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%.

 

So, in essence I guess what he is saying is that you can precisely tune your spark, fuel, and boost, but in failing to take the ambient pressure, temp and humidity into account; if you have tuned your car to the ragged edge, you could be jeopardizing your engine.

 

I bet that he read the post about water injection that was posted the other day, (yesterday?) and was thinking about it... But he did give a very detailed explanation. Now everybody should know why the atmospheric conditions are posted on the bottom of a dyno sheet.

 

To be honest, it's directly out of his book.

Alex has been cracking down on us talking about tech issues in our Hawaii regional forum, and I wanted my friends to discuss it (since it gets so humid here). I just double posted.

 

yeah, i read about that also, i have the same book. interesting yeah!

 

I know a guy who actually had one of the water injection systems. I t did actually work.. Well was a band aid (like posted on the other post) for a distribuator problem. I saw his and made my own from a needle valve from a radio control airplane engine. Both of ours worked fine.. But both of us went through alot of exaust systems, the rust was the killer

 

Shoot, I never thought about that. Hmmm.

 

You plumb the water injection AFTER the air filter.

 

Wow.

So to the original issue by Mr Bell, does the thesis stand as he stated? More humidity = natural octane and so room to subtract fuel. Conversely, the humidity in the air means slower flame front?

I know that, along with what you said, Jehcpa, Mr Bell states that when folks artificially add water to the charge air, they run into corrosion of vital parts because they take the shortcut of not adding distilled water.

 

you're correct. i think that Mr. Bell is saying that humidity is like a natural way to supress detonation, however, like he states that if humidity decreases, and an increase air temp. or intake temp. will result in the ending of the life of an engine. on the other hand, water injection is the ability to subtract fuel and add more boost based on the fact that the water is doing what the extra added fuel, or rich a/f ratios is basically doing is cooling the piston and keeping heat and pressure in the cylinders down. now, water injection is just another alternative to keeping heat to a minimum in the combustion chamber, therefore, instead of running richer fuel conditions, the water acts as the heat suppressor so you don't need to add more fuel, hence subtracting fuel and keeping your IDC down. so basically, if you have a very good water injection set-up, you can run smaller injectors and more boost because you will be using less fuel.