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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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We all deal with engines, so we should know about their relationship with air pressure and density. Query any serious bracket racer, and you'll find he's become a virtual closet meteorologist. What is it they know that you should? Let's find out.

What You Can't See Does Hurt
This amazing planet we live on is surrounded with a delicate bubble of life-sustaining air that breaks down to 78 percent nitrogen, 21 percent oxygen, and another 1 percent of less important stuff. Oxygen is what we crave since it supports combustion. Keep in mind that gasoline won't go whoosh without oxygen. That's why we have to mix 12.9 or so parts of air with one part of fuel to get best horsepower and torque. So it makes sense that stuffing more air into an engine will make more power. But that air has a nasty habit of changing like, well, the breeze. It's not the percentage of oxygen that changes, but rather atmospheric conditions that make this such a volatile subject.

Soon after Rudolph Diesel cranked up the first internal-combustion engine running on peanut oil, car guys realized that the air they use to make power is in a constant state of temperature, pressure, and humidity change. Let's take a look at pressure first.

Earth's atmosphere extends hundreds of miles above sea level, but 99 percent of the atmospheric mass is concentrated in the first 20 miles and is held in place by gravity, which is a good thing. While air is light it does have mass, and therefore a column of air that extends 20 miles or more into space will have weight. Folks who care about stuff like that long ago established that a column of air 1 square inch extending from the outer region of our atmosphere to sea level will have a weight of 14.7 pounds. This weight becomes what we measure as a pressure of 14.7 psi. That is considered standard pressure. If you were to measure the air pressure in the mountains at 10,000 feet, you'd find it has dropped to something closer to 10 psi because that column of air is that much shorter. Besides a pressure change, with increasing altitude you also get decreasing vapor pressure and temperature. But that's not always a given since the high desert gets plenty warm in the summertime.

Air Density
When it comes to engine performance, you'll hear a lot about air density. What this refers to is the amount of oxygen molecules present in a given volume of air. Let's say we have a cube that measures 1 foot in each of its three dimensions, which equals 1 cubic foot of air.

The variables of pressure, temperature, and water vapor all play a part in determining the amount of oxygen present in that cubic foot. It's pretty obvious that as pressure increases, the density will also increase. In addition, as temperature decreases, the molecules are less active, which creates less room between them-so air density in terms of oxygen content increases. Now, let's add in the variable of water vapor. As the amount of vaporized water decreases in that 1 cubic foot of air, it increases the oxygen content and the air becomes more dense.

The combination of these three variables contributes to the oxygen content, and as you can imagine there are a staggering number of combinations possible. The ideal combination for good engine performance is high pressure, low temperature, and zero water-vapor pressure. This combines to create maximum air density that's packed with those great molecules of oxygen that will help burn fuel.

Density Altitude
The problem with atmospheric conditions is that you are forced to juggle more than one variable when evaluating air density. Perhaps you've seen an old air-density gauge that calculated density as a percent based on air temperature and barometric pressure. The problem with that gauge is that it ignores humidity (vapor pressure). Pilots were the first to work out a way to juggle two of the three variables by using a system called density altitude. For aviation purposes, vapor pressure is ignored. The formula (which is way too complex to dive into here) takes temperature and pressure conditions into account to come up with a single equivalent elevation above sea level.

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