Also known as "Dynamic Cylinder Air", airmass is the amount of air (by weight not volume) in a cylinder at any given time. This is a very important number for tuners because the higher the mass of air is in the cylinder, the more fuel can be added. Essentially, more air = more power.

Displacement on Demand (DoD)

Also known as "Active Fuel Management", DoD is GM's name for the ability of some engines to deactivate half of their cylinders when operating in light load conditions. Theoretically, this feature enhances fuel economy and reduces emissions slightly.

Dynamic Airflow

The mass airflow obtained either from reading the MAF sensor or from speed-density calculations, after corrections are applied.

Electronic Throttle Control (ETC)

In vehicles that employ ETC, there is no longer any mechanical linkage between the accelerator pedal and the throttle. The engine (or powertrain) controller dictates the amount of throttle applied based on a number of factors including (among other factors) current engine rpm and vehicle speed as well as the position of the accelerator pedal.

Engine Control Module (ECM)

In some vehicles, the ECM (sometimes called the Engine Control Unit / ECU) is the computer that is primarily responsible for controlling the operation of the engine. Other vehicles have a Powertrain Control Module (PCM) that controls both the engine and the transmission.

Fuel Trims

Although the MAF sensor and/or speed-density tuning will give the engine controller a rough idea of the amount of fuel that needs to be injected in a variety of circumstances, it's not always perfect. Changes in engine load and other conditions may cause the air/fuel mix to be less than ideal. For this reason, the engine controller constantly monitors the amount of oxygen in the engine's exhaust and applies adjustments to the amount of fuel injected. These adjustment are called "fuel trims".

There are two types of fuel trim:

Short Term Fuel Trims (STFT): These trims are an immediate response to the amount of oxygen seen in the exhaust at the moment. As such, they are being adjusted constantly.
Long Term Fuel Trims (LTFT): These trims are the result of monitoring changes in the exhaust oxygen content over long periods of time and determining the average amount of adjustment required based on long term trends.

Intake Air Temperature (IAT) sensor

IAT sensors measure temperature of the air coming into the engine. In speed-density tunes, this data is used in conjunction with a pressure (MAP) sensor and the VE tables to determine the mass air flow.

Manifold Absolute Pressure (MAP) Sensor

MAP sensors measure the air pressure in the intake manifold. In speed-density tunes, this data is used in conjunction with a temperature (IAT) sensor and the VE tables to determine the mass air flow.

In naturally aspirated engines, the pressure measured will be roughly 1 bar (14.7 psi / 100 kPa), which is the normal pressure of the air we breathe at sea level. Therefore, these engines typically use a "1 bar" MAP sensor. Components such as turbochargers and superchargers that force more air into the engine increase the pressure measured and may require a MAP sensor rated for more than 1 bar.

Mass Air Flow (MAF) Sensor

MAF sensors measure the mass of air entering the throttle body. Some engines use this type of device to measure the mass air flow directly rather than (or in addition to) using speed density techniques to determine it by calculation.

Engines with MAF sensors typically have a MAF Calibration Table to translate the signal level (a voltage or a frequency) coming out of the MAF sensor to the amount of airflow represented by that signal.

Powertrain Control Module (PCM)

In some vehicles, a single computer is responsible for controlling both the engine and the transmission. In such vehicles, this computer is called the PCM. Other vehicles have a separate Engine Control Module (ECM) and Transmission Control Module (TCM).

Pulse Width

The amount of time (in milliseconds or microseconds) a fuel injector remains open. Varying pulse width is typically how engines control the amount of fuel being injected into the cylinder.

Spark Advance

The timing of the spark provided by the spark plugs. Spark plugs typically ignite while the piston is moving upward towards the top of the cylinder. Spark advance is how far before the piston reaches the top that the spark plugs ignite. It is usually measured as a number of degrees of crankshaft rotation. So, a spark advance of 10 would indicate that ignition occurs when crankshaft rotation is 10 degrees before the piston reaches top dead center.


A mode of operation that calculates the mass of air present in the cylinder rather than measuring the mass air flow. The calculation is based on air temperature and pressure gathered from the sensors as well as the values in the VE table(s).

Transmission Control Module (TCM)

In some vehicles, the TCM is the computer that is primarily responsible for controlling the operation of the transmission. Other vehicles have a Powertrain Control Module (PCM) that controls both the engine and the transmission.

Variable Valve Timing (VVT)

In a typical engine with fixed cams, the valve timing remains the same, regardless of engine speed and other conditions. This tends to create a sweet spot, where performance and emissions are best at a particular rpm.

Engines with variable valve timing can vary the timing of their intake and exhaust valves in order to achieve more optimal results across the board. This is typically achieved by rotating their cams to a position appropriate for the current rpm.

Vehicle Control Module (VCM)

The computer that is primarily responsible for engine and transmission operation. In some vehicles, there is a single Powertrain Control Module (PCM) that controls both the engine and the transmission. In other vehicles, there are actually two modules: an Engine Control Module (ECM) and a Transmission Control Module (TCM).

Volumetric Efficiency (VE)

The actual amount of air flowing through the cylinders, compared to the theoretical maximum they can hold.

In naturally aspirated engines, VE is normally less than 100%. Components that force air into the cylinders (turbos, superchargers, etc.) may boost VE above 100%.