Our local track guru/drag school teacher/photo guy/weather man on our local boards just posted some DAMN good info on weather and fuel/performance stuff.
Below is a copy of most of the thread.
As many of you fellow weather freaks know, I usually post a note when the corrected elevation gets insane during the winter months. However, I've got an all-timer to report tonight.
I've been tracking the monstrous high pressure system which has been driving our recent weather and, as of 3:00 AM, St. Louis has some very impressive air.
22 degrees
59% humidity
30.65 Hg barometer (massive!)
That's a corrected elevation of 3341 feet BELOW sea level. Awesome, but not a record for us.
However, the center of the high just passed over Williston, North Dakota and produced something simply mind-boggling....a barometric pressure of:
30.99 inches!
To put that in perspective, the highest ever recorded in St. Louis was 31.01 back in 1918.
Currently, Williston is reporting
-17 degrees
90% humidity (!)
30.99 Hg barometer (!!!)
Corrected elevation?
6,738 FEET BELOW SEA LEVEL!
On a 14-second car, that's worth nearly 1.3 seconds!
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Something that I've noticed about these super high fronts..
When you go from atmospheric conditions of say 75 degree ambient air temp, with 65% humidity, and 30.00 inches hg of barometric pressure.. To say 55 degrees, with 40%, and 30.20 inches of mercury.. The power increase is something that you can REALLY feel.. I wouldn't be surprised if those air changes alone were worth 50 horsepower..
On the other hand.. When the corrected altitude drops even further.. To conditions of say 35 degree air, with 25% humidity, and a baro of 30.40 or more.. You don't seem to get another big step in power.. In fact, an engine sometimes may even make LESS power than on a day that the conditions seem to be not as good..
I noticed this when I ran a Kenne-Bell supercharger on my car.. I keep detailed records of the atmospheric conditions on each run.. After a night of racing at Gateway, I would get right on my computer and go here
http://weather.noaa.gov/weather/current/KCPS.html
And compare the clock time I wrote on my time slip, of when I ran.. To the hour by hour conditions listed at Cahokia Airport ( a stones throw from the track)..
I'd then take those figures I got from Cahokia, and enter them here
http://www.************.com/density2.php
To get a "Density Altitude" figure.. A better density altitude didn't always equal a better thru the traps MPH .. That's something I have never understood..
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"Probably needed more fuel to get the most out of the best conditions."-from earlier in post
That's really all it is. In below sea level air, 99% of all carbureted cars need to be rejetted. Not only are they slowing down because they're lean, they're beginning to incinerate pistons and valves. Anybody who's been through the Drag Racing School knows how I STRESS using a digital water temp guage. In below sea level air, the engine temp rises substantially versus normal operating temp in similar ambient temps.
My '76 Corvette, a basic L48/Quadrajet combo, would run like hell to 400 feet above. After that, it would slow down. I could always tell when the air was below 100 feet or so because the engine temp would RISE 8-15 degrees, almost the exact same amount it would DROP if I had a fouled plug! popcorn.gif
By the way, ALWAYS get your weather data from Parks Air College. It's within six feet of the elevation of Gateway's starting line and only 4.76 miles from the track. I presume that's what Jeff meant by "Cahokia".
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The performance gains or looses I was talking about would happen on fuel injected cars.. These vehicles are equipped with BAP (bar absolute press) and MAP (manifold absolute pressure) sensors.. Along with a MAS (mass air sensor) and ACT (air charge temperature) sensors.. They are suppose to automaticly compensate for air density and temperature changes..
And not only can you sometimes feel the performance fall off, when conditions are suppose to be extra good.. You can sometimes get extra good performance, when the conditions seem less than ideal..
I'm sure that temperture, air pressure, and to a lesser degree humidity.. All matter when it comes to an engine making power, calculated in terms of air density.. I'm also think'n there is something else that's coming into play.. Maybe something like the size of the air molecules on a certain day.. Or maybe the ratio of the nitrogen to oxygen mix, that's in the air we breath.. Changes independently of the air density..
Some mornings, while driving to work, my truck feels like a torque monster (Dodge Dakota 4x4 with a 360 and 3.91 gears).. This feeling doesn't always jive with what the weatherman says the conditions are on the morning news..
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The MAF sensor can identify air below sea level but the base jet size can't help you much at that point. They do fine in the normal range of operation, (between 1000-5000 feet), but even FI cars still run like crap in Denver in the summer without rejetting.
How many folks are old enough to remember when carbureted cars sold in Denver had completely different carbs, jets and gears than those sold everywhere else? Same principal.
Jeff, the effect comes from (in order) temp, humidity, and pressure. The "somethin' else" you're trying to describe is grains of oxygen or grains of water. It's not the size of the molecules but the number of them. Wanna learn more? Drag Racing School starts in May
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I would have never thought that humidity had a greater effect on performance than Barometric pressure..
Tell me this Brett.. Have you ever had your personal vehicle, that you drive every day.. Seem to just have more pep, even though the atmospheric conditions seem to say that it souldn't ?? That's what I'm getting at..
Also..I remember when carbs were different for high altitude places like Denver.. In the 70's I work as a Quality Assurance Lab Tech for Carter Carburetor.. We produced some OEM carb with a device called an "altitude compensator".. It would basicly open or restrict air bleeds in the carbs fuel circuits, according to the atmospheric pressure... So you could drive from some place at sea level, up into the mountains of Colorado.. And your carb would still fuel properly.. Well sort of
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A lot of that mysterious "pep" often comes from differences between ambient temp and intake charge temp, which can be affected by stupid stuff like wind and weird radiaion absorption levels. Then again, some of it is just your brain goin' goofy after age 30.
I never knew about the "altitude compensator"...pretty cool '70s tech! I'm sure it worked horribly
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Brett, on those digi guages what would you target as any kind of "best guess" as to how richer or leaner a combo should go in relation to temp change in weather/water temp due to weather? Would I be correct in saying that the best A/F would change like a car that runs best at sea level with a 12.8 would now need a 12.2 or something?
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It takes a fair amount of logbook entries (notes) to figure it out, blksn955.o. Since ambient air temp has the most effect on power, you have to determine a baseline engine temp reading at any given air temp. In example, if the engine runs at 185 degrees at 80 degrees on one run and 187 degrees at 80 degrees on another, you'd need to determine what changed between them. If you can discount erratic procedure, (which is where most people fail miserably), then you can figure it to be a barometer drop or a humidity change.
Humidity can be deceiving. While increased water grains displace oxygen grains, increased water grains also drops engine temp; it's nature's water injection. However, oxygen displacement will almost always decrease performance more than decreased temp will inprove it.
Yeah. That's actually a pretty good way to look at it.--"about Would I be correct in saying that the best A/F would change like a car that runs best at sea level with a 12.8 would now need a 12.2 or something?"
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Sorry for the lenght but its good info about weather anyway.
