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Glossary of Tuning Terms

Load Control

Volumetric Efficiency





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Glossary of Tuner Terms

Acceleration- The rate of change in velocity with respect to time. According to Newton's second law of motion, acceleration is equal to the force, divided by mass  (A=F/M).

Accelerator pump- Accelerator pumps are found in cars equipped with carburetors. When you accelerate, the accelerator pump delivers extra fuel through the accelerator pump circuit to allow the engine to deliver more power.  

Actuator- An electrical mechanism for moving or controlling something indirectly instead of by hand, such as a door lock. Output device the PCM controls such as solenoids, relays, fuel injectors and stepper motors.

AE - Acceleration Enrichment, the enriched mixture provided when the throttle position sensor signal changes at various rates.

AFR - Air Fuel Ratio, the mass ratio of air to fuel in the combustion chamber. See NB- and WB-EGO sensors, below.

Air filter- This device filters the air that goes into your engine. Without an air filter, harmful particles would enter your car's engine and cause internal wear and damage.

Air pump  - Many emissions systems include an air pump, which pumps fresh air into a vehicle's exhaust to help complete the combustion process and reduce emissions.  To get accurate lambda measurements with the LM-1, air pumps should be temporarily disabled.

Alpha-N - Alpha-n tuning is often referred to TPS referenced load control, as this method uses just the data from the throttle position sensor with relation to engine RPM and correction factors to control fuel delivery. 

ASE - After Start Enrichment, the enriched mixture provided for a number of engine cycles when an ECU detects that the engine has transitioned from cranking to running.

Carburetor - A mechanism which mixes fuel with air in the proper proportions to provide a desired power output from a spark-ignition internal combustion engine.

Carburetor jet - A fitting inside a carburetor that meters fuel into a metering circuit where it is mixed with air.

Catalyst - A substance that can increase or decrease the rate of a chemical reaction between substances without being physically consumed in the process. A catalyst, which reduces engine emissions, is used in a catalytic converter.

Catalytic converter - An in-line, exhaust system device, containing a catalyst, which reduces engine exhaust emissions. Converters are located near the exhaust manifolds or headers for maximum efficiency.

Closed loop - refers to those times when an EFI computer is using the feedback on the mixture provided by the oxygen sensor to effectively control the injected amounts.

Combustion - The process by which the air/fuel mixture burns within an engine to create power.

Computer (PCM) - Many modern cars have a central computer called an engine control unit (ECU) or power train control module (PCM). This controls the car's fuel and ignition systems by taking information from various sensors to determine how to run the engine with the most efficiency and power.               

Converter (Torque) - A fluid coupling device which multiplies torque between an engine and automatic transmission/transaxle. When a vehicle is stopped, a converter allows enough fluid slippage, so the engine can idle without stalling.

CTS - Coolant Temperature Sensor. Usually the CTS is an NTC (Negative Temperature Coefficient) thermistor, or a resistor whose resistance varies with temperature (NTC means the resistance goes down as the temperature goes up.

DMM (digital multi meter) electronic current/resistance/potential measuring tool.

Double overhead cam (DOHC) - A DOHC engine has two camshafts in the cylinder head - one for the exhaust valves, and one for the intake valves. This allows greater efficiency and greater power.

Driveline - The system of components that connects the transmission to the wheels. The driveline consists of axles, differentials, constant velocity (CV) or universal joints, and a driveshaft.

Driver - A switched electronic device housed in a computer that controls output state. For example, a driver controls how long a fuel injector remains open.

Duty Cycle (DC)– A number indicating the amount of time that some signal is at full power. In the context of an ECU, duty cycle is used to describe the amount of time that the injectors are on, and to describe the “hold” part of the peak and hold injector drivers (see Low Impedance Injectors, below).

Early Fuel Evaporation - Used on carburetor-equipped engines only, a system where heat is used to help increase early fuel evaporation of the cold-start air/fuel mixture to achieve more efficient combustion and lower emissions. GM used an electric grid system.

EGO Sensor - Exhaust Gas Oxygen sensor, used to describe the sensor in the exhaust that measures the lean/rich state of the AFR. Used to control the via a feedback algorithm called “closed loop”.

Emissions - Emissions are the byproducts of combustion. After combustion is complete, water, gases, and carbon are released through the car's exhaust system as emissions.

Emissions equipment - Emissions equipment is equipment required by the government to keep a car's exhaust emissions to a minimum. Emissions equipment includes catalytic converter, air pump, and oxygen sensor.

Engine - A machine designed to convert thermal energy into mechanical energy to produce force or motion. Connected to a drivetrain, an engine's mechanical energy, or torque, moves a vehicle. An engine can run by using gas, diesel fuel, steam or other fuel sources.

Engine accessory - An engine accessory is a peripheral piece of equipment that runs directly off of the engine's power to supply energy or a fluid to another part of the car. Engine accessories include the alternator, power steering pump, air pump, air conditioning compressor, as well as many others.

Engine block - The engine block is where the cylinders and pistons reside. The block is the strongest part of the engine and withstands tremendous pressures while the engine is operating.

Engine temperature sender - The engine temperature switch and sending unit measure the temperature of the engine's coolant. They send this information to the engine temperature warning light and engine temperature gauge, respectively. Compare to coolant temperature sensor (CTS) which transmits the coolant temperature to the computer, and the radiator fan switch which engages the radiator's cooling fan.

Fuel injection - Fuel injection is a system by which fuel is directly sprayed into the intake manifold or intake port at high pressure. Fuel injection is often controlled by a computer, allowing precise monitoring of efficiency and performance by the car's computer.

Fuel injector - A device for delivering metered, pressurized fuel to the intake system or individual cylinders. An injector sprays fuel, which helps atomization for a more dense mixture, when combined with incoming air.

Fuel pump - The fuel pump moves gas from the gas tank and delivers it to the fuel injection system or carburetor.

Fuel starvation - Fuel starvation occurs when fuel, for one reason or another, is prevented from reaching the carburetor or fuel injectors.

Fuel system - The fuel system is the system by which fuel is stored and delivered to each cylinder. The fuel system includes the fuel tank, fuel tank level sending unit, the fuel pump, the fuel filter, and fuel lines. For carbureted cars, the fuel system also includes the carburetor. For fuel injected cars, the fuel system also includes injectors, fuel pressure regulator and often a main computer.

G-Force - Unit of measurement used to describe lateral acceleration generated while the vehicle is driven in a steady state turn on a skid pad circle. An average sedan generates 0.60 G of lateral acceleration. Measured in "gravities", one G equals the earth's gravity at sea level.

Ground - An electrical conductor used as a common return for completing an electric circuit(s). Car batteries contain a ground terminal, usually the negative terminal.

Head gasket - The head gasket seals the cylinder head to the engine block. It is subject to tremendous pressures, and often fails if and when an engine overheats.

Headers - Constructed from steel tubing, headers provide a smooth and efficient exhaust flow path from the exhaust port to the exhaust system. Headers are frequently used in performance engine applications and are generally less restrictive than the stock exhaust manifold, resulting in increased power.

High Impedance Injectors - (a.k.a. hi-Z) Fuel injectors designed to work with a simple switch in a 12 volt circuit, no special signal conditioning is required to drive them. The resistance of a high impedance injector is about 10-15 ohms.

IACIdle Air Control. Typically a “stepper motor”.

IAT sensor - Intake Air Temperature sensor, same as MAT, see below.

Idle circuit - This is a special kind of circuit found in a carburetor that only operates when the engine is at an idle.

Ignition - Complete system used to step up battery voltage to a higher voltage and deliver it to the spark plug to complete the combustion process. When the key is turned on, the ignition system is energized.

Ignition Advance/Retard - The advancing or retarding (in crank degrees) of ignition spark relative to the piston location in the cylinder. In performance applications, the goal is to set ignition timing such that peak cylinder pressure occurs at 16-18 degrees after top dead center (TDC).

Ignition module - Part of the ignition system which instructs the ignition coil to send current to the distributor.

Ignition system - The ignition system contains the components that supply spark to the vehicle's spark plugs. These include the battery, the ignition coil, the distributor (including the cap and rotor), the spark plug wires, the ignition module, and the spark plugs themselves. Older cars also have ignition points and an ignition condenser.

Knock (Engine) - The sharp, metallic sound produced when two pressure, or flame fronts collide in the combustion chamber. This could be the result of incorrect ignition timing, incorrect air/fuel mixtures, or the wrong grade (octane rating) of gas. Also known as Detonation.

kPa (kiloPascals) - the measurement of air pressure used in some ECU computations. Average pressure at sea level is 101.3 kPa.

Lambda – the ratio between actual air/fuel ratio and stoichiometric ratio.  Lambda of less than 1 is rich, and greater than 1 is lean.

Load Control - Load is essentially a measurement of airflow since, as discussed in our Volumetric Efficiency article an engine is essentially a large air pump. Since airflow determines load and is directly correlated to volumetric efficiency, and it’s operating parameters, including fuel and ignition requirements, it is critical that we have an understanding and a methodology for calculating, measuring and or programming the load of their particular engine configuration. Once airflow is known, fueling and other operating parameter simply become trivial scientific calculations.

Low Impedance Injectors - (a.k.a low-Z) Fuel injectors that are designed to run at a much lower current than would be supplied by a direct 12 volt connection. They require a special signal that is initially at full current (4-6 amps, a.k.a. “peak current”) for about 1.0-1.5 ms, but then drops down to about 1 amp (“hold current”) for the rest of the opening pulse. The resistance of a low-impedance injector is typically 1-3 ohms.

MAF Sensor - Mass Air Flow sensor. Sensor, normally mounted directly in the air intake system to extract a measurement of the actual air flow (in units of mass/time)  See Load Control 101 for how MAF is used in the calculation of engine load

MAP sensor - Manifold Absolute Pressure sensor. Measure the absolute pressure in the intake manifold (related to the engine vacuum), to determine the load on the engine and the consequent fueling requirements.

MAT Sensor - Manifold Air Temperature sensor, the same as IAT. The MAT circuit is identical to the CTS circuit, see CTS, above.

NB-EGO Sensor - Narrow Band EGO sensor, gives a switch at the stoichiometric ratio (the chemically correct mixture of air and fuel), but unreliable for AFR other than stoichiometric.

OEM (original equipment manufacturer) - refers to parts produced for initial assembly of a new vehicle.

Open Loop - refers to those times when ECU ignores the feedback from the oxygen sensor.

P&H Injectors - Peak and hold injectors; see Low Impedance injectors.

Pulse Width Modulation (PWM) - A signal with a fixed pulse width (frequency), which is turned on for part of the pulse. The percent of time that the signal is on is called its duty cycle. PWM is used to control voltage (and consequently current) to fuel injectors.

Required Fuel – For some ECUs and EFI systems, the injector pulse width, in milliseconds, required to supply the fuel for a single injection event at stoichiometric combustion, 100% volumetric efficiency and standard temperature.

Speed Density - Speed-density is one of the most common methods of load control and airflow calculations. This method uses an equation relating the manifold absolute pressure (MAP) and the intake air temperature with the known volumetric efficiency characteristics of the engine to calculate airflow, and thus makes it possible to calculate fueling requirements.

Stoichiometric Ratio- The ratio at which all available fuel is combined with oxygen during the combustion process.  This theoretically ideal ratio produces minimum emissions, however maximum power is achieved at an AFR 10-15% richer than stoichiometric, while maximum efficiency is achieved at an AFR 3-5% leaner than stoichiometric (depending on many engine variables).

TPS - Throttle Position Sensor, a voltage divider that provides information about throttle opening, from which it computes rate of throttle opening for acceleration enrichment.

VE - Volumetric Efficiency. The actual amount of air being pumped by the engine as compared to its theoretical maximum. A 200 cubic inch motor will theoretically move 200 cubic inches of air in one cycle at 100% efficiency. If the engine is actually running at 75% VE, then it will move 150 cubic inches of air on each cycle.  Ref:  Volumetric Efficiency 101

WB-EGO Sensor - Wide Band EGO sensor, can be used to derive real AFR data with mixtures from 10:1 to 20:1, i.e. anything you are likely to be interested in.

WOT - Wide open throttle.

WUE - Warm Up Enrichment, the enriched mixture applied when the coolant temperature is low.




Load Control

Methods of calculating, measuring and determining Load: Speed-Density, Mass Air Flow and Alpha-N

Load is essentially a measurement of airflow since, as discussed in our Volumetric Efficiency article an engine is essentially a large air pump. Since airflow determines load and is directly correlated to volumetric efficiency, and it’s operating parameters, including fuel and ignition requirements, it is critical that we have an understanding and a methodology for calculating, measuring and or programming the load of their particular engine configuration. Once airflow is known, fueling and other operating parameter simply become trivial scientific calculations.

There are several methods for which load can be measured, each with their own advantages, disadvantages and applications. These methods, however, are not nearly as trivial as the equations which follow. It is, however, methodical and with some study, time and proper analysis possible to determine and understand each method and how it relates to your engine and tuning approach.


Mass Air Flow

This method relies on measurement of the actual air flow (in units of mass/time) to calculate fuel flow directly. This is the most flexible and powerful system of load calculation, but is not without limitations and complications. This method relies on the data from the MAF (Mass Air Flow) sensor to calculate airflow and send this data to the engine's control unit (usually with the application of a transfer function.) The advantage in this method is that VE need not be known, since it can be calculated from the mass air flow using the equation:

VE = MAF/(Displacement x ρ(stp) x Speed)


            MAF = Mass Air Flow

            Displacement = SAE Displacement of the Engine

            ρ(stp) =  Air density (at standard temperature and pressure)

            Speed = Engine Speed


The advantage of being able to calculate volumetric efficiency directly from measurable parameters allows for a much more accurate fuel map, minimizing long and short term fuel trims thus maximizing power and consistency. The MAF method also corrects for boost and changes in throttle as the values are always positive and seen as simply higher or lower total mass flow.
As you can see in the following equation fuel flow rate can be directly calculated from the MAF data. Using the known static injector flow at the system fuel pressure makes calculation of injector pulsewidths trivial. Fuel is calculated using the equation:

Fuel Flow Rate = MAF x Target Air/Fuel Ratio

One very crucial point to the success of mass air flow systems is ensuring that the measured output from the MAF sensor accurately reflects the real time flow data of the engine and intake system. As mentioned previously, MAF systems use a transfer function in the ECM/PCM to control the variations of the signal and attempt to ensure an accurate reading (this is where the MAF calibration takes place.) Errors in the actual vs measured mass air flow can lead to incorrect fuel maps and reduced and even potentially harmful engine conditions. Placement of the MAF sensor is critical as well, as the MAF sensor expects clean and laminar air flow to accurately measure the true airflow of the engine. This may complicate the intake system and may not be possible in some applications as tight bends and compact setups may not allow for proper placement of the sensor.

It is also possible in some cases to exceed the limit of a particular sensor. This is common in applications running very high levels of boost or with any application requiring a very high limit (such as very high reving engine.) Once at the limit, the MAF sensor will be outputting it maximum value, telling the engine that airflow is steady, where in reality airflow is increasing. This will lead to lean conditions, and potential engine damage. In applications where a blow-off valve is used instead of a bypass valve the release of air which has already been metered can create issues in transient and on/off throttle responses if too extreme or not taken into account in the tune. In some cases these can be solved with creative tuning, replacement with more robust sensors or other correction factors. In some cases, however, it may warrant using a different load measuring approach. This will vary depending on the vehicle, tuner, operating conditions and even driver preferences.


Speed-density is one of the most common methods of load control and airflow calculations. This method uses an equation relating the manifold absolute pressure (MAP) and the intake air temperature with the known volumetric efficiency characteristics of the engine to calculate airflow, and thus makes it possible to calculate fueling requirements. While this method is not as robust and flexible as the MAF approach, it is not as sensitive to placement, errors and limitations in range, and in some applications can calculate airflow almost as accurately as a direct measurement. Speed-Density systems also have the benefit of not requiring an obtrusive sensor directly in the intake stream, as the MAF sensor can often actually impede airflow. The cost of implementation of such a system is also significantly cheaper. In speed-density systems the volumetric efficiency must be known and recorded in a reference table and will be used in air flow calculations. Using the universal gas law (PV = nRT) it is possible with the MAP, IAT and volume filled to calculate the mass of the air. This method is simple, but accurate. With the mass airflow now known fuel requirements can be calculated using the same equation used with the MAF method.

Speed-Density systems are very sensitive to temperature changes. As such, it is critical to pay close attention to temperature correction factors. This is due to changes in air density at differing ambient air temperatures (recall the VE and MAF equations use density at standard temperature and pressure.) Speed-Density systems also require more time to calibrate, as the entire V.E. table must be programmed into the ECM/PCM, and will be affected by engine component changes, especially parts that drastically change airflow behavior such as forced induction, cams and intake manifolds. Care should also be taken to ensure the VE map is as smooth as possible while maintaining adequate air/fuel ratios. An additional drawback with Speed-Density system can be a decrease in airflow resolution (sample rate) due to the calculation in lieu of direct measurement. Note that installation of forced induction or large increases in boost may also require the MAP sensor to be upgraded to a unit with a higher range.


Alpha-n tuning is often referred to TPS referenced load control, as this method uses just the data from the throttle position sensor with relation to engine RPM and correction factors to control fuel delivery. This method is popular in many race cars, notably those with independent throttle bodies, where consistent and repeatable mass air flow and manifold pressures are not able to be measured. In this method empirical data is used to determine airflow at a given throttle position vs rpm, and is subsequently used to calculate fuel delivery. This method is the simplest, using only the TPS and RPM data. The drawbacks can be many though. On street cars, it often lacks the resolution to provide proper drivability and emissions controls. Additionally, since there is no direct correlation to airflow and throttle position, any changes and tuning require significant time and testing to recalibrate the tune. The addition of boost will create issues, since a steady state throttle position may see changes of airflow due to the additional air provided by the turbo or supercharger. Additionally, Alpha-N tunes are sensitive to changes in barometric pressure, since the air density changes with altitude, and as such require a barometric calibration, in addition to temperature and other correction factors. This approach can often be used in a hybrid system however, such as a partial speed-density/Alpha-n tune, as can be seen in forced induction cars with independent throttle bodies. The alpha-N tune is used in “off-boost” condition, where consistent MAP signals may be difficult to obtain. As boost increases, the system will blend, and often switch entirely to a MAP based system, thus creating a more robust and accurate system.

Correction Factors

In all systems, certain correction factors will be necessary to ensure consistent and reliable operation in all conditions. Cold starts, changes in ambient temperature and air density, and even changes in driving conditions can all impact the fueling requirements of an engine. Cold starts will use the engine coolant temperature can determine how much fuel to add to aid in starting and running of the engine before it is at proper operating temperatures. Battery voltage compensations will ensure that fuel mass flow will remain constant, even with a drop in voltage (which may impact how long the injector actually opens.) Acceleration enrichment can often be important (especially in speed density systems) to ensure that fuel is added during initial throttle tip in so lean conditions are not encountered during acceleration These are just a few, and which correction factors you use, AND the order in which they are applied to your calculations will vary depending on vehicle setup, driving requirements/conditions and even your engine management system. It is best to determine which corrections are most important to your tuning method and how they change your required fueling, then apply these in the proper location in your engine management algorithms.

So which one do I use?

All this may leave you asking, “So which method should I use?”

As you may have already figured out, there is no set answer to this. Each system has its advantages and disadvantages and can work on virtually any setup, providing similar final results. The decision is ultimately, one that should be decided by yourself and your tuner. Choose the system which fits your budget and requirements, but is flexible and accurate enough to safely tune your vehicle, and most important of all, the method which you and/or any one else tuning your car is most experience and comfortable tuning with. The best results will always come from the system which is best understood by the user, regardless of the data and methods used to achieve the results.



Volumetric Efficiency

by Brian Barnhill

This can actually be a quite tricky subject, mostly due to confusion and differing opinions among many people. Volumetric efficiency (VE) is typically defined as "the actual amount of air being pumped by the engine as compared to its theoretical maximum."

Basically, VE is a measure of how "full" the cylinders are.

As most of us will know from basic science, gas will expand to fill its container. Seemingly, that would suggest that the cylinder is always full. And, in the pure volumetric sense, that is correct. A 0.5 Liter cylinder will always have 0.5 liters of air in it. The measure we are looking for here is air density. A cylinder with 500 mols/liter of air in it is said to me "more full" than one with 400 mols/liter.

Now, where is this air density measured?

This is one of the points of disagreement. The point at which air density is measured is crucial. Many will claim that you must take the measurement at a standard, such atmospheric density. This, however, can cause many issues with VE measurements. Forced induction cars will have skewed VE values due to the simple fact that they are forcing more air into the manifold. With more air available to the engine, it will receive a larger/more dense amount. This is not a pure measurement of the efficiency of the engine,

To correct for these factors, air density available at the intake manifold should be used. This will correctly measure the VE based on the amount of air available to the engine. As a simple example: Take a 4 cylinder, 2.0 Liter engine (assume even flow to each cylinder) each cylinder will be 0.5 liters. If the intake manifold has a density of 100 mols/liter (this gives 25 mols/cyl), at 100% VE, the cylinder will have 25 mols/Liter. This comes from the equation:

VE = Densitycylinder/Densitymanifold * 100%

Lets look at this another way. Say the cylinder in a single cylinder engine has 186 mols/Liter. Now, the density of at the manifold is measured at 213 mols/Liter. The calculation of VE gives: VE = 286/213 * 100% or 87.32%

It is upon this principle that variable valve timing and similar technologies rely.

They will change the flow aspects of the engine to best match the particular RPM range. An engine is typically only maximized for a particular rpm range. By allowing the change in parameters, this can be overcome. This can easily be seen when looking at DYNO charts for any Vtec equipped engine (the S2000 is a good example). In these charts there will be a "double peak." The horsepower will begin to fall off at one point, and then climb again. This rpm point will correspond to the "Vtec" point.

Volumetric Efficiency plays a large role in how your engine operates. By understanding this parameter one can begin to grasp the details required to properly tune any engine.


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