Air Density - Playing It By Air

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.