E T vs weather change

[Lenny5160]

Originally Posted by Stan Weiss

Looking at % Humidity at different temperatures will get you in to trouble. It is called Relative Humidity for a reason. You want to look at vapor pressure or gains of water.


103 - 40% - 0.843184 vp - 126.26 gow
89 - 70% - 0.962109 vp - 144.66 gow

Yes! Relative Humidity can be extremely misleading. Use grains, vapor pressure, or dew point which are all pretty much the same thing but expressed differently.

If I know absolutely nothing about the weather other than grains/dew point, I can predict my Nova within .05. If you have all of your runs in a spreadsheet along with several weather measurements (I use an export from Crew Chief Pro) it is pretty easy to run a correlation analysis that will tell you which weather elements most closely follow your changes in ET. For my alcohol carbureted door car, Grains/Vapor Pressure/Dew Point are #1. HP Correction Factor is #2, just slightly better than Density Altitude.

To answer your original question, no it is not unusual at all for a 200 foot change in DA to impact the ET .02. I also agree with others that not all 200 foot changes in DA are created equally. That's the great challenge in ET prediction; there are all kinds of variables and they are constantly changing.