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Old 06-20-2010, 10:31 AM
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Density Altitude vs quarter mile times....

I moved from Seattle to Las Vegas a couple of years ago and I knew my cars would run a little slower in the desert.... My first time out here with my Vega I ran a 12.1@108 and it felt like a great pass(temp was in the 80's). My times in Seattle were usually 11.3-11.4 on any given day, car was usually very consistent. Plugging my timeslip and that days weather into a Density Altitude Calculator the Vegas run would have been an 11.3(corrected). I got out some old timeslips from Seattle and used the same calculator, there was virtually no correction since Seattle is at sea level. My question is this to those with some real world experience.... Does the air here in Vegas really slow the cars down that much? Even when its not all that hot out? I wasn't expecting the car to go 8 tenths slower... According to the calculator it does... I just want to check with someone that really knows. I don't like being the guy with a roll cage that runs 12's! Haha...

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Old 06-20-2010, 12:13 PM
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Less humidity You moved from an oxygen rich dense air climate to a hot dry climate.
I raced in ariz. but drove to San Diego to visit family on some week ends and when I got to San Diego, it felt like I gained 100 cu in.
It boils down to the more dry the air and higher elevation has less oxygen and density.
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Old 06-20-2010, 12:53 PM
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Does anyone have numbers on how much their car slowed down running it in the desert vs elsewhere? I'm very curious to hear some examples.
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Old 06-20-2010, 10:03 PM
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Anybody have any other examples from personal experience?
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Old 06-20-2010, 10:30 PM
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Dry air= good, humid air= not good, cool air= good, hot air= not good. Low altitude- good, high altitude= not good.

Cool, dry, low altitude best, and vice versa.

In FL, our cars would run considerably better in the winter months than summer.

The "cure" for the lack of air at the Las Vegas altitude is more CR, more timing, boost, N2O.
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Old 06-21-2010, 07:14 AM
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Atmospheric conditions very definitely have an impact on the performance of internal combustion engines. Density altitude is a standard altitude corrected for temperature, elevation, barometric pressure, and water vapor content (humidity).

I like to think of it this way (this is an oversimplification but adequately illustrates the point). You have a box of air. The top of the box is open. The size of the box does not change. The temperature is 59 degrees F, the barometric pressure is 29.92", and you are at sea level with zero humidity.

Now start to change some of the conditions. Raise the temperature and the molecules of air start to move around a little faster. They need a little more room between them as they speed up, so some of the air molecules (some of those being oxygen molecules) spill out of the box, resulting in fewer oxygen molecules left in the box. Lower the barometric pressure and more molecules spill out because there is less atmospheric pressure holding them inside the box. Start walking up a hill and the atmospheric pressure will drop even more the farther you go up. Add some water vapor molecules to the box and these will displace some of the air molecules which will spill out of the box. All of these conditions change (in the above, all reduce) the amount of actual air molecules in the box, impacting performance. In the above, the engine 'feels' like it may be at an altitude higher than it actually is. The calculation of density altitude is a convenient way to quantify this impact.

As Cobalt has said, to overcome the effects of DA is to find a way to 'sqeeze' more air into the box (cylinder) or find a way to increase the ratio of oxygen molecules in the air that you put into the box.
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Old 06-21-2010, 07:34 AM
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vegas air

Years ago I had a buddy that was partners in a hemi dragster.. They were un beatable years ago in the high rockies. altitude 5 to 6 k above sea level , when they went to calif they had trouble tuning it. They said they were equal to the coast guys when they ran at the old track in Henderson, NV. He said tweeking the blower overdrive wasn't enough, To get an engine to run right He had to start with the static compression ratio then blower speed, fuel- air ratio, timing cam, exhaust tube length. Etc. One of the partners had a Dr degree in engineering, the dragster was his test vehicle for his ideas

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