How important is humidity.

rocketman

Member
I used to run a Jet Dragster and Humidity meant a lot. I gave a higher false reading on air density, but the heavy air gave you something to push against, like having a backstop on the starting line.
 

alvin l nunley

Site Supporter
I used to run a Jet Dragster and Humidity meant a lot. I gave a higher false reading on air density, but the heavy air gave you something to push against, like having a backstop on the starting line.
is it any wonder how people believe the stories about humidity. When whoppers are judged, this one is going to have to be right up there with the best.
 

paulkish

old fart
I ran some calculations with 50% relative humidity
Shouldn't you be using absolute humidity or better yet specific humidity?


edit: ... :) And everyone knows humidity refers to water vapor a gas, so how and why would you expect to see anything different then .01 of change? Or is this a trick question?

edit again: Next I suppose you'll be asking us about how much humidity effects a rainbow? ... :)
 
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Pete_Muller

Moderator
there's a calculator here that will show you how little humidity affects HP. I ran some calculations with 50% relative humidity to 90% relative humidity in the correction factor only change .01.
Al,

Try re-running your test at 110F and see what happens. (fwiw: I've raced in Hallet, OK in the summer when it was 110F and 98%RH). Air density takes a very significant hit when high temp is combined with high humidity.

This chart might actually be more useful as far as visualizing what's going on:

water_vapor.jpg
 

alvin l nunley

Site Supporter
even without the humidity as a factor, I have to believe that the biggest effect on HP would be the air density. If, from 50 to 90 it only changes .01, 9 more points not to make much difference. But I do get your point, those are miserable racing conditions!


if
 

Pete_Muller

Moderator
Al,

As you can see by the chart, those are not straight lines. The amount of water vapor that air can "hold" does not go up linearly with temperature, which is why air density takes a very significant nosedive at high temp and high humidity.

PM
 

paulkish

old fart
I think what most relate to humidity is when humidity is a factor to make a fuel change when in the evening "Good Air" is felt.
It's the "Good Air" feeling which usually goes hand in hand with a slight increase in humidity which makes the difference.

You can nit pick that relative humidity is not a factor because it makes such a slight change but humidity is just one part of the "Good Air" feeling racers get. If it's not anything but a very minor factor then why care about it when the density gauge tells the real tale?

That is unless your interest is only in banter back and forth.
If it's a non issue and you know it then ... why?

Maybe only racing out west your not aware of cool evenings when the water comes up and you get "Good Air"?

The feeling is usually worth at a minimum a pill/jet change if not also a gear change.
And if you don't you usually won't keep up at least as you did before.

Al, racers are not changing fuel mixture because of humidity, there doing it because of the experience change to "Good Air".
 
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alvin l nunley

Site Supporter
I'm not getting my point across.
70° - 80° equals 0.01 change in the correction factor.
60% - 90% change in relative humidity equals 0.01 change in the correction factor.
 

paulkish

old fart
Is it about humidity not being a Major factor or about you thinking everyone else is dumb and stupid not being able to comprehend it.
An ah ha moment for me is usually a so what for most others. ... :)
 
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