E T vs weather change

[Kdw1403]

Lt 1 stocker. Is it possible for a weather change of 200 ft of density altitude ( worse) to cause a 2 hundredths slower ET. Every 100 ft = .01 et change. My air fuel ratio is 12.88 to 13.1 most of the time. Is this possible

[Jeff Stout]

I'm not an LT1 guy but yes it's possible. When I started tracking I was 110 ft for every .01. Right now I'm at 160ft for every .01. Makes for easy dialing throughout the day. I tuned best performance on a bad weather day and then on good weather days car was lean and would run similar ET. All this is a bracket 400sbc with Holley.

[Adger Smith]

It all depends...
What caused the density altitude to change?
Baro, temp or vapor pressure.
Vapor pressure (grains of Water) can really sneak up on you.
A good prediction program will show you it is not all DA...
Yes, it could..to your question

Stan where are you with an example??

[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

[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

[Stan Weiss]

Originally Posted by Adger Smith

It all depends...
What caused the density altitude to change?
Baro, temp or vapor pressure.
Vapor pressure (grains of Water) can really sneak up on you.
A good prediction program will show you it is not all DA...
Yes, it could..to your question

Stan where are you with an example??

Adger,
Different cars will react differently. Don't know if this will help or not.


Stan

[Andy Friar]

As we have tracked runs on our TBrake equipped low 11 sec SBC Chevelle we are seeing close to the 100'/ .01 change. Not changing setups, parts or routines was key to getting 'good' data for us. We currently use at track calculated DA and wind speed/direction at the start of the track. Which driver was in the car as I am about 50# heavier than Dad, along with tires pressure, launch and shift rpm and engine and trans temp at turnout of the lanes. Looking to add a data recorder this winter to dial in our program as well.

Here is a link to a pretty good article on the topic from Hot Rod. I will give that Hale's Jr Website a go sometime soon to hopefully gain some more insight.



https://www.hotrod.com/articles/ccrp...ensity-tuning/

[Adger Smith]

Right Stan.. Combustion chamber design and flame front cause some combos to be good air engines or engines that fall off a lot on bad air days.
In the years of Dyno testing I have also noticed some combinations respond differently as to where the tune up is in the lean rich window.
That is why we test on the track...

[Stan Weiss]

Adger,
Have you found that some engines up to a point actually like some 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.