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Old 09-22-2019, 08:58 PM
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Mike S Mike S is offline
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Default Slowing up at night

Now the days are getting shorter my index race runs into night time. This Saturday I was dialed in pretty good all day. Humidity was 37%, temp 82 and a DA of 1700. Went into 3rd round after the sun went down and a delay and fell to a 10.04. That is leaving way to much for an index race. Needless to say got beat. Humidity was 57%, temp 73 and da still at 1700.
I looked through my logs and my car always seems to slows up when the sun goes down. 60' remain the same or very close.

Could this be a tuning issue? I would think it would go faster in cooler air. Is the humidity affecting performance at night? Anyone else have this problem?

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Old 09-22-2019, 09:11 PM
Scott Roberts Scott Roberts is offline
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Quote:
Originally Posted by Mike S View Post
Now the days are getting shorter my index race runs into night time. This Saturday I was dialed in pretty good all day. Humidity was 37%, temp 82 and a DA of 1700. Went into 3rd round after the sun went down and a delay and fell to a 10.04. That is leaving way to much for an index race. Needless to say got beat. Humidity was 57%, temp 73 and da still at 1700.
I looked through my logs and my car always seems to slows up when the sun goes down. 60' remain the same or very close.

Could this be a tuning issue? I would think it would go faster in cooler air. Is the humidity affecting performance at night? Anyone else have this problem?
Yes, more humity= less oxygen... it will definitely slow you up even with the same d.a..... you can lean on it more if the humidity goes up..
I am more in tune with power adders but I would think it affect n/a the same...with an nitrous car we always leaned on it when the humidity was high...


Last edited by Scott Roberts; 09-22-2019 at 09:17 PM.
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Old 09-22-2019, 09:12 PM
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Mike, maybe you are a tic lean? I'm always kinda fat for the summer early fall and I usually pic up a few hundred when the sun drops.

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Old 09-22-2019, 09:13 PM
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What ignition box do you have?
Add 1 maaaaybe 2 degrees of timing. When the sun goes down the humidity goes up, adding a degree or 2 will help.
If it's a grid or a 7AL 3 it would be easy to change the timing

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Old 09-22-2019, 09:14 PM
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Quote:
Originally Posted by blaktopr View Post
Mike, maybe you are a tic lean? I'm always kinda fat for the summer early fall and I usually pic up a few hundred when the sun drops.
I would say he's fat with more humidity....

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Old 09-22-2019, 09:22 PM
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It's not just cooler air though.
The humidity or grains of water are more which hurts ET.
Also another reason DA is not usually good for ET comparison.
I would say your running lean also and the cooler temp would need a richer setting? (if the grains of water were the same)




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Old 09-22-2019, 09:26 PM
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Quote:
Originally Posted by johnta1 View Post
It's not just cooler air though.
The humidity or grains of water are more which hurts ET.
Also another reason DA is not usually good for ET comparison.
I would say your running lean also and the cooler temp would need a richer setting? (if the grains of water were the same)



He said it went from 37 %to 57% humidity... that being said ,the water displaces the oxygen! He need to lean on it, not go fatter...I dont care about D.A. or the temp.... I care about the oxygen available in the air....


Last edited by Scott Roberts; 09-22-2019 at 09:34 PM.
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Old 09-22-2019, 09:35 PM
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Sure but I was kind of going by this:

Quote:
Could this be a tuning issue? I would think it would go faster in cooler air.
If it runs good in the summer heat, becoming cooler should need to be richer. Unless the grains of water were higher, with same DA.

So, better data would be a good thing.


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Old 09-22-2019, 09:41 PM
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Quote:
Originally Posted by johnta1 View Post
Sure but I was kind of going by this:



If it runs good in the summer heat, becoming cooler should need to be richer. Unless the grains of water were higher, with same DA.

So, better data would be a good thing.

I see reading comprehension eludes you as well... he gave you a clear scenario...

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Old 09-22-2019, 09:47 PM
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I was running an air cleaner instead of the sealed air pan. The air cleaner slows me up a tenth but chokes air into the engine. Makes it richer which in turn didn't help when the humidity went up. I have a Mallory box that has a timing retard that I wired to a toggle switch so I can take timing out.

I am going to go out Tuesday for test and tune and try some restrictor plates to slow it up instead of the air cleaner. usually I can get 2 runs before the sun goes down and then see what happens after it sets.

I was wondering if others had problems with the car slowing when the sun goes down.

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Old 09-22-2019, 09:53 PM
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Quote:
Originally Posted by Mike S View Post
I was running an air cleaner instead of the sealed air pan. The air cleaner slows me up a tenth but chokes air into the engine. Makes it richer which in turn didn't help when the humidity went up. I have a Mallory box that has a timing retard that I wired to a toggle switch so I can take timing out.

I am going to go out Tuesday for test and tune and try some restrictor plates to slow it up instead of the air cleaner. usually I can get 2 runs before the sun goes down and then see what happens after it sets.

I was wondering if others had problems with the car slowing when the sun goes down.
I would say focus more on the water grains than the sun and D.A..... as the grains go up, it will displace the oxygen coming into the engine. That being said, you can counteract that with the tune...i.e. timing or jets...
As you know, an engine needs fuel, oxygen and spark to run... if you have less oxygen in the air what do you do to maintain the same power level? Not being a smartass...

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Old 09-22-2019, 10:15 PM
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I understand. I need to get some dat/night testing in. If I know what the car will do in certain conditions and I can then adjust for those conditions.

For what it's worth, I spent a lot of last year with a A/F meter setting up the carb. Idle, air bleeds , jetting and such. I found my car likes an A/F right around 13.2 using VP c-12 fuel. I will usually jet it a little richer when the air temp gets in the 60's.

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Old 09-22-2019, 10:19 PM
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Quote:
Originally Posted by Mike S View Post
I understand. I need to get some dat/night testing in. If I know what the car will do in certain conditions and I can then adjust for those conditions.

For what it's worth, I spent a lot of last year with a A/F meter setting up the carb. Idle, air bleeds , jetting and such. I found my car likes an A/F right around 13.2 using VP c-12 fuel. I will usually jet it a little richer when the air temp gets in the 60's.
Dont worry as much about air temp as water grains.... just a suggestion...
Granted if the air temp drops and the humidity drops, jet it up!
However, in my simple opinion, it is all about the oxygen in the air at a specific time.....
But as bugs bunny says, I'm a maroon...


Last edited by Scott Roberts; 09-22-2019 at 10:34 PM.
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Old 09-23-2019, 10:12 AM
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My car always slows about the same amount. One thing I have noticed is once the temperature drops to dew point, is will run the same number for the rest of the night. Even after the temp continues to drop, is will run the same number it ran when it first reached dew point. That is because the water vapor in the air is maxed out, it will only hold so much, and then it becomes saturated. So, when you see the water condensing on your car’s roof & trunk it should be telling you something.

Do you have a weather meter and phsycometric chart or wheel? It will convert temperature and humidity into grains per pound, a very useful tool to have.
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Old 09-23-2019, 10:38 AM
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Unless temperature stays the same percent relative humidity does not tell the whole / true story.

percent relative humidity = Vapor Pressure / Saturated Vapor Pressure

Vapor Pressure = Amount of water in the air

Saturated Vapor Pressure = Amount of water the air will hold. This changes with temperature.

85F 25%RH 0.3027VP 1.2108SVP 44.50 Grains of water

65F 48.702%RH 0.0302702VP 0.621538SVP 44.50 Grains of water

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Old 09-23-2019, 12:57 PM
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A really good weather station plus Crew Chief Pro is a good investment. I’m not smart enough to race without it

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Old 09-23-2019, 05:50 PM
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From what I see my car starts slowing down when the humidity goes past about 70%. This is with alcohol. I can watch the gas guys pick up after dark with the temps dropping but I'm slowing down. With you being on gas I wouldn't expect much change with the numbers you gave above. I would expect it to pick up a touch. Does everyone else in the class slow down or do they change jets to compensate?

I see where Scott is coming from and agree with his thinking unless you are lean to start with and the air is actually better, and that's what's slowing you down because it's now leaner. I haven't run your numbers to know wether the car should pick up, slow down, or run the same even with more humidity. With the cooler air I would expect it to be almost a wash or pick up a touch.


I also notice that if there is a delay the car will slow some till the transmission temps come back up as well as engine oil temps and such.

I would jet it up a couple numbers and see if that helps consistency.

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Old 09-23-2019, 06:09 PM
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Quote:
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......
I would jet it up a couple numbers and see if that helps consistency.
....x2

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Old 09-23-2019, 07:01 PM
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This stuff really isn’t that hard once you get your head wrapped around it.

82 degrees at 37%= 61 grains per pound. DA @ 1700’

73 degrees at 57%= 73 grains per pound. DA @ 1700’

Which scenario above would produce the fastest ET? I’ll take the middle of the day, hot temperature low humidity weather any day of the week. It happens every day, as the temperature warms, water vapor goes down, then every night it cools off, and water vapor rises. In the summer it usually reaches Dew Point (saturation) and can’t hold any more, so it condenses on any cooler surface. If the surface is warm, it will not condense. That’s why your trunk and roof will have water condensation, before your hood will.

Have you ever wondered why, When your AC in your car is on, and it’s blowing some of the cold air into the defrost duct, because you didn’t adjust the levers right, and water condenses where the defroster duct is hitting the inside of the windshield. Have you ever considered the physics behind that? The water condenses on the outside of your windshield, because the cooler air on the inside is cooling the glass. When the humid on the outside of the glass hits the cool glass, temperature drops, and because cool air can’t hold water like warm air, it turns into liquid immediately, hence the condensation only where the defrost duct is blowing cold air. (At least that is my observation in the South).

Farmers park their tractors under open air sheds because it they shield against this phenomenon. Sublimation or Chinook winds are a result of this basic function in the atmosphere. (Google it if you want to learn about it) the bottom line is, Cold air can’t hold water vapor like Warm air.

If you log weather each pass, a change of 12 grains per pound would probably predict the increased ET of .04 that you saw.
Make a chart that takes all important parameters into consideration, density altitude #1, water grains in #2, and then compare it to past performance at each parameter, average it over several similar passes and put your ET in the chart. This will give to a “go to” off the trailer prediction, no time trials required however; track conditions vary, making each time trial important.

The atmosphere is a living, breathing, constantly changing animal. Learn how it changes, and use a hand held weather meter (and chart) and help you to predict your ET.
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Old 09-23-2019, 07:05 PM
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And watch out for Probird.... he can drive.

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