Air Density, what is it and why is it important?

If you spend a small fortune on your supercharger, turbocharger or intercooler, how do you evaluate them… boost? All those power adders are designed to increase air density flowing into your intake manifold! Boost is interesting but can also be misleading, you need a manifold air density gauge, and here it is… interested?

Banks iDash 1.8 Super Gauge
The Boost Gauge is dead to me!


Banks Boost Air Density:

Something so NEW that it’s Patented!
U.S. Patent 7,254,477 B1
U.S. Patent 7,593,808 B2

Horsepower is directly dependent on the number of oxygen molecules available for combustion. Simply put, the more oxygen your engine has, the more fuel it can combust and the more Horsepower it can make. Boost pressure has been the go-to performance measurement to predict Horsepower changes because it directly affects the amount of oxygen available to the engine. However, it is only part of the story and leaves out the temperature effects on oxygen content,which is a critical element that can greatly alter your engine’s Horsepower.

So if Boost isn’t the best performance indicator, what is? Air Density is the most direct measurement of oxygenmolecules being used by your engine for combustion. Unlike looking simply at Boost pressure, Air Density accounts for the effect that temperature plays on the oxygen content of the air. Air Density is defined as the lbs. of air mass per ft^3 of volume. We display this parameter as lbs./1000ft^3, which scales it to an easier-to-read range of typically 0-300.

Because the Air Density changes based on pressure, temperature and humidity, the power output of your engine will also change based on your current ambient conditions. The Society of Automotive Engineers (SAE) has defined a set of ambient conditions that all engines must be corrected to when measuring Horsepower. SAE J1349, which is the most common correction factor, uses an ambient pressure of 14.4 psia, an ambient temperature of 77 deg F and a relative humidity of 0%, which results in an ambient Air Density value of 72.2 lbs./1000 ft^3. Another common Air Densitystandard is SAE J607, which specifies an ambient Air Density value of 76.4 lbs./1000ft^3.

All Banks Air Density measurements can be viewed in their raw value form, or normalized against a selectable standard day and read as a percentage. This makes it quicker to compare to standard conditions. If Ambient Air Density (AAD) is over 100% then there is more oxygen per unit volume in the air and if it is below 100% then there is less oxygen content than the J1349 standard day.

Manifold Air Density (MAD) is calculated just like AAD except it uses the Manifold Absolute Pressure and Manifold Air Temperature data. This reading represents the mass of oxygen per unit volume available in the manifold. Boost Air Density (BAD), which is calculated as MAD – AAD, is another useful measurement. BAD is the additional Air Density available in the manifold that is greater than the current ambient conditions, and is a more insightful performance measurement than simply using Boost pressure.

 Density Definitions:

AAD – Ambient Air Density
Mass of air per volume of the air surrounding the vehicle.

BAD – Boost Air Density
Mass of air per volume in the intake manifold greater than the Ambient Air Density.

MAD – % Manifold Air Density
Manifold Air Density expressed as a percentage of the selected standard day.

AAD – % Ambient Air Density
Ambient Air Density expressed as a percentage of the selected standard day.

BAD – % Boost Air Density
Boost Air Density expressed as a percentage of the selected standard day.

DCF – Density Correction Factor
Ratio of the actual Ambient Air Density to the selected standard day.

D-RAT – Density Ratio
Ratio of Manifold Air Density compared to Ambient Air Density.

MAD – Manifold Air Density
Mass of air per volume in the intake manifold.

MAN RH – Manifold Relative Humidity
Relative humidity in the intake manifold (a value greater than 100% indicated condensation).

RH* – Relative Humidity (B-Bus)
Percentage of actual water vapor in the ambient air compared to the maximum potential water vapor.


Watch some videos below to find out more: