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.

Many of us are very in tune with the weather and can tell when the weather changes. What may be slight, nearly unnoticeable changes to some people, others can feel. If it's too hot or too cold, your body lets you know right away. If the humidity rises or falls, your body lets you know right away. But barometric pressure is a bit different. Some of us have a hip or a big toe that hurts when the weather changes. While this is a unique and interesting condition, it does not always work as needed at the track, where you have to rely on instrumentation for your data points.

But why does the weather have an effect on how your engine or car performs? Just what is happening with the air that leads you to change how you adjust your engine? First we need to look at air a bit differently. Air is actually a fluid. It has many of the same characteristics as water, as it flows in much the same way that water would flow through a tube or between the banks of a river. Yes, there are some very distinct differences, but the mixture of gases we call air is a fluid. Unlike water and many other liquids, air is compressible. This is where things start to get a bit more complex-interesting, but complex.

If we stand back and look at the big picture, we see the earth is surrounded by air. All of the air surrounding the earth has mass, no different from your car or you. For the purpose of illustration here, mass indicates weight. We can measure this mass, and we'll call it measure weight.

We all know about weight. In this country, we measure weight in pounds. The weight of air at sea level is about 14.2 psi. Just what does that mean? The air is pressing down on the earth with a weight of 14.2 pounds per square inch.

Let's look at a single slice of air from ground level and go straight up about 15 or 20 miles. Within our imaginary slice of air, we can remove segments and measure the mass of the air in our slice at various levels or altitudes. We can know how much that section of air weighs. The higher we go, the less mass each segment has and the weight is less. The lower we go, the more each segment weighs. Why does it weigh more? It weighs more at the lower levels because the weight of the air in the upper levels are additive to the whole, so we have a greater amount of pressure being applied to the bottom of the pile than the top.

It would be no different if you started to pile rocks on a scale: The higher the stack of rocks, the heavier the total load. The pressure on the bottom rock is greater and greater, while the density of the rock does not increase because it is a solid and is not compressible (at least for this illustration). Air is compressible, and the pile of air compresses the air at the bottom due to the weight of the air above. It is just that simple.

We have established that the air has weight. The weight of air at ground level, where we race, is controlled by the weight of the air above. It is this component of weight that makes us faster. And the weather is what controls the air above. Are you starting to see the connections? We call this atmospheric pressure, but more on that later.

As stated earlier, the air is composed of a mixture of gases: nitrogen, xenon, helium, carbon dioxide, several other trace gases, and the biggie, the one we racers are looking for, oxygen. It is the oxygen component of the air that makes our engines run better. If there is less than 19 percent oxygen, the air will have a difficult time supporting higher life forms, such as people.

We have established that air is a fluid, it has weight, and it is composed of a mixture of gases. So how do we use this information to improve our position on the track and ultimately reach Victory Lane?

We have established that oxygen is required to develop power. It is not just the oxygen but the weight and/or pressure that are good things. Contrary to popular opinion, air is not sucked into our engines. Air is pushed into the engine. As the pistons move down in the cylinder, a low-pressure area is created. The valve opens, the carburetor is opened, and the air of a higher pressure outside the engine is pushed into the engine. The trick is finding a way to get more of this good thing.

It is not all that simple since you do not control how much you get. What you do get is the opportunity to make adjustments around the amount of oxygen you are given. We measure the weight of air in terms of pressure. The most common method for measuring atmospheric pressure is in inches of mercury, ranging, again, from 27 to 31 inches. The higher the pressure, the better, within reason.

Let's go back to our example of slicing air into segments. As we went higher up the column, we found that the weight or pressure was lower. The air is not lighter per se, but rather less of it occupies the same space. Consequently, at lower pressures, we have less air in a given volume. So, as our car is moving down the track, the engine is consuming large amounts of air. If the measured volume of air is at a lower pressure condition, we will have less oxygen to burn within the given volume of air we are pumping through the engine. Less oxygen, less horsepower-simple. It is not that we have less oxygen in an air mixture, as the homogeneity of the mixture stays the same. Dalton's Law of partial pressure states, "The total pressure that a mixture of gases exerts is equal to the sum of the separate pressures which each of the gases would exert if it occupied the whole volume." Translation: The percentage of oxygen in the air is the same at 27 inches of mercury as it is at 31 inches of mercury.

What we need to remember is that at lower pressures our engine is not going to run as well as when the air pressure is higher, because in a given volume of air the oxygen content will be less than when the pressure is higher. To recap, for a given volume of air at a lower pressure, the total of the content of all the gases that make up the air is reduced. That means you will have less oxygen to help burn fuel, and burning fuel is where your power comes from.

If the barometric pressure starts to fall, not only will you have a reduced volume of oxygen to your engine, but your wing or spoiler (assuming you are using one of the two) will not generate the same amount of downforce at a given speed because the air has less weight. This could create a loose condition on the track. It may require you to dial-in more angle of attack to the wing or increase the angle of the spoiler to generate the same amount of aero forces you may be used to getting. This may create a higher-drag aero package than you may be used to running. If the barometer starts to rise, then the wing or spoiler will work better.

What types of changes will you make as the air pressure changes? If we look at a typical day at the races, for example, you may find after practice that the air pressure has dropped from 30.2 inches of mercury to 28.2 inches of mercury. Or if you're using an air density gauge, you may have seen a drop of 12 to 15 points. That drop is almost 1 psi. In either case, you need to make an adjustment to keep your engine running correctly. The first thing you should do is consult your notes to see if this type of thing has happened on prior race or test days. In any case, your engine may now be running rich. Remember, you will not have the same volume of oxygen in each gulp of air the engine takes. If you do not have notes, shame on you.

Your first change may be something as small as one jet size, and this could be all it takes. It may take a larger drop in jet size to compensate for a drop in air pressure, and to avoid running lean you may want to richen the engine if the air pressure goes up.

Great care should be exercised when making changes that will result in large changes to the jetting. You could very easily go too far and damage engine components.

What about the gearing? You may be making less horsepower. Can you still pull the same gear? You may have to change the gear ratio by a few tenths to compensate for the lower or higher horsepower. Will engine performance change the way you drive the car? Only you can answer that question.

Just remember the change in the air pressure will affect the engine. Lower air pressure will almost always equal less horsepower. Conversely, higher air pressure will mean that the engine will have the oxygen, based on the volume of air, to burn more fuel; that almost always means more horsepower. It is very simple. This is not rocket science.

How will we know what the air is doing? First we can observe. Look at the sky. Are clouds forming, or is the cloud cover dissipating? Is the wind blowing? Remember that wind always blows from a high-pressure area to a lower-pressure area. Where are you in relation to how the wind is blowing? These are signals that the weather is changing. Have you noticed a change in the temperature, or has the humidity changed?

The first tool you'll need is not in your toolbox. No, it is you, using your powers of observation. Open your eyes, look around, and be aware of the world around you!

There is an abundance of tools available for you to use to monitor the weather. Many performance shops sell weather stations. You need only look as far as Wal-Mart, Kmart, Home Depot, Lowe's, Sears, Radio Shack, or look in any of the automotive racing magazines to find barometers, thermometers, and/or air density gauges. Many furniture stores sell decorative weather stations that really work, and many times they include some very good gauges that individually cost much more than the whole set when purchased separately. They include a hygrometer for measuring humidity, a barometer to measure barometric pressure, and a thermometer to measure temperature. And they all come mounted on a decorative display that you can mount to the wall in your trailer for that homey touch.