Practical Engine Airflow. John Baechtel

more traditional methods of coaxing more air into the engine via manipulation of the flow path dimensions through ram tuning, head porting, and valve work.

      For the most part, I concentrate on examining and understanding airflow in the naturally aspirated sense. Moreover, it is a fortunate coincidence that most airflow enhancements are pretty much complementary to any type of supercharging.

      Next, you need to quantify some basic properties of air and think about how you can influence or employ them to fill your engine’s cylinders more effectively. As shown in the chart (at right), the basic properties of air are characterized by mass, volume, density, specific volume, pressure, temperature, and viscosity. Their relationships seem complicated for something as seemingly intangible as ordinary air, but they are pretty well defined according to scientific principles.

Oxidizers and Oxygen-Releasing Compounds

      Atmospheric oxygen is the most plentiful oxidizer available for IC engines. It offers the considerable advantage of an unlimited supply that does not require onboard storage. There is no weight penalty and the power-to-weight ratio is favorably increased for free.

       Although my focus here is on engine airflow and the techniques that can be employed to improve it, you should note that in addition to mechanical supercharging (and turbos) more oxygen can also be supplied to enhance IC engine performance in two primary forms: nitromethane racing fuel and nitrous oxide injection. Both release extra oxygen during the combustion process and thus require more fuel to ensure the proper air/fuel ratio for best power. This is often referred to as chemical supercharging because the added oxygen is artificially introduced via a tube and not directly supplied by atmospheric pressure.

       A secondary means of supplying the additional fuel to support the increased oxygen content is required. Kits to support this are plentiful and easy to apply. Numerous other oxygen-releasing compounds are available, but most are too toxic, corrosive, or difficult to store and handle. This makes nitromethane and nitrous oxide the most popular secondary sources of oxygen.

       This color-coded chart illustrates common combinations of molecules. For the purpose of this engine discussion, only oxygen and nitrous oxide are useful. In a nitrous oxide–injected engine, the oxygen separates at 565 to 575 degrees F, thus providing additional oxygen that must be supplemented by additional fuel for more power.

      Most of this has a much greater effect on tuning issues than airflow, but it’s important to understand the fundamentals. Maximizing airflow is your goal, but some properties of air can impede this to some slight degree.

Mass and Weight

      Mass is generally represented by air density, which is a function of temperature and pressure. Cooler air is heavier and denser; hotter air is lighter and less dense. When discussing these properties it is advantageous to recognize the distinction between mass and weight. Scientifically, mass is defined as the amount of matter contained within an object. It does not change. Weight is the force of gravity acting upon a given mass. It is represented as:

      W = mass × g

      Where:

      W = weight (force)

      g = gravity (acceleration)

      The mass of an object times the acceleration equals the active force or weight with acceleration or gravity equal to 1 g.

      Weight is a force. Gravity causes the upper levels of the atmosphere to press down on the air underneath it, creating pressure relative to the particular elevation point in the vertical air column. Air surrounding an object presses against all sides of the object with equal force. The specific weight of air varies with elevation, temperature, pressure (elevation determines pressure), and the amount of water vapor and/or fuel contained within a given air mass (volume of air). Standard air at sea level weighs 0.0763 ft-lbs³.

      Basic Properties of Air

      

      Dry air at 15 degrees C (59 degrees F) and minimal or dry moisture content is the standard (ideal) reference. Dry air contains little or no moisture so there is more room for fuel vapor in a given volume. Cooler air also promotes greater density (oxygen content), which requires the addition of more fuel to burn completely.

      These terms are frequently misused because the mass of an object is commonly (incorrectly) referred to as its weight. For example, the volume of an air/fuel mixture in a given cylinder is referred to as the “trapped mass” when the valves are closed. The greater the trapped mass, the higher the theoretical power. That mass has a specific weight and volume according to its density, which varies according to temperature and pressure.

      The weight of air depends on where the air is located within the local column of air that extends from sea level to the edge of space. Air on a mountaintop is less dense and lighter because there is less air above it pressing down on it; the temperature is also lower. So you have a temperature and pressure reduction and a lessening of water vapor pressure as well.

      Pressure, temperature, and water vapor content are important components of engine performance from a tuning standpoint. Higher pressure (density) improves performance; higher temperature lessens performance. And water vapor is the joker that spoils everything by displacing oxygen. Your charge as an engine builder is to achieve optimal airflow despite the influence of these components, which are generally identified as weather conditions.

Density

      Density is the mass (weight, in a sense) per unit of volume, or the specific amount of matter contained within a specified volume (density = mass ÷ volume). Density increases with pressure; high density increases the mass-per-unit volume. Higher density equals greater trapped mass and, thus, more power from the increased amount of oxygen, provided you can supply the correct amount of fuel to encourage effective combustion.

      Water with a density of 1 is the common reference, or 1.000 g/cm³ by definition.

      Air density is a huge factor in operating high-performance engines. Denser air contains more oxygen molecules unless it is saturated with water vapor. Density is primarily influenced by elevation, temperature, and humidity, but not pressure. A given volume of colder air contains more air molecules than the same volume of warmer air, thus more oxygen is available to burn more fuel. Your car feels peppier on a cool, dry day as opposed to a blisteringly hot day with lots of humidity displacing what few oxygen molecules are available.

      To a lesser degree, high humidity also affects air density by displacing oxygen molecules. Air saturated with water vapor is less dense than dry air and because it contains water vapor and fewer oxygen molecules, it doesn’t support greater power output. Cool, dry air is the most desirable. It is denser, thus providing ample room for additional oxygen molecules.

Coping with Variables

      If you look at a cubic foot of air at any given location, you are required to contemplate its density based on its temperature, pressure, and amount of water vapor. When any of these variables changes, the density varies. So a hotter cube of air is less dense than a cooler cube. Oxygen content changes with these variables. Vapor pressure is also a factor by displacing some amount of oxygen molecules within the given volume.

      Dealing with these atmospheric variables makes it difficult to accurately judge air density. In the absence of an “oxygen meter” you have to rely