Air Density: The Pillar Of Performance Revealed With Gale Banks

Air Density: The Pillar Of Performance Revealed With Gale Banks

Author’s Note: Most articles I write are technical, and I try to take the personal aspect out of it. This article is a bit different. The truth is the research for this article grew my knowledge of engine performance more than any other I have written. My journey of understanding air density led me into a deep study of multiple subjects. I was determined to understand the concept and how our love for performance comes to fruition. Unfortunately, I only have a small amount of space to relay what I’ve learned, but I did my best to encapsulate the entire picture into something that most performance enthusiasts will appreciate.

Air density is the pillar of engine performance. It is all about the amount of oxygen you can stuff inside the combustion chamber, so you can add more fuel to create more heat energy, which is converted into mechanical power. Sometimes, we put the cart ahead of the horse and forget about understanding the most basic points in detail.

After a conversation with Gale Banks, owner of Banks Power, who often refers to himself as a data junkie, I began to understand the importance of air density. I would argue that Banks is not only a data junkie but a density junkie as well.

He is so much of a density junkie that he patented the display of air-density values and its effect on engine power to drivers in real-time with the iDash system. At the same time, his data addiction is fed by the DataMonster, which can log up to 100 parameters at 10 samples per second for an unlimited amount of time.

The Banks Power website states, “Don’t just monitor your vehicle’s performance — define it!” I believe Banks has given us the tool to do that. Within this article, I will provide insight into what air density is, what affects it, and some examples of how we modify it. Finally, I will discuss the iDash system: what it is, why it’s different, and how it’s used to define your vehicle’s performance.

All induction is forced. That includes naturally aspirated engines. The force is Mother Nature. — Gale Banks

What is Air Density?

Air density is the mass, or weight in general, of air divided by the volume it occupies. As the weight of air increases per unit of volume or it is condensed into a smaller volume, air density increases. If the weight is reduced or it occupies a larger volume, then air density is reduced.

Consider a box that measures one foot on all sides. The volume of this box is one cubic foot (1ft x 1ft x 1ft = 1 ft³). Next, the box is filled with marbles. Suppose the weight of the marbles inside the box is seven pounds. Density is a comparison of weight and volume. It is determined by dividing seven pounds by one cubic foot. The result is expressed as 7 lb/ft³ (seven pounds per cubic foot). If the volume of the box increased to two cubic feet, then the density would be reduced to 3.5 lb/ft³ (7lb / 2 ft³ = 3.5 lb/ft³).

What is Air Density

Air density is found by diving its weight by the volume it occupies. A box of marbles is used to illustrate density. Illustration courtesy of Prestige Motorsports.

However, the effective volume of piston displacement per operating cycle is fixed. A 427 displaces 427 cubic inches every two rotations of the crankshaft (four-cycle engine). A 540 displaces 540 cubic inches. This holds true for variable displacement engines as well, although the displacement may vary from one operating cycle to the next.

What does change significantly is the mass of air which fills that volume. Air mass can be measured directly by a Mass Air Flow (MAF) sensor, or calculated using pressure, temperature, and volume. Changes in air mass flowing into the engine per operating cycle results in corresponding changes in air density within the engine. As air density increases, the amount of oxygen available for combustion is also increased.

Combustion: Combining Oxygen and Fuel

Combustion is the chemical combination of a flammable substance with oxygen and an ignition source. The flammable substance is considered fuel, whether it be gasoline, diesel, methanol, or any other common petroleum-based substances used in internal combustion engines. These fuels are all hydrocarbons. As the name implies, a hydrocarbon is made of hydrogen and carbon atoms. The amount of each element differentiates one fuel compound from the next.

Oxygen combines with hydrogen and carbon elements to form oxides when an ignition source is introduced. This chemical combination generates heat during combustion that is converted to mechanical power by the engine. The amount of oxygen required to complete the combustion process is specific to each type of fuel. Chemical equations are used to calculate how many parts of oxygen are necessary per part of a particular fuel compound to reach complete combustion.

Air-Fuel Ratio: Weight of Air Per Pound of Fuel

Fuel provides the carbon and hydrogen required for combustion. Oxygen is provided by atmospheric air and can be supplemented via nitrous oxide. However, air isn’t made of oxygen alone. Consider the box of marbles. Imagine that there are two colors of marbles in the box, red and blue. The blue marbles are contributing only a percentage of the total weight. These represent oxygen, which makes up approximately 23-percent of air by weight. In order to get 1 pound of oxygen, it requires approximately 4.35 pounds of air (1 divided by 0.23).

Oxygen content in air

Oxygen makes up only a percentage of the weight of air, represented by the blue marbles. Illustration courtesy of Prestige Motorsports.

Since it is possible to calculate the parts of oxygen required for combustion and the percent of oxygen in air by weight is known, it is also possible to calculate the weight of air required per pound of fuel for combustion. The ratio of the weight of air used per unit of fuel in the combustion process is referred to as the air-fuel ratio.

Now that the requirements for combustion are known, the importance of air density in terms of engine performance is clear. Air density affects the number of oxygen molecules available to combine with fuel for combustion. Adding fuel alone isn’t enough, because we must satisfy the air-fuel ratio. That ultimately starts with air.

Density Machines: Changing Air Density

Air density is vital to engine performance. It determines how much power an engine can produce by determining how much oxygen is available for combustion. Of course, there are other factors like friction, heat loss, and volumetric efficiency. However, changing air density is the primary way of modifying power in internal combustion engines.

“Engines pump cubic feet [of air] per minute. How much does it weigh? That determines engine power. Nothing else,” says Gale Banks, owner and chief engineer at Banks Power.

Think about the box of marbles. The size of the box is fixed and represents engine displacement. If the weight of each marble does not change, the only way to increase the total weight of marbles in the box is to add more marbles.

Density Machine Illustration

Intake throttles control power output by varying air density in the intake manifold. Illustration courtesy of Prestige Motorsports.

This is precisely how spark-ignition engines control power output. The throttle is adjusting the amount of air entering the intake manifold, effectively controlling manifold air density and, consequently, power output. Think of the box filled with only a few marbles at idle and completely full at wide-open throttle. Once wide-open throttle is achieved, the manifold air density will be equal to ambient air density.

Compression-ignition engines, void of an intake throttle, operate with ambient air density in the intake and use varying air-fuel ratios to control power output. Intake air density must be modified by other means in order to increase power once the ideal air-fuel ratio is satisfied at ambient air density for either type of engine.

Ideal Gas Law

A change in air mass within a fixed volume results in a change in air density. Banks has dubbed any device that modifies air density as a Density Machine. Modifying air density is somewhat intuitive, like in the example given above. But a look at the Ideal Gas Law is necessary for a better understanding of what affects air density.

The Ideal Gas Law is used to describe the characteristics of air and other ideal gases in relation to pressure, temperature, and volume. If the equation is rearranged to solve for n, and volume is fixed, you can see that the only way to increase the number of air molecules is to increase pressure, decrease temperature, or both.

Ideal Gas Law


“All induction is forced,” states Gale Banks. “That includes naturally aspirated engines. The force is Mother Nature.”Atmospheric pressure is the primary force inducing air into the engine. As the piston moves down the bore, cylinder volume increases, creating low pressure. As the intake valve opens, higher pressure air in the intake manifold is forced into the cylinder, equaling out the pressure difference.

At the same time, the throttle opening is allowing air at atmospheric pressure to force its way into the intake manifold to equalize the low-pressure created by filling the piston displacement. At wide-open throttle, the pressure inside the intake manifold is equal to atmospheric pressure.

Altitude and weather conditions cause changes in atmospheric pressure, and therefore, changes in ambient air density. It follows then, that changes in atmospheric pressure affect maximum power. However, intake air must be compressed to increase the pressure above what Mother Nature has supplied.

All Induction is Forced

Superchargers and turbochargers are used to compress the intake air in order to raise pressure above atmospheric, greatly increasing air density. Boost is a measurement of pressure above atmospheric. That is important to understand.

One pound of boost at sea-level is not the same as one pound at 2,000 feet in terms of absolute pressure. Boost is relative to atmospheric pressure, not inclusive of it. Therefore, boost alone is not enough to determine air density in the manifold.

“Guys compare boost numbers like it matters,” says Banks. “Boost is misleading and incomplete. Ambient pressure is missing.” Manifold Absolute Pressure (MAP) is atmospheric pressure plus boost, and is used to calculate Manifold Air Density, or MAD, as Banks calls it.

“The beauty of MAD is that it’s comparable anywhere in the world. Run 25 pounds of boost on a hot day in Long Beach. Take it to Denver. Does it make the same horsepower? No. Therein lies the fallacy of boost.”


Earlier, it was stated that decreasing temperature will increase air density. To fully understand why, it is important to define the temperature at a molecular level. Temperature measures the average kinetic energy (or motion) of molecules. As heat (energy) is added, the molecules making up the air mixture vibrate faster, bounce off one another, and ultimately take up more space. As heat is removed, the molecules have less motion and take up less space. Therefore, air density increases as temperature decreases.

One last point of importance is that the relationship between pressure and temperature is directly proportional. That means as air pressure increases within a fixed volume, air temperature also increases. Boosted applications add pressure to increase air density, but temperature also rises, counteracting the density increase to some degree.


Banks Air Intercooler

Intercoolers increase air density by removing heat from the compressed intake charge.

However, the good news is that the air temperature under boost is typically higher than ambient. Heat can be removed by passing ambient air through an intercooler which contains the charged air. Other methods of intercooling include water-to-air coolers or chemical intercooling by injecting nitrous or a water-methanol mixture directly into the intake air. Each method will decrease temperature and increase air density.


Before we move on, it is important to talk briefly about humidity. Humidity is water vapor suspended in the air. The addition of water vapor affects the amount of oxygen in (and weight of) the air mixture. Humid air conforms to the ideal gas law the same as dry air, so air density is still affected by the same changes. However, oxygen content should be corrected based on the percent of humidity in the air mixture.

Since water vapor takes up space, reducing the amount of oxygen available, humidity has a negative effect on producing power. Mother Nature is responsible for humidity. As of today, there is no device widely used to modify the amount of humidity in the intake air.

Banks Air Mouse

Humidity can be monitored with the Banks AirMouse sensor module.

Monitoring Air Density, Defining Performance

Air density plays a huge role in determining how much power an engine will produce. Altitude and current weather conditions affect how much oxygen is available for combustion. This used to be calculated by paper, pencil and a portable weather station. Once fuel injection came on the scene, the weather station was replaced by sensors that input data into the engine computer.

Density machines such as cold air intakes, throttle-bodies, superchargers, turbochargers, and intercoolers further modify air density. It is critical to monitor density changes so the air-fuel ratio can be satisfied, and meaningful power comparisons can be made.

The Society of Automotive Engineers (SAE) developed standards for dyno facilities to use for power comparisons. These standards allow power measurements to be “corrected” to standardize the ambient air density between different altitudes and under differing weather conditions. The dyno measures observed torque, then a computer software is used to calculate the correction factor based on a comparison of ambient conditions with standard conditions and displays the power output as if it were tested at standard conditions.

While this is great for a testing environment, it doesn’t provide much help when climbing Pikes Peak or on a hot day at the track. Previously, even with an electronic sensor input, the one piece of data that wasn’t available without manual calculation was air density. On top of that, the operator did not have access to available power numbers in real-time, or the ability to monitor the power contribution and efficiency of individual components. Enter the iDash system from Banks Power.


The iDash Super Gauge displays OBD-II data along with proprietary air density and engine performance values, exclusive to Banks.

What is the Super Gauge and DataMonster from Banks Power?

The iDash Super Gauge from Banks Power is the centerpiece of the patented Banks Air Density System. This digital gauge integrates into your vehicle’s OBD-II network to display sensor data that your engine computer is already monitoring. Then it adds proprietary information based on calculations or direct data from additional Banks sensors and modules.

The monitor provides real-time air density and engine performance information to the driver. It also can integrate with several aftermarket ECUs or operate as a stand-alone system with Banks expansion modules. The DataMonster version adds data-logging capabilities.

Each gauge fits into a standard 2-1/16-inch mount, allowing for mounting just about anywhere. Up to four gauges can be daisy-chained, giving the driver access to up to 32 parameters at one time. Gauge layouts, colors, and displays can be customized to satisfy individual preferences. Data logs are saved on a micro-SD card for transferring to a computer or can be played back on the iDash gauges. Logs are recorded in the universal .CSV format, making them compatible with most third-party programs.

Why is the iDash 1.8 SuperGauge Unique?

The iDash monitor is the only system available that displays air density measurements and potential engine power to the driver. Boost has been the most popular way of predicting power. As we covered, boost only provides one piece of the pie, because temperature and ambient pressure are left out. The iDash system provides the rest of the pieces. Even without boost, air density is the most effective way of predicting power changes.

Banks has defined proprietary parameters that display air density in useful ways. Banks Air Density measurements can be viewed as raw values scaled to pounds per 1,000 cubic feet, for an easy-to-read range of 0 to 300. They also can be displayed as a percentage of a selectable standard day for quick comparisons. Patented parameters include Ambient Air Density (AAD), Manifold Air Density (MAD), Boost Air Density (BAD), Manifold Relative Humidity (MAN RH), and others.


Up to four gauges can be daisy-chained together, in addition to connecting to other Banks sensor modules.

Defining Performance

We’ve covered what air density is, how it’s affected, and what we do to change it, and we have looked at the iDash system’s capabilities in conjunction with the Banks air density values. Now we get to the heart of the system — defining performance. When we talk about defining performance, we usually think about the engine, all its components, and any power adders as one packaged system.

But, how often do we look at each piece individually to see what type of contribution it is making to the overall power number? That is exactly what the iDash allows us to do. By placing sensors in strategic locations, we can now monitor the contribution of individual components to overall power.

For example, if you wanted to know the contribution of a turbocharger, you could measure the air density before and after the turbo inlet. Unlike the boost gauge, measuring air density is going to tell you the effectiveness of the turbocharger, by not only a pressure increase but also accounting for any temperature increase.

By adding another sensor post-charge-cooler, we can now see the efficiency of our intercooler as not only a temperature drop but also by the pressure drop. By measuring changes in air density at select locations, we see the performance of each individual piece.


Placing sensors in strategic locations allows the iDash system to be used as an R&D tool.

If engine power is known at a given density and speed, then changes in air density can be used to calculate maximum horsepower available at the current altitude and under current conditions. In addition, volumetric efficiency can be determined by calculating the potential power if the cylinder volumes were filled completely at MAD and comparing it to maximum horsepower available. The iDash makes it possible to display all of this to the driver in real-time.

We can also use the iDash system as a research and development tool. It is now possible to know if the billet impeller you just installed performs as good as it looks by monitoring the air density change. Compressor efficiency can be monitored and compared to any component changes through temperature and pressure rise readings.

Intercooler effectiveness can also be observed by adding another sensor to measure the temperature of the air or water being used to cool the charged air. The iDash system handles all calculations and displays the result to the operator.

Air density is the pillar of performance. A change in air density is what propels the power potential and performance we crave. Understanding what air density is, what affects it, and how to modify it, is the basis of building performance. Banks’ air density information displayed through the iDash system bridges the gap between understanding effects on performance and defining them.

Article Sources

About the author

Eric Labore

Eric LaBore's extensive background includes a solid education in automotive and high performance motorsports technology and 10 years of working in the industry. Currently, he is a full-time ASE master technician and advanced engine performance specialist. As a former dyno operator and engine assembler, he is passionate about custom and performance engines.
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