Altitude Adjustment

Pilot to crew. We're passing 10,000 feet, time to go on oxygen.

That’s a line from the WWII movie Twelve O’Clock High , with Gregory Peck talking to the crew of his B-17 bomber on a mission over Germany. The B-17 was a four-engine bomber that required the crew to wear oxygen masks at high altitudes because of the lack of oxygen at extreme altitudes.

Just as humans need air to perform at their peak, engines need good air to make power, and changing weather patterns have a tremendous impact on performance. Altitude, temperature, barometric pressure, and humidity all play a part in the total oxygen content of air. Recently, we tried very hard to push our Agent 87 350 engine in a ‘65 El Camino into the 12s and missed by a frustratingly few hundredths of a second. That could easily be attributed to weather conditions. We decided we needed to know more about the air that we use to make power.

Most racers use portable weather stations to monitor the constantly changing atmospheric conditions. Kyle Seipel at Helmet City sent us a PerformAire machine to use whenever we go to the track so we will no longer have to guess about the air.

Current Conditions

Almost since the first hot rod, racers have paid particular attention to the weather to get a handle on estimating track performance. This is especially important for bracket racers where even a seemingly insignificant change in the humidity can drastically affect the car’s e.t. While all racers track the weather, few do it with the religious intensity of bracket racers.

Over the years, racers have employed a complex combination of aircraft altimeters, barometers, and wet/dry bulb thermometers to measure the weather. These pieces have been replaced by the simple digital weather station from companies like PerformAire. These machines measure the four most important factors in determining the oxygen content of the air our engines breathe: altitude, barometric pressure, temperature, and humidity.

How High Is Up?

Altitude and barometric pressure are closely related. As altitude increases, barometric pressure decreases, or stated another way, barometric pressure is inversely proportional to altitude. This is because at any one place on earth, you stand under a column of air that at sea level is roughly 50 miles high. Even though air weighs very little, stacked 50 miles high, it creates pressure, measured in inches of mercury (in/Hg). At sea level, this pressure is 29.92 in/Hg, which also equals 14.7 pounds per square inch (psi).

If you were to climb a mountain carrying a barometer starting at sea level, you would notice that as you climb, the pressure indicated by the barometer decreases. This is because as you climb, you are reducing the height of the column of air above you. If you reached the summit of Mount Everest, you would be at 29,028 feet above sea level and the pressure would be very low compared to the sea-level reading. Since the air is very thin, the oxygen content is very low.

Heat Kills

The next most important portion of our automotive atmospheric equation is temperature. This is relatively easy to understand—hot air is less dense than cold air. There will be more oxygen in one cubic foot of air at 60-degrees F than there will be at 90-degrees F. You’ve probably experienced this because engines make less horsepower as the temperature increases. That’s why drag racers like to qualify at night when the air is usually cooler.

Wet Weather

The final major component of atmospheric conditions is humidity, or vapor pressure. Humidity refers to the amount of water in the air and is expressed most often as relative humidity in percent. Relative humidity means that at a given temperature, there is a maximum amount of water that the air can hold. For example, 40 percent humidity means that the current conditions contain 40 percent of the total amount of water the air can hold. Since water doesn’t burn, greater humidity means this water will displace a certain amount of oxygen and that this water in the cylinder will tend to quench a portion of the combustion process. The bottom line is reduced horsepower.

The Package

Now that we know what all these things mean, it’s time to put them all together. Racers have used several different combinations to establish where the air is, but the most common is a single standard called density altitude. Density altitude is expressed as an altitude figure, such as 1,000 feet. This refers to the level of oxygen present in the air at that altitude with the temperature at 60 degrees F, with a standard pressure of 29.92, and dry air, or zero percent humidity. In classic hot rodder fashion, the shorthand for this would be “The air’s at 1,000 feet.”

Perhaps you’ve been to the track and overheard a racer say, “Man, the air’s really good. We’re at 200 feet below sea level!” This can occur with the right set of circumstances where the pressure is high and the temperature is below 60 degrees with low humidity. Plug all these factors into the equation (we’ll spare you the math) and the density altitude can come out to a below–sea-level figure. That means the air is exceptionally dense and the engine should make great power.

Use The Numbers

Density altitude numbers don’t tell us very much until we start to apply it to our street or race car. For example, let’s say you have a ‘68 Camaro with a 400ci small-block that runs high 11s. Over a period of months, you’ve accurately recorded the density altitude at the track every time you run your car. After awhile, you’ve noticed a direct correlation between the density altitude and how well your car runs. This is the value of a unit like this PerformAire weather station.

Let’s say you make a change to your car and it doesn’t pick up the e.t. and speed that you think it should. If you kept track of how much your car is affected by density-altitude changes, you can then predict how much e.t. and speed was lost because you chose a high density altitude day to test your car.

We spent some time with multiple-time national event winner Kyle Seipel discussing how important weather is to drag racers and learned some extremely useful information. Kyle runs both a Super Stock Camaro and a dialed-in 9.90 Super Gas Corvette. According to Kyle, a typical race car like his Super Gasser will lose 0.01-second for every 100 to 120 feet of change in density altitude. This may not be as solid for street cars, but a 500-foot change in density altitude will still be worth as much as 0.03 second.

Kyle also gave us some interesting insights into the relative strengths of temperature, pressure, and humidity. Temperature has the strongest affect on density altitude with a one-degree change worth a 100-foot change in density altitude. Pressure is next with a 0.01-in/Hg change also worth a 100-foot change in density altitude. Finally, a 10 percent difference in humidity can also affect the density altitude by 100 feet.

We’ve only offered a whiff of the esoteric details involved in the atmospheric-changes/engine-performance equation. But it’s clear that you can be a winner or a loser depending on how much you pay attention to the winds of change that blow around your car. It’s those details that make the difference.