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Altitude Density Tuning - Horsepower In The Air

What every car crafter needs to know about air, pressure, temperature, and humidity-because there's horsepower in the air

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More on Density Altitude
Let's get into some specifics to see how density altitude works, and where it doesn't. We took our car to Pomona, California, which is roughly 900 feet above sea level. With 72 degrees F, 55 percent humidity, and an uncorrected atmospheric pressure of 28.72, we had a density altitude of 2,400 feet. This is the equivalent of a 60-degree dry-air day at 2,400 feet of elevation. The next day we traveled to a dragstrip at 2,000 feet of elevation with 49 percent humidity and a chilly 55 degrees F. It turns out this was also exactly the same 2,400 feet of density altitude. While the temperature was much cooler, the air pressure was lower since we were at a higher elevation. Even though the tune-up might be slightly different with the same density altitude, most material out there claims a car should run the same since the density altitude is the same. According to Patrick Hale at Racing Systems Analysis, the car will probably not run the same at both tracks.

On his Web site quarterjr.com, Hale has created two very simple programs that are free for anyone to use. We plugged both track conditions into his Weather Station program to get the density-altitude numbers. Then we used his Dragstrip Dyno program to get an idea of how our 3,550-pound street car with 400 hp and an automatic would run at both tracks. The dyno estimates the car will run slightly slower at the higher-elevation track despite the identical density altitude, cooler air, and reduced humidity. Hale's Dragstrip Dyno shows the difference in performance is 12.18 at 113.60 mph versus 12.13 at 114.00 mph. Part of the output of the Weather Station is a horsepower correction number. At the 900-foot track the correction was 1.079, while the higher-elevation track's horsepower correction factor was a larger 1.092, which means the engine was making less power at the higher elevation despite the lower temperature. Part of the problem with the density-altitude formula is that it was originally developed for aircraft and solving for proper lift with a given atmospheric condition. Unfortunately, engine power does not respond as strongly to changes in temperature as density altitude predicts.

The Dragstrip Dyno program uses inputs from Hale's Weather Station program to estimate power based on pressure, temperature, and vapor pressure. The Dragstrip Dyno then calculates a simple horsepower correction factor that can then be used along with the variables of horsepower and weight to come up with a simplified e.t. and speed estimate. The beauty of this is not that it is accurate for your particular car, but that you can quickly input different atmospheric conditions and see their effects on the performance of any car. As our example above illustrates, lower pressure hurt performance, but the cooler air made up for much of that so the car slowed down only slightly. Had the 900-foot temperature and vapor pressure been the same but at 2,000 feet, the car would have slowed down even more to a 12.30 at 112.60 mph with a density altitude of 3,750 feet-that's 0.17 second and 1.4 mph slower than what the car ran at 900 feet of track elevation. It's all about knowing how atmospheric conditions affect engine performance. We have abbreviated this explanation for the sake of space, but if you want to learn more go to Hale's Web site, which also includes some simple math that will make it all very clear.

NHRA Elevation Correction Factors
Use this chart to convert your e.t. and mph to sea-level performance by multiplying either e.t. or mph performance at a given altitude by the indicated factor. These are NHRA's correction factors for normally aspirated Stock and Super Stock race cars, which equate the best for a typical normally aspirated street car. These factors are used to "correct" back to sea level as a way of establishing class indexes.

As an example of how to use this chart, if your car runs 12.00/114 mph at a 3,000-foot-altitude track, use the correction factors (e.t. x 0.9640 and mph x 1.0381) to generate altitude-corrected numbers equaling 11.568 at 118.34 mph. These are roughly 3 1/2 percent corrections for that altitude. Keep in mind that these factors correct only for track altitude, they do not address the existing weather conditions. This is a way to roughly compare a normally aspirated car's performance at Denver's Bandimere Speedway (at a wheezy 5,800 feet) to your buddy's Englishtown, New Jersey, rocketlike e.t.'s running at a track that is virtually at sea level. It's also worth noting that NHRA uses a less aggressive correction factor for the quicker Competition Eliminator cars. Plus, most supercharged and turbocharged cars (AA/Altered Turbo for example) use half of the Comp correction factor. Alcohol dragster and Funny Cars are not factored.

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