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Discussion Starter · #1 ·
I am running my Golf at Cecil County Drag way tonight (black 4 door 2.slow). It is going to be hot and humid so I may not beat my old time but I have to try before I turn the car in.
I know the heat hurts your performance by reducing the density of the air and a given engine may be more likely to knock due to the higher temps so timing may be pulled. Would extra octane help in the heat? I normally run on 87... and higher octance only hurts performance on a stock 2.0 normally... but I don't know what the effect will be in 90 degree heat.
How does the humidity help or hurt? I know higher humidity may actaully reduce the chances of an engine knocking.... but how might it hurt?

If you are there I am the geek with the Team 2.slow sticker and the 17 second terror sticker
well... not they are not really stickers.... more like pieces of paper from my computer that I printed on......


[Modified by Golf 2 Slow GLS, 4:25 PM 8-2-2002]
 

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Re: Humidity, friend or enemy? (improvius)

Humidity is very bad for performance. As it was mentioned, water vapor takes up space. Oxygen is key to making power, any atmospheric condition which lowers the amount of oxygen (heat, humidity, elevation, pressure) reduces HP.
If you want, get an air density gauge someday. It will give information on the "quality" of air at the track. You can use it to assist you in determining whether the car is getting faster/slower or if the track is faster/slower.
 

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Re: Humidity, friend or enemy? (Golf 2 Slow GLS)

Andy & Maxx are correct ... liquid water is heavier, or more dense, than air. But, the water that makes the air humid isn't liquid. It's water vapor, which is a gas that is lighter than nitrogen or oxygen.
Compared to the differences made by temperature and air pressure, humidity has a small effect on the air's density. But, humid air is lighter than dry air at the same temperature and pressure.
Humidity effectively lowers air density and raises Density Altitude , which makes your engine feel as if it were operating at higher altitude, in thinner air. Bad.



[Modified by Tornado20v, 5:23 PM 8-2-2002]
 

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Re: Humidity, friend or enemy? ([email protected])

I have not had chemistry in a while.... but I do have a minor in it. I believe you need to use electricity do break down H20 into hydrogen and oxygen (electrolysis). I don't believe it will break down in the combustion chamber. I think it comes out the exhaust as water vapor. But my understanding is that it does effectivly reduce detonation because it slows the advance of the flame front (I need to look up what that is really called).
 

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Re: Humidity, friend or enemy? (Golf 2 Slow GLS)

Water is a byproduct of any hydrocarbon combustion, that's why you see water or (on cold days) steam coming out of your tailpipe.
For the reasons already stated, humidity is BAD for performance.
 

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Discussion Starter · #12 ·
Re: Humidity, friend or enemy? (kewl20v)

An increase of absolute humidity of 1.0 g water/kg of dry air lowers the
octane requirement of an engine by 0.25 - 0.32 MON
Yes, water is a byproduct of combustion but what does that to do with humidity? We are talking about two different things.


[Modified by Golf 2 Slow GLS, 5:55 PM 8-2-2002]
 

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Re: Humidity, friend or enemy? (Golf 2 Slow GLS)

If combustion breaks the hydrogen/oxygen bond in the water IN humid air, where more water is present...then you've released more oxygen into the air mixture (albeit a little) and more is better than less.
 

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Re: Humidity, friend or enemy? (Golf 2 Slow GLS)

Found this on the net:
"Another variable that affects the performance and octane requirements of your racing engine is humidity and barometric pressure. Low humidity or dry air will lean out the mixture, requiring a higher octane gasoline to prevent detonation. While high humidity will lower the octane needed to prevent detonation because the added moisture in the inlet air will cool the mixture making it more dense or richer. The ideal air/gasoline ratio at sea level is 15:1 at 60°F - 50% humidity and a barometer reading of 29.92 inches of Hg. This is referred to as the Stoichiometric air-fuel ratio which is the exact ratio required to completely combust a fuel to water and carbon dioxide. Fluids are directly affected by pressure and temperature, and air is a fluid just like gasoline. Therefore temperature, humidity, and barometric pressure all influence octane requirements."
I guess the water helps cool the mixture because it has a high specific heat.
 

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Re: Humidity, friend or enemy? (Jman5000)

drier air is better. even though h2o has oxygen in it, that hydrogen is still taking the space that more oxygen could be taking.
the real question should be... how does humidity affect our I/C's performance...
 

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Re: Humidity, friend or enemy? (John A)

How does the hydrogen take more space? Hydrogen atoms are very very small. Heck, I am still not convinced that the water vapor in the air is converted into 02 that the engine can use.
I am not trying to sound like I am a know it all.... I just would like someone to explain things to me if I am wrong.
 

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Re: Humidity, friend or enemy? (Jman5000)

What Tornado said is exactly correct. Humidity IS BAD(in terms of engine perf). I'd bet my life on it. You need 02, not just 0 as part of some other compound. If nothing else, just think in terms of density. You know that the more dense the air, the better right? Humid air is less dense than dry air. If you want you can figure this out by taking the atomic weights of all the elements. Basically the water vapor is taking up the space that would allow more air in that same space. You can look this info up. This was discussed with the water-injection talk. I'm touching that subject, but the above is true.
 

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Re: Humidity, friend or enemy? (Golf 2 Slow GLS)

from:
http://content.honeywell.com/uk/air-quality/air_we_breathe/air_we_breathe.html
"By volume, the air we breathe consists of approximately 78% nitrogen, and 21% oxygen, together with traces of argon, carbon dioxide and other rare gases."
there's not a lot of oxygen in the air that your car takes in... do you want more hydrogen in there taking up the space that oxygen could be taking up?
also:
"Moist air is less dense than dry air ... Under high humidity conditions, the air density may be reduced by as much as three per cent."
from: http://www.copanational.org/non-members/safety/safetyPPSept99.htm
less dense? and less oxygen? no thanks...
 

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Re: Humidity, friend or enemy? (Jman5000)

Looks like people are jumping all over this one.
quote:[HR][/HR]If combustion breaks the hydrogen/oxygen bond in the water IN humid air, where more water is present...then you've released more oxygen into the air mixture (albeit a little) and more is better than less.
[HR][/HR]​
Even if that were true, if it happens at the combustion process, isn't that a little late as it's on it's way out the exhaust at that point? If it were broken down further, you wouldn't get water/water vapor from the exhaust.
 

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Discussion Starter · #20 ·
Re: Humidity, friend or enemy? (Redline18T)

Ok, so we have 3 things figured out here.
1) High humidity lowers your octane requirement. It does this by cooling the combustion chambers temperature due to its high specific heat.
2) Even if the 0 in H20 vapor was able to be converted into 02 during the combustion process (which I don't think it is), it would be too late to increase power because combustion would have already taken place and it would be expelled in the exhaust. If the water vapor would turned into 02 and hydorgen, in theory fires would burn hotter in humid conditions. This is not true. I believe water vapor passes through the combustion process unchanged.
3) Humidity (water vapor) takes up space in air (may be negligible) and in theory, makes the air less dense and therefore reduces power.
[Modified by Golf 2 Slow GLS, 8:20 PM 8-2-2002]


[Modified by Golf 2 Slow GLS, 8:23 PM 8-2-2002]
 
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