E T vs weather change

[Stan Weiss]

Originally Posted by Robert Simpson

On this question. If your car picks up when the humidity goes up and the temp goes down some, what does that reflect on the tune up? I am talking going from 103deg 40% down to 89deg and 70% and car pick up? Sorry to hijack the question.

Robert

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


Stan

[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.