strick

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#3
Thanks Rich, this is the type of explain I was looking for. Much better than the bench racing explanation - 'just cuz'.

I have been lost in the additional oxygen contained in humidity (water vapor). For some warped reason I saw that addtional oxygen molecule contained in water as more oxygen being available for combustion. Man was I off. Thanks again!
 
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#4
Something else to consider. Although the effects may be minimal, humidity will change with temperature. If it starts out cool in the morning, as it warms humidity goes down. The term relative humidity is a % of air saturation. As air heats up it expands, if the same amount of water vapor is present, %RH goes down. Dew Point is a more accurate measure of amount of moisture. It is the temperature at which the air will become saturated(can hold no more water vapor).
 
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#5
"Basic rules HOT and HUMID lean it out a lot"

Rich,

Your above post may have just save my sanity! I've been having all kinds of problems with my '96 Husaberg FE501E, from hard starting to just not very snappy. The bike ran great through the winter & spring. It's been really humid here in KC this year, temps in 90's, dew point 72-73 sometimes! After reading your post I went out & pulled off the ol' Dellorto. I went from a 260 needle jet to 256 and took the pilot jet from 40 to 33, left the 180 main alone. Runs great compared to where it was, the snap is back!! All I had was a 33, so I may get a 35 as it feels a little lean on the bottom now, but not by much. Man, this is the same setup I ran at 9000' in Colorado earlier this year, I can't believe there's that much difference with the heat & humidity! I am going to order some Dell'horrible parts today and try some different needles and get a few more jets too. My last bike was 650L with a CV carb, so all this carb dicking in new to me.
 
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#6
Cool. I'm glad the thread helped out. We've had similar effects here in Chicago. It's amazing how little effect humidity will have at lower temps and how profound the effects are as the temperature climbs. I guess it's part of what makes tuning fun :)
 
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#7
Now that it's warming up this might be worth bringing back up to the top.
 
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