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Let's talk Density Altitude (DA) / TL;DR included!

- - - - - da density altitude fbo e85

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#1
RAIDTNK_GTR

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I'm looking to really understand DA and how it affects our cars at the drag strip.  I have a basic elementary knowledge on what DA is and how it works and I have looked at the general formula's many use to calculate DA as well as the various websites that do that for you, (Dragtimes, Airdensity, etc)  In a nutshell, lower DA = good, higher DA = bad.

 

So, that brings me to really my main question.  How does it REALLY affect our cars, being that they are FI?  Let me give you a better idea of what I'm after.  The past few weeks at my local track here in Tampa/Brandenton (Bradenton Motorsports), the DA has been anywhere from 900 to 1700 (last night).  My car has been trapping consistently 5mph (130 vs 135) slower during those conditions.  However, on the dyno the car is making solid power for a FBO+e85 car.  Now, with my basic understanding of DA, at sea level, the car should be making full power and trapping what it would normally trap as 900 DA isn't too bad.  Additionally, the car ran a 10.5 only trapping 130mph with a 1.61 60ft. 

 

Last night, the DA was 1700.  With a set of fresh tunes to test, my first time out was 10.4@130 with a 1.64 60ft.  Each run thereafter was also 130mph up to the 4 runs I got in.  So what I'm thinking, and please feel free to chime in, is that I can't just look at DA.  Because when you break down DA around here, you have to look at air temp AND humidity.  The past 3 weeks the temps have been around 73-80 degrees out at night.  But, the humidity has been 75-100%!  So now I'm "assuming" that my problem with missing MPH is the humidity.  Does that seem to be an accurate assumption?  We're still fine tuning the car but I feel as if the car is running strong and its consistently hitting the same numbers run after run. Other FI car owners that I talked to last night said they were having the same issue with their cars trapping 5-7mphs less but still pulling in really good ET's.  That's the part I really don't understand.  Does that mean the car is making good power down-low to mid then falling on its face at top end because of the humidity?  This is on a FBO+e85 stock turbo setup.

 

TL;DR

 

Does humidity rob our cars of MPH and if so at which percentage (50%+?, 70%+, etc..)?  Can you still have a good ET even though you're trapping lower than usual?


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#2
Tim

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I have my own theories but ill wait till the experts chime in.


 

 


#3
2highpsi

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5-7 MPH is A LOT of difference when you are talking about FBO level, in my opinion.

 

My old car which was full bolt on never experienced more than a 2 MPH difference (Once I stopped changing parts and started using the same tune) no matter the DA.  Almost 0 - 3000.  The worst the car ever trapped was 136 and the best it ever trapped was almost 138.

 

Now my ET would change a lot, but it depended more on track prep than it did DA


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#4
RAIDTNK_GTR

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5-7 MPH is A LOT of difference when you are talking about FBO level, in my opinion.

 

My old car which was full bolt on never experienced more than a 2 MPH difference (Once I stopped changing parts and started using the same tune) no matter the DA.  Almost 0 - 3000.  The worst the car ever trapped was 136 and the best it ever trapped was almost 138.

 

Now my ET would change a lot, but it depended more on track prep than it did DA

 

5mph is what I'm missing.  5-7 mph was for the ZR1 who traps 145-147.

 

2highpsi, how much humidity are you usually looking at? That's what I'm really trying to nail down to considering if its a determining factor or not. 



#5
RAIDTNK_GTR

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And if it helps/matters, my 1/8 times were around the 106.xx mph mark so I'm basically only picking up 24mph the back half.



#6
2highpsi

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I use air density online, and I assumed it took humidity into consideration, as their calc feature asks for it:

 

http://airdensityonl...com/free-calcs/

 

Elevation is required in order to provide an uncorrected baromter value. Leave the elevation field blank to use the provided baromter value as the uncorrected value. Further explanation can be found here.

temperature:  deg f
humidity:  %
barometer:  Hg
elevation:  ft

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#7
RAIDTNK_GTR

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I use air density online, and I assumed it took humidity into consideration, as their calc feature asks for it:

 

http://airdensityonl...com/free-calcs/

 

Elevation is required in order to provide an uncorrected baromter value. Leave the elevation field blank to use the provided baromter value as the uncorrected value. Further explanation can be found here.

temperature:  deg f
humidity:  %
barometer:  Hg
elevation:  ft

 

 

I use the same but that's what I can't wrap my head around.  How can a DA of 900 be "good" when it reads a temp of 73 with a humidity of 75%+?  Elevation and barometer play a factor as indicated by your reference to airdensityonline and being that the track is I believe only 53ft above sea level, that still makes me think the humidity is playing a huge factor in missing MPH even though the "DA" reads low.



#8
2highpsi

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I use the same but that's what I can't wrap my head around.  How can a DA of 900 be "good" when it reads a temp of 73 with a humidity of 75%+?  Elevation and barometer play a factor as indicated by your reference to airdensityonline and being that the track is I believe only 53ft above sea level, that still makes me think the humidity is playing a huge factor in missing MPH even though the "DA" reads low.

 

Go to that calculator, and leave everything the same except for humidity.  As humidity increases, DA will as well.


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#9
RAIDTNK_GTR

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5-7 MPH is A LOT of difference when you are talking about FBO level, in my opinion.

 

My old car which was full bolt on never experienced more than a 2 MPH difference (Once I stopped changing parts and started using the same tune) no matter the DA.  Almost 0 - 3000.  The worst the car ever trapped was 136 and the best it ever trapped was almost 138.

 

Now my ET would change a lot, but it depended more on track prep than it did DA

 

 

I started thinking about what you said here and I'm trying to understand how you could have a swing from 0 DA to 3000 DA and only see a difference of 2mph (unless I'm understanding what you're saying here).  That seems like a HUGE difference in DA to only see a difference of 2 mph.



#10
2highpsi

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I started thinking about what you said here and I'm trying to understand how you could have a swing from 0 DA to 3000 DA and only see a difference of 2mph (unless I'm understanding what you're saying here).  That seems like a HUGE difference in DA to only see a difference of 2 mph.

 

 

At FBO level to go from 130 MPH - 135 MPH (which you were saying you were seeing from a change in DA) in my opinion isn't possible.

 

That's a decent change in power.  I'm not saying you are lying by any means, I'm just saying there are other things coming into play.  If you have a newer GTR with Dynamic Advance maybe on the higher MPH runs, the car was seeing more timing than the other.(even though it is the same tune)  Or if you are going from one strip to another, maybe one just reads high and one reads low.

 

Maybe one of the experts can chime in, and maybe I'm wrong.  But I keep all my slips, and pay attention to the DA and everything pretty close.  2 MPH is the biggest difference I noticed (when EVERYTHING ELSE stayed the same except DA).


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