gpatmac

iTrader: (3)

We do not tend to think much about humidity until our clothes begin to stick to us and perspiration drips from us. This indicates that the air is saturated with water vapor. However, not only are there a lower number of oxygen atoms being dragged into the cylinders when the humidity rises, but there are also a much larger number than usual of water molecules interspersed between the oxygen and fuel molecules. The effect is that the space between the two increases, which slows the combustion flame speed within the combustion chamber because rather than progressing rapidly from fuel molecule to fuel molecule, the flame front comes up against water moleculesthat do not contribute to the combustion process. Thus the water suppresses violent combustion - detonation - just as an octane booster does by chemical means. This means that increases in humidity will often compensate for increases in ambient air temperature, which typically demands a 1.0 octane increase for every 13dg C that the mercury rises.
Conversely a big decrease in humidity and an increase in intake charge temperature or air temperature could see a race engine being destroyed by detonation if octane levels are not increased, or spark advance reduced and fuel mixture richness increased.
A rise in barometric pressure of 1" of HG calls for an octane boost of 1.0 octane, which alternatively can be compensated for by richening the mixture or reducing the spark lead. However, a steep rise in the humidity level decreases the degree of compensation required.
We determine humidity levels by taking temperature readings from two thermometer, one with a dry bulb and one with a wet bulb covered with water-soaked cotton wick. When both thermometers read the same temperature the humidity is 100%. However, if the dry one reads 25dg C and the wet bulb reads 24dg C, a depression of 1dg, then the humidity is 92%.

gpatmac

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IS2Scooby

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Gimme 5% humidity any day over 88% like they had on Saturday.
I don't care how much it helps resist knock, airborne moisture is displacing oxygen!

My theory for power = Suck in the most air possible (compress it then cool it) and use octane and a tailored ignition advance curve to resist detonation.
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Nolimit

iTrader: (1)

^^

gpatmac

iTrader: (3)

Well, that's my point.

You can't tailor the weather, and if you refuse to consider the weather then you have to accept the lesser times you're running unless, on a humid day when supposedly you're less knock-prone, you lean out your mixture a little bit to exptrapolate a little more power.

IS2Scooby

iTrader: (3)

What does humidity being an octane booster have to do with leaning your car out to make more power?

I think you have 2 sound concepts here.

1. Humidity in the air can help resist detonation.

2. Humidity content in the air reduces the amount of oxygen that can mix with the fuel (gasoline) in the intake charge reducing the potential energy.

Leaning the mixture out to regain lost power doesn't have much to do with #1, but everything to do with #2. (That's why carburetor carrying drag cars are frantically re-jetted to take into account the weather/ambient conditions on race day before the brackets begin).

On a very humid day you CAN increase timing tentatively without knock - but that's pretty much just getting you back to square 1 after taking a hit in performance from the air-starved rich mixture being injected into the cylinders.
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IS2Scooby

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I think this discussion is Hawaii-centric because of the humidity angle but I bet Joey or Alex is gonna have to move this to a tech section soon.
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gpatmac

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It's already there. I double posted.