For the past 15 years or so, I have been using the Density Altitude reading from my weather station to help predict what the car will run. For the most part, it has been a reliable input, (of course I factor in other variables beyond the air).
This past Saturday race began with 2 time runs in late afternoon/early evening, (5:45 & 7:52 PM). Then eliminations began, I ran Rd 1 at 10:13 and lost Rd 2 at 12:24 AM.
Even though the temperature steadily cooled, (relatively speaking - It is summer in TX, so temp was still 81.5* when I got trailered), the DA did not change all that much from 1st pass to last. Total difference was less than 500' - It steadily dropped from 3629' down to 3140'.
Here is the crazy thing - My car slowed after dark, with lower DA and cooler track temp. Not a lot, not even enough to make me think something was wrong. But I really thought it should have picked up a number or 2. Wound up running same ET last pass as 1st pass, within 6 thou.
Not that I should be complaining about that, but normally when the sun is off the track for 2 hours, and DA is 400' better, I can dial down a couple, still carrying what I normally do.
So I throw down a .004 light, but my opponent went .000 and Dead 2, and my "carry pocket" was empty, it made for a short night. Not saying I could have managed to pry that tiny window open a little bit more, but with no spray, and no taters to toss, I had no tools in my box at that point.
After analyzing every useful bit of data I have, it has finally clicked that I may need to look at using Dew Point, or Vapor Pressure from my weather station. The temp dropped 8.4* from Pass 1 to 4. Barometric Pressure had gone up .06". But the humidity had soared from 57.2%, up to 78.1%. So the only logical conclusion I can draw, is that the increase in humidity hurt my car more than the cooler air and higher pressure helped it, regardless of the slightly better D.A. reading.
Again, this is on Methanol. Are any of you guys using some weather parameter other than DA as your guide? Which of the other 2 options (on my unit) do you think might be more useful, VP? or DP? (I will admit I know next to NOTHING about either one - Although I do have the option to predict using either one).
While doing my follow-up, I thought I would go back and look at the couple of runs I did log into the W.S. to see if it showed what the DP & VP numbers were. It does not, only shows them live while I am looking at the current weather conditions. I suppose if I were predicting based on one of the other parameters, it would show that in the run data.

Anyway, I will appreciate any and all comments.
Thanks guys.

This message has been edited. Last edited by: FootbrakeJim,

Relative Humidity going from 57.2% up to 78.1% may or may not represent an actual increase in the amount of moisture in the air.

You say the temp dropped 8.4*, but what are the actual temperature numbers? We can calculate the Dew Point using Temperature and Relative Humidity.

Hey Tony,
I think I get what you are saying - Could that have something to do with the grains of Oxygen content?

My old Altronics Performaire WS does not have the Oxygen Altitude (OA) option on it.
Here is the full set of weather data I logged for all 4 runs:

Run # Time Temp Hum B.P. DA
1 5:45 89.9* 57.2% 29.41" 3629'
2 7:52 87.6* 60.7% 29.37" 3524'
3 10:13 82.6* 69.2% 29.40" 3165'
4 12:24 81.5* 78.1% 29.43" 3140'

Oh, and to be honest, I don't know if the W.S. reads humidity in "Relative" terms or not.
(Not a meteorologist, and I did not stay at a Holiday Inn Express last night)
Thanks for helping me try to figure this out.
Same W.S. since 2003, it has been to Altronics for updating a couple times, but not in recent years. And I have only been running Alky for 4 years.

Dew Point for those 4 runs is:

1. 72.7
2. 72.3
3. 71.5
4. 74.0

Vapor Pressure is:

1. 0.811
2. 0.800
3. 0.777
4. 0.847

The DP (and therefore the actual moisture content in the air) did increase somewhat, but probably to much less than most would assume by looking at the Humidity % change.

Dew Point and Water Grains are essentially the same thing expressed in different ways, but both give you an absolute measure of water content. I suppose Vapor Pressure is also, but those small numbers don't stick as well in my brain.

As a rule of thumb, my 10-second alky-carbureted door car will move .01 with a change of 2 degrees of Dew Point or 4-5 Water Grains. Now, these things don't move in a vacuum, everything else is changing too. But I look at Dew Point first if I'm on the edge of a dial-in number or to see which way things are going to move, if any.

What you had happen is the really tricky part in all of this. The Baro and DA got better, the moisture indicators got worse. How much did each change, how much is each worth, do they cancel each other out or does one factor override the other? If the DA and the moisture numbers both say .01 quicker, are they just agreeing with each other or are they additive and you will pick up .02?

Tony,

Thanks for the thoughts, and for doing those detailed calculations.
The W.S. had been inching the predicted ET down a hair each pass. Normally I gain/lose about 5 thou for each hundred feet of change in the DA.
The ET prediction had gone down a total of .017 from 1st to last run. I went into that last round thinking I had 2 in my pocket, but actually had nothing. Knew I hit a good light. Was dialed 4 tenths slower, so didn't get to see his leave. Knew he is usually good, so only a teeny bit surprised when he drove around me, and backed into a perfect half-wheel. As your brain goes through all the estimates, calculations and options on the fly, the big red arrow was saying "he is on a breakout run, so are you, ya can't quite get the stripe back, do you want to push him into a double B/O and hope you were better, or send him at the cone?".
We all know sometimes in this sport even really good ain't good enough, He left me such a small window a mouse couldn't squeeze through it. And even if my weather station had more accurately predicted the effects of the change in weather, it might have just given me the opportunity to screw up differently.
But next outing I think I will begin logging the Dew Point and Vapor Pressure as well, and maybe even looking to see how different the predicted ET is for each mode.

With alky the best predictor I have had is DP. And there is a point in the night where the DP gets within a few degrees of the temperature that the car will throw a number. I figure it could be dew on the track. Get past that round and it is predictable again. Temperature can't be ignored all together. And barometric pressure swings will affect things as well.

Is 'horsepower correction factor' an option? Or maybe 'air density ratio'?

jim there are times i start out useing my prefered choice (hp adusted) but when really humid conditions are in play i use late night with humidity,even in some daytime conditions

Track conditions are my biggest factor. It will change much quicker than the weather.

George Rupert would always say "when the dew first appears on the car your going to loose 1-2".

This is alky and it was 100% correct for us.

quote:

Originally posted by Bad Nusz:
Is 'horsepower correction factor' an option? Or maybe 'air density ratio'?

Yes, that one is pretty good with my stuff. Nice that it rolls a bunch of stuff into a single factor.

Sort of similar to what I was saying above with some factors getting better and some getting worse, if the HP Factor makes a move for the better and your car slows down, then you can look at all the individual weather elements (HP Factor uses Temp, Barometer, and Dew Point) to find out which things did get worse, and maybe that element will affect your car more than some other things. In fact, that appears to be what happened to Jim in this case.

Thanks, Tony.

Hey, does anyone else own a copy of the book "Motorsports Standard Atmosphere and Weather Correction Methods" 2014 by Patrick Hale?

http://www.dragracingpro.com/Books.html

In this book, Hale declares that HP correction factor is the number to use for ET prediction.

I learned a lot from that book, though the math described in there looks pretty hairy to me. However, I believe even the cheapest hand-held racing weather stations crunch those numbers for you.

I've been recording horsepower correction factor myself after every pass, but the results cannot always be trusted for ET prediction. However, I draw my intake air from under the hood.

Footbrake Jim himself had posted an excellent treatise on that subject in the thread "External air or underhood air?".

Cheers,
Troy

This message has been edited. Last edited by: Bad Nusz,

Here is a HP Correction Factor formula for Microsoft Excel, with Temperature in B25, Barometer in B27, and Dew Point in B29:

=IF(B29=0, 0, 1.18*((990/((B27*33.865)-(6.1078*10^((7.5*((B29-32)/1.8))/(237.3+((B29-32)/1.8))))))*((((B25-32)/1.8)+273)/298)^0.5)-0.18)

To predict ET, you need a past run's ET and that run's HP Correction Factor, along with the HP Correction Factor for the current conditions.

Predicted ET = (((850/[Past Run HP Factor]) / (850/[Current HP Factor))^0.333) * [Past Run ET])

I like underhood air. The conditions don't change as much under there. But then the ET may not move as the raw weather numbers say it will.

You guys are all obviously smart, and have given me a lot of options to consider.

One other variable I failed to include in the weather data, was the wind. It was a steady angle, crossing the track nearly 90* from Right to Left, and had some moderate fluctuations in wind speed. At it's peak it was about 15 MPH, and slowly lost velocity through the evening. My prediction data showed the cross-wind itself having no effect on ET at all. But I DO know that the breeze was coming across a very large corn field before it hit the track, and I think it was swift enough to knock the humidity down quite a bit. When that wind became a gentle breeze of 5 MPH around midnight, that is when the humidity jumped way up.
I remember racing at Brainerd MN up in Tony's part of the country about 15 years ago, and there is a huge lake next to the track. Man, you wanna talk about a slight change in wind making a difference!

quote:

Originally posted by FootbrakeJim:


Oh, and to be honest, I don't know if the W.S. reads humidity in "Relative" terms or not.

Jim, I think that if your weather station reads in a percentage, that it would be relative humidity. It is of course the percentage of the amount of humidity that the air is holding, compared to what it could hold (at the temperature of that particular air sample).

You will recall that warmer air can hold more moisture. Air at 100* with 70% RH holds more water than 60* air with an RH of 70%.

Does your weather station give a reading too in "grains"? That would be the actual grains of water per pound of air, air temp notwithstanding (although of course warmer air can hold more moisture). That is the actual amount of water in the air, diluting your oxygen.

An old drag racer once told me to pay attention to 'grains' on my weather station.

I'm not a mechanic or engineer or anything -- I just play one on TV. ;^)

This message has been edited. Last edited by: Bad Nusz,

From what a friend of mine told me, that the DA has to change at least 500ft for his car to change .01, but humidity is another thing altogether! He said that changes his combo far more than DA or Temp alone, and he runs Alky.

As Greg K said above, you see dew on the windows, dial up 2 to be safe!

I know hot air can hold more moisture and once it cools the air feels "wetter" to our skin, can show on our cars, though it is now denser because it is cooler. Has the actual moisture content changed? I don't know that answer, but with the cooler air there is more moisture per cubic meter than if the air was hotter.

I know the rare times I race at Island Dragway in NJ I dial-up after time shots because the sun is going down, the air is cooler, but the humidity really starts to climb, and in the fall you better have towels to put on your windshield!