E T vs weather change
 

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

Re: E T vs weather change
 

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.

Re: E T vs weather change
 

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??

Re: E T vs weather change
 

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

Re: E T vs weather change
 

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

Re: E T vs weather change
 

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


Stan

Re: E T vs weather change
 

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/

Re: E T vs weather change
 

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...

Re: E T vs weather change
 

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

Stan

Re: E T vs weather change
 

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.