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Dragstrip and Launch Techniques Discussion 1/4 mile, 1/8 mile, launch discussions.

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Old 08-23-2009, 08:39 PM   #1
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Effects of Heat & Humidity

I know in hot weather my car gets slower run times then it does in cooler weather,even with 10 degrees difference.
What about Humidity & heat ?
Would high Humidity alone ( at same temps. ) make a car run slower ?

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Old 08-23-2009, 11:00 PM   #2
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Yes, it is puling timing on the car and this is not me saying this it is the guys at Vararam.
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Old 08-24-2009, 09:35 AM   #3
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Yes, it is puling timing on the car and this is not me saying this it is the guys at Vararam.
I guess you are talking about the Heat ?
Can you explain what Vararam said ? .... Or give me a link to it.

What about the Humidity ?

Thanks.........
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Old 08-24-2009, 01:42 PM   #4
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yes, humidity will make your car slower even at the same temp.

Hot AND humid is the worst combo
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Old 08-24-2009, 02:05 PM   #5
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Talked to a tuner about this last week , IAT above 88* will drop timing on a stock tune .
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Old 08-24-2009, 03:06 PM   #6
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through air pressure in the mix too... hot, humid day with a low pressure front.. ugh! LOL
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Old 08-24-2009, 10:16 PM   #7
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Quote:
Originally Posted by 97one View Post
I guess you are talking about the Heat ?
Can you explain what Vararam said ? .... Or give me a link to it.

What about the Humidity ?

Thanks.........
This is that i have fond out after 6Hr of talking with Patrick at Vararam on the phone if you want to call i think he will talk to you on heat and humidity he has done the unit with the two, Its that way in Tx for almost 2 months.
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Old 08-26-2009, 02:09 PM   #8
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Density Altitude or DA is what effects ET. Temp, Hum & Baro psi are all factors that effect DA. The more one factor changes DA the more it will effect your ET.
On average a NA car on gas will change about .0053 in ET for every 100ft change in DA. To give you a example 70 deg, 80% hum & 28.9 baro psi = 2189.3 DA now say the temp go's up 80 deg, 80% hum & 28.9 baro = 2920.04 da
Back to 70 deg but hum jumps to 90% & 28.9 baro = 2222.02 da.
Where you see real big changes is in Baro psi.70 deg, 80% hum & 30.0 baro = 917.19 da and a fast day at the track
Also wind plays a large roll. A typical door car will change +.03 with a 1-5 mph head wind and +.05 with a 6-10 mph head wind. And pick up about the same with a tail wind.

There are allot of other variables but that should give you ruff idea of how the weather effects your ET.
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Old 08-26-2009, 03:45 PM   #9
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Quote:
Originally Posted by jbsrace1 View Post
Density Altitude or DA is what effects ET. Temp, Hum & Baro psi are all factors that effect DA. The more one factor changes DA the more it will effect your ET.
On average a NA car on gas will change about .0053 in ET for every 100ft change in DA. To give you a example 70 deg, 80% hum & 28.9 baro psi = 2189.3 DA now say the temp go's up 80 deg, 80% hum & 28.9 baro = 2920.04 da
Back to 70 deg but hum jumps to 90% & 28.9 baro = 2222.02 da.
Where you see real big changes is in Baro psi.70 deg, 80% hum & 30.0 baro = 917.19 da and a fast day at the track
Also wind plays a large roll. A typical door car will change +.03 with a 1-5 mph head wind and +.05 with a 6-10 mph head wind. And pick up about the same with a tail wind.

There are allot of other variables but that should give you ruff idea of how the weather effects your ET.
Very Good Info.
So the Baro.Psi. is the most inportant factor ?

How do you get the DA using the temp.hum & Baro.? +,-,X,or divide ?
What is the formula ?

You are talking about 1/4Mi times ,right ? w/ .0053 in et per 100ft. change in DA

Thanks
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Old 08-26-2009, 04:52 PM   #10
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Yes 1/4 mile times.
There are several formulas and they are all long and can be a pain to work with.
Several years ago I worked up a formula taking parts from several and built a easy predictor app for drag racing. That works fairly well. I will try and dig it up tonight when I get in.
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Old 08-26-2009, 05:56 PM   #11
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Quote:
Originally Posted by 97one View Post
I know in hot weather my car gets slower run times then it does in cooler weather,even with 10 degrees difference.
What about Humidity & heat ?
Would high Humidity alone ( at same temps. ) make a car run slower ?

I stole this quote, because its worded better than I could do.

Rick Martin, an engineer with Yamaha Marine (800/526-6650; yamaha-motor.com), explains, “Humidity is water vapor. In a given amount of air, it displaces oxygen content, making the air less dense. As a result, the engine ingests less oxygen, and its output is reduced.
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Old 08-26-2009, 06:28 PM   #12
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I was sent this by one of the Airaid engineers.

". 10 degrees of higher inlet air temp can result in as much as 14 degrees of spark retard. Everyone on the forums refer to this issue as “heat soak.”

The canned tunes found in many handheld programmers simply rewrite the low octane tables to match the high octane tables and reduce spark retard vs. air inlet temp.
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Old 08-26-2009, 06:46 PM   #13
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Quote:
Originally Posted by SGOS252382 View Post
I stole this quote, because its worded better than I could do.

Rick Martin, an engineer with Yamaha Marine (800/526-6650; yamaha-motor.com), explains, “Humidity is water vapor. In a given amount of air, it displaces oxygen content, making the air less dense. As a result, the engine ingests less oxygen, and its output is reduced.
Not 100% true. If everything else stays the same (temp, Baro) then yes more humidity will reduce engine output. But drop the temp by 10 degrees and raise the humidity by 20% leaving the Baro the same and the engine will output more not less.
When the temp drops so does the dew point and the vapor psi (both saturated and actual) along with the dry air psi. When humidity go's up Actual Vapor psi will go up but if the temp stays the same then the Saturated Vapor psi will also stay the same.
It gets confusing as hell at times.


This is a simple program to help predict ET from one pass to the next. Just enter the temp, humidity and baro psi from your last pass along with the ET (do not put anything in the prev DA) Then enter the currant temp, hum & Baro psi, Select the fuel Gas or Alcohol and click predict. It will fill the DA in automatically. This is just the .exe file if you already have VB runtime files on your computer it should run fine. If not I can dig up a installer version that will install them for you.
Attached Files
File Type: zip EZpredictor.zip (13.0 KB, 44 views)
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Old 08-26-2009, 06:48 PM   #14
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Actually, density altitude or air density ratio is best used to adjust air/fuel ratio. The SAE
horsepower correction equation is the most accurate for predicting performance.

http://www.computechracing.com/forum...isplay.php?f=5
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Last edited by Roger Ramjet; 08-26-2009 at 07:06 PM. Reason: Add link
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