.

Air Density (the weight of 1 cubic meter of air ) is a valuable tool for racing enthusiasts, because it helps determine the optimal jetting under current weather conditions.

Note:  Air density calculations for Monitor and Perception stations uses the uncorrected bar value. Air density calculations for Vantage Pro station use the corrected/final bar value.

Internal combustion engines operate at their peak efficiency (producing the most power or using the least fuel) when the correct ratio of oxygen and fuel are introduced into the combustion chamber. The carburetor jets (or fuel injector setting) control the ratio of how much fuel is introduced into a given airflow. Carburetors do not automatically compensate for changes in the amount of oxygen in that airflow and therefore changes in oxygen concentrations can result in inefficient operation. Changes in the oxygen content of air result from changes in the weather (barometric pressure, temperature, and humidity).

Measuring oxygen concentration in air is difficult; other measures are commonly used to estimate oxygen content. For example, determining changes in the density of the air (i.e., how much a given volume of air weighs) can produce a reasonable estimate of changes in oxygen concentration.

The formula used to calculate air density in the WeatherLink Software requires measures of absolute pressure (barometric pressure uncorrected for altitude), relative humidity, and temperature.

Air Density (in Kg/m3) = 1.2929 X 273.13 X ( AP – ( SVP x RH ))

( T + 273.13)  760

Where:

T  = Temperature in Celsius

AP = Absolute Pressure (mm of Hg)

SVP =  Saturation Vapor Pressure of air over water at temp T (see Table 1 below)

RH  = Relative Humidity (decimal)

Table 1. Saturation Vapor Pressure at Different Temperatures

One must develop “rules of thumb” to use air density effectively in tuning carburetors for optimal performance. One such rule of thumb is that a five percent (5%) change in air density may require re-adjustment of the carburetor jetting. High air density readings indicate more oxygen in the air, thus more fuel is needed (richer tune), while low readings indicate less oxygen, requiring less fuel (leaner tune).