Water Injection

[John Hofstetter <hofs@domain.elided>]

>George, 
>
>I'll throw out one possibility why your engine might run hotter when it's
>humid.  
>Assuming all other atmospheric conditions are the same, there are less
>oxygen molecules in a given volume of air when it's humid than when it's
>dry.  Water molecules are much larger that O2 molecules, and take up a
>lot of "space" in a given volume of air.  With less O2 molecules
>available in the air on a humid day, your engine has to work harder to
>draw in a larger volume of air to produce the same amount of power as it
>would on a dry day.  Harder working engine = higher temperatures. 
>If you would like, I can dig through my pilot meteorology or
>reciprocating engine books and try to give you a more detailed answer.  
>
>Mike Ryan
>Pilot Instructor, America West Airlines, Airbus A320
>Gilbert, AZ

On the other hand, George and Mike, most of us have experienced our 
engines running better when it is raining. Years ago, when I was messing 
around a lot with flat head Ford V8's, I devised a water injection system 
that essentially made my Fords run as if it were raining. Then about 10 
years ago, I bought an Edelbrock Water Injection system and put it on my 
Terra. The Terra ran much better with the water being injected by the 
computer. No question in my mind that the water injection increased 
power, eliminated pinging, and generally helped the engine run well.

John H.