Density Altitude is only a function of the local atmospheric weather conditions. It cares nothing about what type of racing you are doing, or what kind of fuel you are burning. Density Altitude only describes the ambient air density in the units of feet (of altitude). Simply stated, Density Altitude is the elevation having the same air density as the local, measured atmospheric weather conditions.

For example; if the temperature is 60 deg F, and the "absolute" barometric pressure is 29.92 in Hg, and the relative humidity is 0% - the Density Altitude is 0 ft. Consider another example with a temperature of 42.2 deg F, a barometer of 24.9 in Hg, and 0% relative humidity - this has a Density Altitude of 5000 ft.

However, when you add in the effects of water vapor, things start to get a little more complicated! Water vapor takes up space that would otherwise be occupied by dry air (which normally contains almost 21% oxygen). Generally, motorsports is concerned with air-breathing, oxygen burning engines . . . so the amount of oxygen that is in the air is a key feature. Water vapor takes up space otherwise occupied by oxygen molecules. How much? The space occupied by the water vapor is proportional to the water vapor pressure.

However, there really is nothing mathematically incorrect about the definition of Density Altitude . . . it's all about what you're going to use it for. To help illustrate this point, please consider a common tool in our racer toolbox - a standard 6 inch caliper. A 6 inch caliper is a pretty complicated device, having several components, all put together in a way that allows very accurate measurements of length - actually to the nearest thousandths of an inch. Calipers are manufactured by several different companies, and all will give basically the same answer when used properly. However, a caliper is not the proper tool for measuring the weight of the pistons in your engine.

The same idea or concept is true for Density Altitude. It is an excellent tool for determining the elevation having the same air (or oxygen) density as the local, measured weather conditions . . . but it won't give you the correct answer for predicting the performance of your race car.

Why not? First, we must go back and examine the history of term "Density Altitude". Density Altitude was originally invented decades ago for use in aircraft flight manuals. Pilots are critically concerned that they will be able to safely liftoff the ground during the takeoff roll within a given runway length. The lift generated by the wings (at a critical speed) must be greater than the aircraft weight in order to actually get off the ground. Lift is directly proportional to the local air density.

Motorsports initially used Density Altitude for jetting Holley carbs, and for this purpose, it does a really good job. Air (or oxygen) and fuel must be mixed in the proper ratio for best engine performance. The "Problem" with Density Altitude started with the advent of bracket and super class drag racing where the need to accurately predict ET under varying weather conditions became critical to winning races. The erroneous assumption of physics used by many at the time was that drag racing ET is proportional or otherwise clearly related to Density Altitude.

The way that an engine's power varies with changing weather conditions is thoroughly documented in the Society of Automotive Engineers (SAE) standards. These are the same procedures used for correcting observed engine dyno data to the standard day conditions of sea level, 60 deg F and dry air.

OK, in Bubba terms what does all this mean in relationship to our seven runs on the salt at Bonneville.