Help with DA information

[66Valiant528]

So on August 24th with a DA of 2200 at NE Dragway my Valiant ran 137.5 mph.
Yesterday with a DA of 1620 the car ran a best of 134.5. Is this telling me that the carb needs to be fattened up?

 

[71Demon540]

Something I have learned regarding weather, DA is only one factor when dialing in the car. Air temp, humidity, barometric pressure, water grains, and wind are all factors when it comes to what the car runs. My weather station uses the correction factor setting. Which equates all of the weather factors and bases its number off of that. I don’t use my weather station to dial my car for me. I use it as a tool to give me an idea which way I should go. By looking at correction factor instead of independent variables I have found to be much more predictable for my ET dial in. Like all things, it’s a tool that took time to learn. When you see big swings in correction factor and the car doesn’t vary much, I’d say your tune up is spot on. When weather gets worse and car slows down, you know your car follows the weather which is good for dialing. If car goes faster when weather is worse, you may be on fine line of little too lean on carb setup.

 

[furrystump]

I would try it, it could be another thing different between the days. Like wind, I have also found humidity plays a larger factor than say temperature. I know the DA calculation takes humidity in to consideration, but my car really likes dry air. 3-4 mph and 3-4 10ths difference from heat of summer to fall is par for the course. I can see changes on the AFR log from morning to afternoon during the same average day. I don’t really chase the AFR throughout the year. I run it with a fall tune in it most the year. Fat and safe.

 

[413]

Same track?
What was the 60' time?
What was the wind?
Did the car hook both runs, or spin at all?

Was anything changed on the car?
Shift points
Rear tires
Converter
rear gear ratio
Timing
carb
throttle cable

 

[66Valiant528]

I thank you both. I just couldn't understand why the mph dropped by 3. Weather was suggested. It don't think it will hurt to go 1 number bigger all around. Its square now a 950 ultra xp with I believe 95s

 

[66Valiant528]

Same track?
What was the 60' time?
What was the wind?
Did the car hook both runs, or spin at all?

Was anything changed on the car?
Shift points
Rear tires
Converter
rear gear ratio
Timing
carb
throttle cable

Same track August 24th and Sept 14. Car exactly the same. 60s 1.55s to spinning 1.8s but usually the car will mph at about 135. For whatever reason it went 137.5 twice. So best mph this weekend was 134.5.

 

[67Dart273]

Could it simply be the difference in head/ tail wind?

 

[66Valiant528]

3 mph vs 7 mph. I dont recall it being windy. Here's some info from the 24th

 

[66Valiant528]

Sept 14th

 

[Newbomb Turk]

Something I have learned regarding weather, DA is only one factor when dialing in the car. Air temp, humidity, barometric pressure, water grains, and wind are all factors when it comes to what the car runs. My weather station uses the correction factor setting. Which equates all of the weather factors and bases its number off of that. I don’t use my weather station to dial my car for me. I use it as a tool to give me an idea which way I should go. By looking at correction factor instead of independent variables I have found to be much more predictable for my ET dial in. Like all things, it’s a tool that took time to learn. When you see big swings in correction factor and the car doesn’t vary much, I’d say your tune up is spot on. When weather gets worse and car slows down, you know your car follows the weather which is good for dialing. If car goes faster when weather is worse, you may be on fine line of little too lean on carb setup.

^^^^^^^^this^^^^^^^

 

[Newbomb Turk]

So on August 24th with a DA of 2200 at NE Dragway my Valiant ran 137.5 mph.
Yesterday with a DA of 1620 the car ran a best of 134.5. Is this telling me that the carb needs to be fattened up?

See post 2.

I would add that with that small a DA change and a 3 MPH drop it was a combination of the stuff laid out in post 2.

Sometimes I pull 2 may three degrees of timing out for something like that. You have to test and see how your engine responds to varying weather changes.

I’ve seen engines that a 1000 foot DA change wouldn’t change a thing.

Those are great bracket engines. I hate them, but that’s what you want in a bracket engine.

 

[413]

If it spins the MPH will be higher, and that's the same day, same fuel, same everything, back to back.

 

[ValiantRacing]

Check the water grains, humidity can be close to the same but the water grains can have a bigger impact.

 

[pink340swinger]

I only read the initial post so as not to go down the rabbit hole that is the Internet, but you say the DA got 580' better and the car slowed down. When I figure out the DA I use all the factors, temp, humidity, pressure, etc,etc. so with a better DA, better air overall, the car should/will perform better unless it's spinning considerably more or the tune up I.E. jetting, bleeds timing, etc is not optimal for the conditions. Better air can usually use more fuel if the initial tune up is correct... 3 mph is a lot, at least to me it is.

 

[66Valiant528]

Yes 3 mph slower with a better DA but as pointed out that's only 1 factor.

 

[furrystump]

I would add some fuel, it will be safer for that 200-300 foot day in the fall.

 

[BSB67]

When I figure out the DA I use all the factors, temp, humidity, pressure, etc,etc. so with a better DA, better air overall, the car should/will perform better unless it's spinning considerably more or the tune up

I’m with you. DA factors all those variables. Generally, better DA=better air=better horsepower.

If the car slows down 3 mph in better air, I’d be looking for something that’s not right, including your data.

In my car tire spin will add mph, but not that much. Maybe a combination of things.

 

[Demonx2]

For whatever reason, I've found the air density (with water vapor) to be a tad more accurate/predictive than DA alone.

Here's a snip from the app for racetrack DA which shows all the numbers at the Orlando track at this moment.