Air Density - Playing It By Air

When you started racing, you needed to learn some skills. Not all skills, though, are based in the mechanical or the driving side of the sport. Some skill comes from observation and improving your knowledge base and, as we all know, knowledge is power. In the context of this story, power is in the air.

As racers, we tend to look at the larger, obvious mechanical or human-based variables and minimize the smaller, possibly unseen and seemingly less-influential factors. If our performance changes, we may look to a different shock package, or to the engine that may or may not be running better, or to the track that has better, or worse, grip. Possibly, our driver is on or off. It may seem to be nothing we have any control over-or is it? If we are truthful, it may be something as simple as a change in the air we breathe.

The environment has a great influence on the performance of our racing vehicles. This performance is not limited to engines alone. Any part of the car that sees the air will be impacted, and this will drive our performance variance. This applies to any aerodynamic devices on the car, wings of any type, spoilers, and ducts. It does not matter if you race on dirt or asphalt; your performance is influenced by a given atmospheric condition and how you adjust to this changing condition. We can ignore this factor or we can learn how to read the atmospheric conditions, measure and quantify the conditions, and learn how to adjust the car based on the measurements of these conditions. If you are not adjusting for the weather, you should know that your competition is adjusting.

For example, if you are racing on a dirt surface and the ambient humidity is lower than it is on a normal night for a given time of year, you can be sure the track will dry out faster than if the humidity were higher. And if the wind happens to be blowing, with a low humidity level, you may be looking at a black, slick track by the end of the first heat. This will have an influence on how you adjust the car.

The air can have a large impact on drivers and crews, so we need to be aware of this constantly changing component. The first things we need to understand are components of the air we can measure.

From a broad perspective, let's look at the components of air, as the air that surrounds us is a mixture of gases. For the most part, about 70 percent of it is composed of nitrogen, around 8 or 9 percent is made up of other trace gases mixed with carbon dioxide, and oxygen makes up the balance by coming in at about 21 to 22 percent.

Here are the measurable components that vary and create the conditions we, as racers, must account for in order to gain optimum performance:

Temperature: This is usually measured in degrees Fahrenheit. It's simple enough to measure, as all you need is a thermometer. It's wise to use one you can mount in the pits and keep track of at regular intervals.

Humidity: This is a measure of the amount of water in the air in a gaseous state.

Barometric pressure: The weight or the force the air is exerting over the surface of the planet at any given location. The pressure variance can range from 27 inches of mercury to 31 inches of mercury. While this range may not seem large, it represents a huge swing in pressure.

Wind: This is measured in both speed and direction. The movement of air across the surface and in the upper heights of the atmosphere of the planet has a measurable effect on your car's performance.

Gas Makeup:
We can measure the oxygen content of the air. Oxygen is what we seek. It takes at least 19 percent oxygen content to sustain life. While this is a fairly homogeneous number, it varies with the weather. As stated earlier, the percentage of oxygen in the air varies from 21 to 22 percent.

The weather's effect on your car's performance is huge. The difference can be felt coming off the corner as well as down the straightaway, and the performance difference can be measured by your lap times. It changes how you select carburetor jets, and it can potentially change your gear selection. It may even change the way you drive your car. If you suddenly had 5 percent more horsepower, what would you change? Gear, tire size, or possibly tire compounds-the options are numerous.

The bottom line is that a change in air density can give you a boost in power that you will have to tune around. The components of the weather that have this type of effect on your car are the barometric pressure, temperature, and humidity.