Updated August 2022 by Jerry Hoffmann
EFI Tuners Guide – Glossary of Terms
This is a living document that I’ll be updating as I continue to write future chapters of this EFI Tuning Guide. This Glossary aims to define and to help you understand all of the various terms and TLA’s (Three Letter Acronyms… lol) that you’re likely to encounter as you read deeper and learn the ins and outs of Electronic Fuel Injection and Ignition Systems using a modern Standalone Engine Management System like the MS3Pro/MSPNP/MicroSquirt/MegaSquirt line of ECUs.
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Acceleration Enrichment
Abbreviated to AE. An extra amount of fuel added to compensate for the increase of air as the throttle opens during acceleration.
AE
Abbreviation for Acceleration Enrichment. An extra amount of fuel added to compensate for the increase of air as the throttle opens during acceleration.
AFR
Abbreviation for Air Fuel Ratio. This is the measurement used to tell you how much fuel is mixed with how much air going into your engine. Example would be an ideal AFR of 14.7:1 for gasoline. This means 14.7 parts air to 1 part fuel.
After Start Enrichment
Abbreviated to ASE. Extra fuel delivered to the engine after start up to ensure “wall wetting” and stable engine operation.
Air Fuel Ratio
Abbreviated to AFR. This is the measurement used to tell you how much fuel is mixed with how much air going into your engine. Example would be an ideal AFR of 14.7:1 for gasoline. This means 14.7 parts air to 1 part fuel.
Air Intake Temperature
Abbreviated to AIT, sometime called MAT (Manifold Air Temperature). A sensor used to measure air temperature by the ECU as part of the fueling calculation. May also be used in the ignition timing calculation.
AIT
Abbreviation for Air Intake Temperature, sometime called MAT (Manifold Air Temperature). A sensor is used to measure air temperature by the ECU as part of the fueling calculation. May also be used in the ignition timing calculation.
ASE
Abbreviation for After Start Enrichment. Extra fuel delivered to the engine after start up to ensure “wall wetting” and stable engine operation.
Before Top Dead Center
Abbreviated to BTDC. A crankshaft position before a cylinder has reached the very top of its travel. It is between traveling up and traveling down in the cylinder. A normal ignition event occurs BTDC. Typically, ignition timing advance is expressed in degrees BTDC, where “32 degrees of advance” is referring to an ignition event at crankshaft angle 32° BTDC, or before the piston is at the full top of its travel.
BTDC
Abbreviation for Before Top Dead Center. A crankshaft position before a cylinder has reached the very top of its travel. It is between traveling up and traveling down in the cylinder. A normal ignition event occurs BTDC. Typically, ignition timing advance is expressed in degrees BTDC, where “32 degrees of advance” is referring to an ignition event at crankshaft angle 32° BTDC, or before the piston is at the full top of its travel.
Camshaft Position Sensor
Abbreviated to CMP. Used to monitor the exact position of the Camshaft in an internal combustion engine.
CANbus
Abbreviation for Control Area Network (CAN). A network of automotive devices allowing communications for sharing of sensor data and other control information.
Carburetor
Mechanical way to mix air and fuel in the appropriate ratio for a given internal combustion engine. Often referred to as Carb.
CAS
Abbreviation for Crank Angle Sensor or Cam Angle Sensor. Sometimes used interchangeably for one or the other and must be understood in context. We recommend instead using phrases CKP for Crank Position Sensor and CMP for Cam Position Sensor.
CHT
Abbreviation for Cylinder Head Temperature. A way of measuring engine temperature that may be used in lieu of Coolant Temperature. Common on air cooled and often on newer engines from companies such as Ford.
CID
Abbreviation for Cubic Inch Displacement. Engine size in cubic engines. Can be translated to Liters as 1 Liter = 61 Cubic Inches.
CKP
Abbreviation for Crankshaft Position Sensor. Used to monitor the position or rotational speed of the crankshaft in an internal combustion engine.
Closed Loop
A control method where the ECU uses sensors to monitor conditions and adjust outputs to match actual conditions to targets. Examples would be such as adjusting turbo boost control so that the measured boost (MAP) matches target boost even during changing weather and altitude conditions.
CLT
Abbreviation for Coolant Temperature Sensor. This sensor is used to measure the temperature of your engines coolant system. (Also sometimes referred to as ECT or Engine Coolant Temperature Sensor)
CMP
Abbreviation for Camshaft Position Sensor. Used to monitor the exact position of the Camshaft in an internal combustion engine.
CNP
Abbreviation for Coil Near Plug, or sometimes referred as Coil Per Plug. A coil pack that is in close proximity to the spark plug and is connected with a short spark plug wire. An example would be the IGN-1a race coil.
Coil Near Plug
Abbreviated to for CNP, or sometimes referred as Coil Per Plug. A coil pack that is in close proximity to the spark plug and is connected with a short spark plug wire. An example would be the IGN-1a race coil.
Coil On Plug
Abbreviated to COP . A type of ignition coil where the coil itself fits on top of a spark plug via a boot and does not use a spark plug wire.
Configuration Error
Also known as Config Error. An error message displayed in TunerStudio upon power up when the user/Tuner has made a serious error in the calibration and the ECU can not respond as instructed. A user error. The error must be corrected before the MegaSquirt will allow operation of the engine. The user will also be notified of a Config Error by rapid cycling of the fuel pump when the ignition is turned on.
Coolant Temperature Sensor
Abbreviated to CLT. This sensor is used to measure the temperature of your engines coolant system. (Also sometimes referred to as ECT or Engine Coolant Temperature Sensor)
COP
Abbreviation for Coil On Plug. A type of ignition coil where the coil itself fits on top of a spark plug via a boot and does not use a spark plug wire.
Crankshaft Position Sensor
Abbreviated to CKP. Used to monitor the position or rotational speed of the crankshaft in an internal combustion engine.
Crank Angle Sensor or Cam Angle Sensor
Abbreviated to CAS. Sometimes used interchangeably for one or the other and must be understood in context. We recommend instead using phrases CKP for Crank Position Sensor and CMP for Cam Position Sensor.
Cubic Inch Displacement
Abbreviated to CID . Engine size in cubic engines. Can be translated to Liters as 1 Liter = 61 Cubic Inches.
Cylinder Head Temperature
Abbreviated to CHT . A way of measuring engine temperature that may be used in lieu of Coolant Temperature. Common on air cooled and often on newer engines from companies such as Ford.
DC
Abbreviation for Duty Cycle. A value indicating the percentage of time an item or output is active in a cycle compared to the amount of time the entire cycle took.
Digital Multimeter
Abbreviated to DMM. May also be called Digital Volt Meter (DVM) or Digital Ohm Meter (DOM). A tool used to measure voltage, resistance, continuity, and higher end models may also measure duty cycle, frequency, capacitance, and other aspects of signals or devices. Used for installation or troubleshooting of electronics. One of the most important tools an automotive technician can own.
Diode
A small electrical component that acts kind of like a check valve for electricity. It allows current to flow in a single direction and is a common component in electronics.
DMM
Abbreviation for Digital Multimeter. May also be called Digital Volt Meter (DVM) or Digital Ohm Meter (DOM). A tool used to measure voltage, resistance, continuity, and higher end models may also measure duty cycle, frequency, capacitance, and other aspects of signals or devices. Used for installation or troubleshooting of electronics. One of the most important tools an automotive technician can own.
Dynamometer
A dynamometer, also known as a “dyno”, is a device that measures force, torque or power. For chassis or engine applications, a dynamometer is designed to create a load to duplicate various speed (RPM) and torque requirements. From this data, power (HP or KW) can be calculated. This in turn provides a snapshot of the chassis or engine performance for comparison to the manufacturer’s specifications. Typically, a dynamometer gives the operator the ability to vary the load applied to the unit under test to mimic specific requirements.
Duty Cycle
Abbreviated to DC . A value indicating the percentage of time an item or output is active in a cycle compared to the amount of time the entire cycle took.
ECM
Abbreviation for Electronic Control Module. Refers to the electronic control unit on a given vehicle that takes information from sensors, controls fuel injection as well as ignition, in order to make your internal combustion engine run. Also known as ECU or Electronic Control Unit
ECT Sensor
Engine Coolant Temp Sensor, commonly referred to in these articles as a CLT Sensor (Coolant Temperature Sensor). This sensor is used to measure the temperature of your engines coolant system.
ECU
Abbreviation for Electronic Control Unit. Refers to the electronic control unit on a given vehicle that takes information from sensors, controls fuel injection as well as ignition, in order to make your internal combustion engine run. Also known as ECM or Electronic Control Module.
EDIS
Abbreviation for Electronic Distributorless Ignition System. An ignition used by Ford in the 90s that used an external ignition module that wired to the ECU, crankshaft position sensor, and coil pack(s). Sometimes used in retrofit applications to add distributorless ignition to older engines due to ease and simplicity, but generally not a capable enough ignition system for higher performance applications.
EFI
Abbreviation for Electronic Fuel Injection. EFI replaces your need for a traditional carburetor, using injectors instead and injecting fuel into the intake port using electronic controls. Most modern vehicles have EFI.
EGO
Abbreviation for Exhaust Gas Oxygen. The measurement of left over products of combustion in exhaust to determine AFR. Can be used as Closed Loop EGO to allow measurement feedback to trim fuel delivery to add or subtract fueling so measured AFR can match a commanded AFR.
EGT
Abbreviation for Exhaust Gas Temperature. The measurement of the temperature of exhaust gas for tuning or safety purposes. EGT can be monitored by the tuner to help them determine when the engine is making as much power as it can safely. Overly high EGT will lead to engine damage. Cold EGTs can also be used to help identify a problem with a cylinder, such as a bad spark plug or coil pack, as lack of combustion will result in low EGT.
Electronic Distributorless Ignition System
Abbreviated to EDIS. An ignition used by Ford in the 90s that used an external ignition module that wired to the ECU, crankshaft position sensor, and coil pack(s). Sometimes used in retrofit applications to add distributorless ignition to older engines due to ease and simplicity, but generally not a capable enough ignition system for higher performance applications.
Electronic Control Module
Abbreviated to ECM. Refers to the electronic control unit on a given vehicle that takes information from sensors, controls fuel injection as well as ignition, in order to make your internal combustion engine run. Also known as ECU or Electronic Control Unit
Electronic Control Unit
Abbreviated to ECU. Refers to the electronic control unit on a given vehicle that takes information from sensors, controls fuel injection as well as ignition, in order to make your internal combustion engine run. Also known as ECM or Electronic Control Module.
Electronic Fuel Injection
Electronic Fuel Injection or abbreviated to EFI, replaces your need for a traditional carburetor, using injectors instead and injecting fuel into the intake port using electronic controls. Most modern vehicles have EFI.
EMS
Abbreviation for Engine Management System. Refers to the electronic control system on a given vehicle that takes information from sensors, controls fuel injection as well as ignition, in order to make your internal combustion engine run. Also known as ECM or Electronic Control Module and ECU or Electronic Control Unit.
Engine Coolant Temp Sensor
ECT Sensor – Commonly referred to in these articles as a CLT Sensor (Coolant Temperature Sensor). This sensor is used to measure the temperature of your engines coolant system.
Engine Management System
Abbreviated to EMS. Refers to the electronic control system on a given vehicle that takes information from sensors, controls fuel injection as well as ignition, in order to make your internal combustion engine run. Also known as ECM or Electronic Control Module and ECU or Electronic Control Unit.
Exhaust Gas Oxygen
Abbreviated to EGO . The measurement of left over products of combustion in exhaust to determine AFR. Can be used as Closed Loop EGO to allow measurement feedback to trim fuel delivery to add or subtract fueling so measured AFR can match a commanded AFR.
Exhaust Gas Temperature
Abbreviated to EGT . The measurement of the temperature of exhaust gas for tuning or safety purposes. EGT can be monitored by the tuner to help them determine when the engine is making as much power as it can safely. Overly high EGT will lead to engine damage. Cold EGTs can also be used to help identify a problem with a cylinder, such as a bad spark plug or coil pack, as lack of combustion will result in low EGT.
Fast Idle
Abbreviated to FIdle. The output on a MegaSquirt ECU generally intended to control a PWM style idle valve or IAC, but also may be re-purposed to control other devices if no PWM IAC is desired.
FET
Abbreviation for Field Effect Transistor. A type of electrical component used inside the ECU Field Effect Transistor to control various devices such as solenoids or injectors.
FIdle
Abbreviation for Fast Idle. The output on a MegaSquirt ECU generally intended to control a PWM style idle valve or IAC, but also may be re-purposed to control other devices if no PWM IAC is desired.
Field Effect Transistor
Abbreviated to FET. A type of electrical component used inside the ECU to control various devices such as solenoids or injectors.
HEI
Abbreviation for High Energy Ignition. A distributor based ignition system used by GM for a number of years. Early examples used a 4-pin module that is generally not recommend for electronic control. Later models used a 7 or 8 pin module and are easy to control with MegaSquirt.
Hertz (Hz)
The frequency of an event, measured in times per second. Most automotive controls occur at high frequencies, such as idle valve control that may occur anywhere from 80 to 500Hz. 500Hz is 500 cycles per second.
High Energy Ignition
Abbreviated to HEI . A distributor based ignition system used by GM for a number of years. Early examples used a 4-pin module that is generally not recommend for electronic control. Later models used a 7 or 8 pin module and are easy to control with MegaSquirt.
IAC
Abbreviation for Idle Air Control Valve. Bypasses the throttle body when it is closed to allow air into your engine and help in creating a smooth idle.
IAT
Abbreviation for Intake Air Temperature Sensor. This sensor is used to measure the temperature of the air in your engines intake manifold. Also referred to as MAT or Manifold Air Temperature Sensor.
Idle Air Control Valve
Abbreviated to IAC. Bypasses the throttle body when it is closed to allow air into your engine and help in creating a smooth idle.
IGBT
Abbreviation for Insulated Gate Bipolar Transistor. A type of electrical component used by an ECU and/or in an ignition module to allow the ECU to control a “dumb” coil, or a coil that does not have a built in ignition module.
Ignition Advance
This is when ignition of the air/fuel mixture happens earlier in the compression stroke. Ignition before TDC or Top Dead Center.
Ignition Control
The act of controlling your ignition system with help from your ECU or Electronic Control Unit.
Ignition Retard
This is when ignition of the air/fuel mixture happens later in the compression stroke.
Ignition Timing
Ignition timing refers to the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke.
Insulated Gate Bipolar Transistor
Abbreviated to IGBT. A type of electrical component used by an ECU and/or in an ignition module to allow the ECU to control a “dumb” coil, or a coil that does not have a built in ignition module.
Intake Air Temperature Sensor
Abbreviated to IAT. This sensor is used to measure the temperature of the air in your engines intake manifold. Also referred to as MAT or Manifold Air Temperature Sensor.
Key On Engine Off
Abbreviated to KOEO. The ignition switch is in the “ON” position, but the engine is not spinning or running. The ECU and other electronics will be powered on.
kiloPascals
Abbreviated to kPa. A unit of measurement of pressure as used by MegaSquirt’s MAP and barometric sensors. 101kPa is atmospheric pressure at sea level. If at sea level, MAP < 101 kPa would be considered “vacuum” and over 101kPa would be considered “boost”.
KOEO
Abbreviation for Key On Engine Off. The ignition switch is in the “ON” position, but the engine is not spinning or running. The ECU and other electronics will be powered on.
kPa
Abbreviation for kiloPascals. A unit of measurement of pressure as used by MegaSquirt’s MAP and barometric sensors. 101kPa is atmospheric pressure at sea level. If at sea level, MAP < 101 kPa would be considered “vacuum” and over 101kPa would be considered “boost”.
Lambda
Another term for a measurement used to tell you how much fuel is mixed with how much air going into your engine. It represents all fuel’s stoichiometric value as 1.00. Lean conditions would represent a value higher than 1.00, and rich conditions are lower. These lean (higher) and rich (lower) values are calculated for the Lambda scale by dividing the observed A/F ratio with that particular fuel’s stoich. For example: a 12.8:1 observed reading with gasoline is divided by 14.7 to produce a 0.87 Lambda reading.
Manifold Absolute Pressure Sensor
aka MAP Sensor. Measures the pressure inside the intake manifold, generally in kPa, to report to the ECU for fueling/load calculations. One of the more critical sensors used by the Engine Management System when tuning with a Speed Density algorithm.
Manifold Air Temperature Sensor
Abbreviated to MAT. This sensor is used to measure the temperature of the air in your engines intake manifold. Also referred to as IAT or Intake Air Temperature Sensor.
MAP Sensor
Abbreviation for Manifold Absolute Pressure Sensor. Measures the pressure inside the intake manifold, generally in kPa, to report to the ECU for fueling/load calculations. One of the more critical sensors used by the Engine Management System when tuning with a Speed Density algorithm.
MAT
Abbreviation for Manifold Air Temperature Sensor. This sensor is used to measure the temperature of the air in your engines intake manifold. Also referred to as IAT or Intake Air Temperature Sensor.
MegaLogViewer (MLV)
A software package developed by EFI Analytics to display and manipulate datalogs.
MegaTune
A deprecated software package that was once used to tune MegaSquirt ECUs. It is no longer supported and all users should now use TunerStudio.
milliseconds
Abbreviated to mSec. An amount of time equal to 1/1000 of a second. Typically injector open time and ignition dwell events are measured in mSec.
MPFI
Abbreviation for Multiport Fuel Injection. This is when the engine is being electronically fueled with at least 1 injector per intake port on your engine.
MS
MegaSquirt. A standalone EFI controller or ECU.
mSec
Abbreviation for milliseconds. An amount of time equal to 1/1000 of a second. Typically injector open time and ignition dwell events are measured in mSec.
Multiport Fuel Injection
Abbreviated to MPFI. This is when the engine is being electronically fueled with at least 1 injector per intake port on your engine.
OEM
Abbreviation for Original Equipment Manufacturer. Refers to a component(s) installed in the initial production of a vehicle.
Open Loop
A control method where sensor feedback is not used to adjust outputs. The ECU controls outputs based on calibration alone and will not make adjustments if actual conditions to not match desired targets.
Original Equipment Manufacturer
Abbreviated to OEM. Refers to a component(s) installed in the initial production of a vehicle.
PIP
Abbreviation for Profile Input Pickup. This is what Ford calls the crankshaft position signal in certain ignition systems they used generally in the 80s-90s such as TFI or EDIS.
Required Fuel
May also be known as Req Fuel. A baseline value used by MegaSquirt that should equal the amount of time in milliseconds of an injector event to supply fuel for stoichometric combustion. This value allows the user to scale their tune to allow for larger or smaller flow rate injectors or to accommodate changes in engine displacement or fuel type without full retuning.
Resistor
An electrical component that can be used to reduce current or voltage in a circuit. Commonly used as a pullup resistor or bias resistor in ECU installation.
Rev Limiter
A feature that controls Engine Speed by reducing power such as ignition advance retard or cut, or fuel cut.
Revolutions Per Minute
Abbreviated to RPM. This refers to the speed or revolutions per minute that the crankshaft in an engine is spinning. Could also be used to measure any spinning mechanism in any given machine.
RPM
Abbreviation for Revolutions Per Minute. This refers to the speed or revolutions per minute that the crankshaft in an engine is spinning. Could also be used to measure any spinning mechanism in any given machine.
Sequential
Traditional definition is forming or following in a logical order or sequence. In Automotive terms this could refer to sequential fuel injection which is when fuel sprays per individual injector and only while the intake valve is open for the cylinder that is about to fire.
Sequential Fuel Injection
When fuel sprays per individual injector and only while the intake valve is open for the cylinder that is about to fire.
Speed Density
A control algorithm that calculates fueling or spark advance based primarily on MAP.
Stoichiometric Ratio
In automotive terms this is the “ideal” exact ratio between air and flammable gas or vapor at which complete combustion takes place. Measured in parts of air to parts of fuel. Gas is 14.7:1, Diesel 14.5:1, Ethanol 9:1, Methanol 6.5:1. Often referred to as “Stoich” for short.
Supercharging
The act of installing a supercharger or “blower” on your internal combustion engine.
Supercharger
A mechanically, often belt driven, way to compress and force more air into the intake of an internal combustion engine. Often refered to as Blower.
Table Switching
A feature which allows the user to “swap” from one calibration to another by flipping a switch. An example could be swapping from low power to high power ignition and boost tables when using pump gas versus race gas.
TDC
Abbreviation for Top Dead Center. Refers to the position of an internal combustion engine’s piston when it is at the very top of its stroke.
TFI
Abbreviation for Thick Film Ignition. An 80s-90s Ford ignition system that used an external ignition module and distributor. Commonly found in Fox Body Mustangs and similar vehicles of the era.
Thick Film Ignition
Abbreviated to TFI. An 80s-90s Ford ignition system that used an external ignition module and distributor. Commonly found in Fox Body Mustangs and similar vehicles of the era.
Throttle Position Sensor
Abbreviated to TPS. This sensor is used to monitor how far the butterfly is open on your throttle body. This is determined by how far down you push the gas pedal.
Top Dead Center
Abbreviated to TDC. Refers to the position of an internal combustion engine’s piston when it is at the very top of its stroke.
TPS
Abbreviation for Throttle Position Sensor. This sensor is used to monitor how far the butterfly is open on your throttle body. This is determined by how far down you push the gas pedal.
TPSdot
Throttle Position Sensor Delta Over Time, or rate of change of the throttle position. This is the rate of change of the throttle, either opening or closing, and can be used to open an idle valve or trigger Acceleration Enrichment.
TS
Abbreviation for TunerStudio. Software application used to tune MegaSquirt ECU products.
TunerStudio
Abbreviated to TS. Software application used to tune MegaSquirt ECU products.
Turbocharging
The act of installing a Turbocharger or “Turbo” on your internal combustion engine.
Turbocharger
A forced induction device powered by the flow of exhaust on your engine. It compresses and forces more air into the intake of your engine much like a supercharger. Often referred to as a “Turbo”.
VE
Abbreviation for Volumetric Efficiency. In an internal combustion engine engineering is defined as the ratio of the mass density of the air-fuel mixture drawn into the cylinder at atmospheric pressure to the mass density of the same volume of air in the intake manifold. This is what we are tuning when we are tinkering with the fuel table.
Volumetric Efficiency
Abbreviated to VE. In an internal combustion engine engineering is defined as the ratio of the mass density of the air-fuel mixture drawn into the cylinder at atmospheric pressure to the mass density of the same volume of air in the intake manifold. This is what we are tuning when we are tinkering with the fuel table.
Note this Glossary is a Work in Process– we’re not done yet! See something missing you’d like us to get in here pronto? Hit us up at [email protected]!
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