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#1
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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? |
#2
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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. |
#3
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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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#4
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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
__________________
"You have to evaluate the past,Focus on the future,and that tells you what you have to do in the present"--Lou Holtz “It’s the process it takes to get to goals that sets us apart, the execution on every single play, one play,one life” Notre Dame Head Coach Marcus Freeman 69 GTO NHRA Super Street Car 2860lbs 10.890@157.08 MPH in Iowa in June’23 ,157.56 MPH in Gainesville in March ‘23 |
#5
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I would say he's fat with more humidity....
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#6
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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)
__________________
John Wallace - johnta1 Pontiac Power RULES !!! www.wallaceracing.com Winner of Top Class at Pontiac Nationals, 2004 Cordova Winner of Quick 16 At Ames 2004 Pontiac Tripower Nats KRE's MR-1 - 1st 5 second Pontiac block ever! "Every man has a right to his own opinion, but no man has a right to be wrong in his facts." "People demand freedom of speech to make up for the freedom of thought which they avoid." – Socrates |
#7
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Quote:
Last edited by Scott Roberts; 09-22-2019 at 09:34 PM. |
#8
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Sure but I was kind of going by this:
Quote:
So, better data would be a good thing.
__________________
John Wallace - johnta1 Pontiac Power RULES !!! www.wallaceracing.com Winner of Top Class at Pontiac Nationals, 2004 Cordova Winner of Quick 16 At Ames 2004 Pontiac Tripower Nats KRE's MR-1 - 1st 5 second Pontiac block ever! "Every man has a right to his own opinion, but no man has a right to be wrong in his facts." "People demand freedom of speech to make up for the freedom of thought which they avoid." – Socrates |
#9
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I see reading comprehension eludes you as well... he gave you a clear scenario...
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#10
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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. |
#11
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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... |
#12
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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. |
#13
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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. |
#14
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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.
__________________
1969 Firebird, Tx3-455/468 machined by CVMS E-heads by Dave Wilcox/Comp Cams 300B-6 flat solid 850DP on E-85 by Eric Niefert/T2 1" plastic spacer T-400/PTC 4000/390's/30x9 Hoosier radial slicks,#3400 1.38 60' 6.32 @ 108 MPH at Northeast Dragway NC 5/23/15 (9th pass on new engine) https://m.youtube.com/watch?v=ePCu2v...ature=youtu.be 1.37 60' 6.26 @ 109 half track, 9.86 @ 136 1/4 mile, #3350 11/26/16 at Richmond Dragway (125th pass on new engine) |
#15
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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 Stan
__________________
Stan Weiss/World Wide Enterprises Offering Performance Software Since 1987 http://www.magneticlynx.com/carfor/carfor.htm David Vizard & Stan Weiss' IOP / Flow / Induction Optimization - Cam Selection Software http://www.magneticlynx.com/DV Download FREE 14 Trial IOP / Flow Software http://www.magneticlynx.com/DV/Flow_..._Day_Trial.php Pontiac Pump Gas List http://www.magneticlynx.com/carfor/pont_gas.htm Using PMD Block and Heads List http://www.magneticlynx.com/carfor/pont_pmd.htm |
#16
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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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1965 TriPower GTO, 1967 GTO, 1969 GTO, 1969 Judge, 1972 GTO, 1977 Smokey and the Bandit, 1989 TA ProStreet, 1968 Firebird NHRA 10.90 racecar, 1963 Tempest S/Gas |
#17
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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.
__________________
Come take a ride http://www.youtube.com/watch?v=P7Y8Awfk2I0 2008, 2013, 2015, 2016, 2019 Central Il Dragway Mod track champion and 2015 IHRA Div 5 Mod champion |
#18
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....x2
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1978 Pontiac Grand LeMans.......out next year Previous - 2001 Jerry Haas-built P/S Grand Am - former WJ car, Hoffman Racing 535" IA2, Tiger Heads/Intake, Single Dominator, PG Best ET: 7.776 @ 175.94, 1.096 60', 2375 lbs. |
#19
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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.
__________________
1969 Firebird, Tx3-455/468 machined by CVMS E-heads by Dave Wilcox/Comp Cams 300B-6 flat solid 850DP on E-85 by Eric Niefert/T2 1" plastic spacer T-400/PTC 4000/390's/30x9 Hoosier radial slicks,#3400 1.38 60' 6.32 @ 108 MPH at Northeast Dragway NC 5/23/15 (9th pass on new engine) https://m.youtube.com/watch?v=ePCu2v...ature=youtu.be 1.37 60' 6.26 @ 109 half track, 9.86 @ 136 1/4 mile, #3350 11/26/16 at Richmond Dragway (125th pass on new engine) |
#20
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And watch out for Probird.... he can drive.
__________________
1969 Firebird, Tx3-455/468 machined by CVMS E-heads by Dave Wilcox/Comp Cams 300B-6 flat solid 850DP on E-85 by Eric Niefert/T2 1" plastic spacer T-400/PTC 4000/390's/30x9 Hoosier radial slicks,#3400 1.38 60' 6.32 @ 108 MPH at Northeast Dragway NC 5/23/15 (9th pass on new engine) https://m.youtube.com/watch?v=ePCu2v...ature=youtu.be 1.37 60' 6.26 @ 109 half track, 9.86 @ 136 1/4 mile, #3350 11/26/16 at Richmond Dragway (125th pass on new engine) |
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