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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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Standard motorsports temperature and pressure are established to be 60 degrees F, 29.92 inches of mercury pressure (which equals 14.7 psi), and zero humidity. Aviation standard temperature is 59 degrees, which is why you will see that figure used sometimes. That is considered zero-density altitude. Any temperature or vapor-pressure increase or pressure decrease will contribute to raise the density altitude. This ultimately means a decrease in overall air density.

According to the folks at Altronics, there is one more variable here that we have not touched upon. The science books tell us that the oxygen content of air is roughly 21 percent. But the reality is that this can vary by a few tenths of a percent. This can have a slight effect on performance, and it's significant if you are a bracket racer where every hundredth of a second must be tracked in order to make the car as consistent as possible. For the rest of us mere mortals, however, keeping track of density altitude will suffice to make us better tuners than most of our friends.

The Weatherman Lies
Have you ever wondered why the weatherman in Denver will tell you with a smile on his face that it's going to be a beautiful day with the pressure at 30.12 when your uncorrected mercury barometer tells you it's actually something like 24.80? The government decided that regardless of the altitude, atmospheric pressure readings from the National Weather Service should be altitude-corrected based on similar conditions that would exist at sea level.

In most cases this would not be important, except that you cannot plug the weatherman's data into your density-altitude calculations because the data is incorrect for the altitude. This is why if you are attempting to do density-altitude calculations, you must use what is called uncorrected station pressure. This is actual atmospheric pressure at a given altitude and can be obtained from the nearest airport by asking for uncorrected station pressure. The higher the elevation, the greater the disparity will be between those numbers and what the weatherman gives you.

The Shell Game of Correction Factors
Since internal-combustion engines are directly affected by the constant change in atmospheric conditions, long ago the automotive industry came up with a plan for using correction factors to establish a common ground from which all power numbers could be compared. The original "gross" horsepower correction factor includes the standard of 29.92 inches of Mercury (Hg), which is standard sea-level pressure, combined with a temperature of 60 degrees F with no humidity, or zero vapor pressure. As you can imagine, these are ideal or "gross" horsepower numbers that are not practical in the real world but serve as a common reference point. Through the early '70s, these were the numbers Detroit advertised and the performance industry followed. In 1972, Detroit switched to a net horsepower correction followed by several more changes, the last of which occurred with SAE standard J1349. This current Detroit correction factor uses a lower 29.235 inches of Mercury pressure with a higher air temperature of 77 degrees F and zero vapor pressure. This correction factor reduces the old gross-horsepower output number by roughly 5 percent but is also more realistic. As an example, the new 427ci LS7 Corvette engine rated at 505 hp would probably correct to closer to 530 hp using the performance-industry gross correction factor (C.F.).

To bring this home, let's take a look at a big-block power curve with three different sets of numbers in the chart below. The first column represents the observed numbers generated on a high-pressure day with 30.02 inches of barometric pressure, air temperature of 73 degrees F, and a vapor pressure of 0.35. Using the classic gross correction factor reference, this equates to 1.025 or a 2.5 percent increase over the observed power. The third column indicates power using the SAE J1349 C.F., which uses 29.235 inches of Hg station pressure, 77 degrees F temperature, and zero percent humidity as its reference point. The observed power data came from a dyno operated at very close to sea level. Using the SAE J1349 standard, the correction factor calculated to 0.978, which reduces the observed power by 2.2 percent! This works out to a differential between the gross and SAE J1349 corrections of roughly 5 percent.

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