This is a list of many terms used within the Megasquirt documentation. You may want to use "Find" within your browser to search for terms.

ADC - stands for "analog-digital converter". In this case, it is part of the conversion circuitry in the CPU that translates the varying voltage signal to a digital value that the CPU can understand and operate on. All of the sensors (TPS, MAP, CLT, IAT) send their signal to a particular ADC pin on the processor. The ADC result is used by the processor as a 'count'. MS-I has an eight bit ADC, so the counts can be from 0 to 255. MS-II and MS3 have a more precise 10-bit ADC, so the count can be from 0-1024. Both of these are mapped over a 0-5 Volt range (so the voltage into the ADC = ADC count * 5.0/1024 for MS-II, for example); however, higher external voltage may be brought down to a 0-5 volt range with a 'voltage divider' or boosted with an amplifier (such as for EGTs).

AE - Acceleration Enrichment, the enriched mixture provided when the throttle position sensor signal (tpsdot) or map sensor signal (mapdot) changes at various rates.

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

Alpha-N - Fuelling algorithm that uses throttle position as the primary load. Should not be used on turbocharged engines.

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

AMC - Automatic Mixture Control, a control system available in MS2/BG code to automatically tune fuel within the ECU using feedback from the oxygen sensor without use of a laptop. Should be used with great caution as a defective O2 sensor could cause a good VE table to go bad.

ATDC - After Top Dead Center, the crankshaft position with respect to the piston being at the top of its travel, meaning it has passed it's highest position and is descending.

Baro - Barometer - the ambient air pressure. At sea level this is around 100kPa. At high elevations it may be 80kPa. The fuelling need to take account of this.

%baro - a fuelling algorithm where engine load = MAP/Baro and the speed-density system is used. This alters what values are looked up in the fuel VE table.

Barometric correction - a calculation that alters fuelling based on the ambient air pressure. Important at varying elevations. Of little importance in low-lying countries.

BIP373 - a robust ignition driver transistor from Bosch. Features over-currrent and over-temperature protection. Used in all good Megasquirt kits. Beware of lesser alternatives.

BTDC - Before Top Dead Center, the crankshaft position with respect to the piston being at the top of its travel, meaning it has NOT passed it's highest position and is rising. Most normal spark event occur BTDC.

CAN - (Controller Area Network) - a dedicated automotive networking system to allow different automotive processor to communicate and share inputs and calculated results. Used as a convenient way to extend the inputs and outputs of the Megasquirt ECU. (Not available on Megasquirt-1)

Carbon Monoxide (CO): Poisonous gas produced during combustion process. In an automotive context, generally refers to regulated carbon monoxide (CO) tail pipe emissions.

CAS Crank Angle Sensor. Often used to refer to combined crank and cam position sensors mounted where historically a distributor would have been.

Catalytic converter: a chemical device in the vehicle's exhaust system that can reduce the amount of regulated emissions emitted by converting NOx to N2 + 02, CO to CO2 and HC to CO2 + H20. Catalytic means the converter active substrate facilitates the reactions, but is not consumed (and thus has a long life with no replenishment requirements).

CID Cubic Inch Displacement The imperial measure of the swept volume of the pistons. 61.02 cubic inches = 1 litre.

Closed loop - refers to those times when an EFI computer is using feedback from a sensor to alter outputs

Closed loop boost - Megasquirt uses a PID to keep boost in line with a boost target table

Closed loop EGO - Megasquirt controls the EGO to the target table by varying the injected fuel amounts.

Closed loop idle - Megasquirt controls the idle rpm by opening or closing the idle valve as required.

CHT - Cylinder Head Temperature, used instead of coolant temperature (CLT) on air-cooled engines.

CKP - Crankshaft position sensor

CLT - CooLant Temperature sensor (aka. CTS). Usually the CLT sensor 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).

CM - Contract Manufacturer, the factory that builds MegaSquirt® products for Bowling and Grippo.

CMP - Camshaft position sensor

CNP - Coil Near Plug - See Coil on Plug

Coil On Plug - One ignition coil is installed per spark plug. Either directly on top of the plug e.g. a 'pencil' coil. Or in close proximity with a short plug-lead (actually coil near plug.) This setup can give maximum spark energy and can be the neatest install with least lost spark energy.

Compiler - a program used to generate other programs using a source code written in a computer language (assembly language, C, C++, etc.). People often use the term to refer to the program used to compile, link and locate to create a usable program. Example of Compilers are Microsoft's Visual Studio C++ .NET, GCC or Freescale's Codewarrior.

COP - Coil on Plug - See Coil on Plug

CPS - Possibly ambiguous Crankshaft or camshaft position sensor

CPU - Central Processing Unit, aka. "processor" or "microprocessor" the computational engine that performs the calculations to operate the injection and ignition function in MegaSquirt®. It has a number of support circuits, like the power circuit, the clock circuit, the serial and CAN communications circuits, and various input and output conditioning circuits.

Cross-compiler - refers to a compiler (see) used on one platform (for example a Pentium processor running Windows OS) for use on another processor (MegaSquirt-II™'s HSC12 processor, for example).

CTS - Coolant Temperature Sensor (aka. CLT). 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.

Datasheet A full listing of all the specifications on a particular component, often including sample circuits and other useful info. Useful mainly to software and hardware developers.

Diode A two lead electrical device that allows current to flow in only one direction (see also Zener diodes).

DIY Do-It-Yourself.

DMM (digital multi meter) electronic current/resistance/potential measuring tool. (Required tool for any Megasquirt install.)

DT code (dual table code) Obsolete Megasquirt-1 code derivative.

Dual table A firmware feature allowing different injector outputs to have independant fuel tables. Can be used as an awkward method to achieve per-bank trim. More commonly used (with external relays) to map for regular fuel and LPG.

Duty Cycle (DC)– A number indicating the amount of time that some signal is at full power. In the context of MegaSquirt® EFI Controller, 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).

EasyTherm (ET) - A Windows program that simplifies configuring your MegaSquirt®-1 to accept the substitution of non-standard temperature sensors and to upload software revisions. Not necessarily that easy! Not used with Megasquirt-2 or 3.

ECU - (Electronic Control Unit) is the general term for a fuel injection controller, of which MegaSquirt® is an example.

EDIS - Electronic Distributorless Ignition System is Ford's wasted-spark computer-controlled ignition module, which has a simple two wire hookup to MegaSquirt® for full mapped ignition control.

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

EGR Exhaust Gas Recirculation. Used by OEMs for emissions purposes. Not supported by Megasquirt.

EGT (Exhaust Gas Temperature) is the temperature of the exhaust gases, typically measured with a thermocouple. Megasquirt does not have on-board hardware for a direct connection. An add-on board is required.

Errata A list of all the errors currently known in specific documentation, board layout, etc..

FCCM: Acronym for field-programmable custom computing machine, a computing device in which programmable logic devices replace traditional general purpose processors, or fixed logic, in order to achieve superior performance or other capabilities.

FET (field effect transistor) - In MegaSquirt® EFI Controller, the transistors used to control the activation of the injectors.

FIdle - Fast Idle. A device used to control idle speed with additional air supplied by a vacuum solenoid. MegaSquirt has a simple on-off fast idle control. More commonly a fully variable PWM idle valve is used instead.

Gamma - Used to indicate the change in a fuel amount from the calculated amount.

GammaE - A collection of all enrichments and minor corrections to fuel (see speed density)

Gear tooth sensor - an "active", tooth presence sensor. It is based on the Hall effect. The Hall effect sensor consists of semiconductor material which will conduct current when the material is subject to a magnetic field. These types of sensors include a magnet within the sensor and will detect a steel tooth in close proximity. No external magnets are required. Very simple to use.

Gego - Gego is short for 'Gamma - Exhaust Gas Oxygen'. It is the change applied to the fuelling equation based on the EGO O2 sensor feedback. This feedback can (and is) done in a number of different ways, depending on the type of sensor, etc. The important thing is that this is a result of an external measurement, not a pure calculation, so it appears a bit mysteriously in the equation.

GPIO - (General Purpose Input/Output Board) - A CAN enabled processor equipped expansion board for MegaSquirt-II.

GM - General Motors, the manufacturer for the default coolant and air temperature sensors used with MegaSquirt®.

Hall sensor - an "active", magnetic field presence sensor. It is based on the Hall effect. The Hall effect is the change of resistance in a semiconductor in a magnetic field. The Hall effect sensor consists of semiconductor material which will conduct current when the material is subject to a magnetic field. These types of sensors require a "flying magnet", wheel. Instead of teeth on the wheel, as in a variable reluctor sensor, you must have small magnet and a shutter wheel.

Hydrocarbon (HC): in an automotive context, generally refers to regulated unburned hydrocarbon tail pipe emissions.

HEI - , the distributor based electronically controlled ignition system from General Motors. There are a number of variant, identified by the module they use:

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.

HR code - (high resolution code) Originally a derivative of the Megasquirt-1 code that doubled the injector precision. Now there is also a "hi-res" version of the MS1/Extra code. Low-z injectors are not supported.

Hz (Hertz) the measurement of the frequency of a cyclical event, it represent to number of times per second the cycle is completed.

IAC - Idle Air Controller, though it term is sometimes used more generally, it usually refers to GM's 4 wire stepper motor controller for additional idle air (and hence engine speed) during warm-up and for idle regulation.

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

IGBT - Insulated Gate Bipolar Transistor a particular kind of transistor especially suitable for driving ignition coils. e.g. VB921, BIP373

Incorporate AFR - An MS2 and MS3 feature where the AFR target table is included in the fuelling equation. The Fuel VE table then becomes far closer to a true VE table. Enrichment or enleanment from stoichiometric comes from the AFR table. Ideally this setting should be turned on before any tuning and the AFR table set to your target AFRs. Then the VE table is tuned to achieve that AFR. Turning the setting on/off will require a complete re-tune.

Interpolate - The dictionary meaning is to insert an intermediate value into a series by estimating or calculating it from surrounding known values. On all tables and curves, when you are between points, the Megasquirt interpolates the intermediate values. Some older other brands were unable to do this and required monstrously large tables to overcome this limitation.

kPa (kiloPascals) - the measurement of air pressure used in MegaSquirt® computations. It ranges from 0 (vacuum) to 101.3 kPa (standard atmospheric pressure at sea level) to 250 kPa (21psi of boost) or higher.

Lambda - an alternative term to AFR where 1.0 is stoichiometric. Lambda numbering is fuel independant. 1.0 is always stoich.

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. These may need a "peak and hold" board or a resistor pack.

LSU-4 - Bosch wide-band oxygen sensor.

MAF - Mass Air Flow sensor. A sensor used to measure the mass of air flow through the intake tract (which must be sealed at the senor and downstream).

MAP sensor - Manifold Absolute Pressure sensor. Measure of the absolute pressure in the intake manifold (related to the engine vacuum), to determine the load on the engine and the consequent fueling requirements. The standard MAP sensor in MegaSquirt® is the MPX4250 (2.50 BAR, or 15 psi (vacuum) + 21 psig (boost)). A basis of the "speed density" fuelling algorithm.

MAPdot - rate of change of MAP value. (dMAP/dt) Used mainly to trigger Accel enrichment.

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

.MSL - file extension used by the MegaSquirt® fuel injection controller for storing datalogs (.XLS might also be used for datalogs with older versions of tuning software).

MPX4250AP - the internal MAP sensor used in MegaSquirt.

MS - MegaSquirt, used in this document to refer to the MegaSquirt® fuel injection controller or its embedded software. e.g. MS1 = Megasquirt-1, MS2 = Megasquirt-2, MS3 = Megasquirt-3

MS1/Extra - The most commonly used firmware on the Megasquirt-1 chip. Gives fuel and spark control. Used to be called Megasquirt'n'spark-extra or MSnS-extra. The original basic MS1 code was fuel only.

MS2/Extra - Firmware for the MS2 chip and Microsquirt with multiple spark outputs, many wheel decoders, closed loop idle, boost, nitrous, tableswitching and more

MS2/BG - More basic firmware for the MS2 chip and Microsquirt from Bowling and Grippo.

See the Product Cross Reference for a more detailed comparison.

.MSQ, file extension used for saving MegaSquirt® fuel injection controller user settings (aka. "parameters") on a PC computer. Since approximately 2005, these have been readable XML format, so you can open these in Notepad or similar text editors. This format allows some exchange of tune data between different code versions.

MT - MegaTune, Eric Fahlgren's Windows-based configuration program for the MegaSquirt® EFI controller. Now superceded by TunerStudio.

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.

Ohm's Law A fundamental law of electricity, that states that the current flow (I) is equal to the voltage (V) divided by the resistance (R), or:

I = V/R

Oxides of Nitrogen (NOx): in an automotive context, generally refers to regulated 'mono-nitrogen oxides' (NO and NO2) tail pipe emissions.

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

Open Loop - refers to those times when MegaSquirt® works entirely from its calculations with no feedback from a sensor (compare Closed Loop.)

Open Loop Boost - there is no feedback from the map sensor - boost duty is controlled by the duty table only.

Open Loop EGO - there is no feedback from the oxygen sensor - fuelling is from the VE table only.

Open Loop Idle - there is no feedback loop with rpm - idle valve position is set only from the table.

PCB (printed circuit board) - the fiberglass board that has the MegaSquirt® component layout and circuits imprinted on it.

PC Configurator (PCC) - The original tuning software from Bowling and Grippo. Now superceded.

PID (Proportional, Integral, Derivative) - a method of closed-loop feedback control

Pull-up Circuit A very simple circuit consisting of a voltage supply and a current limiting resistor designed to prevent a signal from floating, it forces the signal to either be high (equal to the pull up voltage) or low (grounded).

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

PIP - Profile Ignition Pick-up is the term used for the signal sent from Ford's Electronic Distributorless Ignition System (EDIS) to the electronic control unit. The EDIS module decodes the 36-1 wheel and sends 2, 3 or 4 PIP pulses per engine revolution depending on EDIS4,6,8. The PIP signal into the ECU is a square wave switched at 12 volts. It provides information about both the engine speed and position.

PW (Pulse Width) is the amount of time a signal is applied during each period. For example, is the amount of time (in milliseconds) an injector is pulled low (grounded) to inject fuel.

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 limit current to low impedance fuel injectors, as well as for solenoids such as idle valves, boost solenoids and VVT

Realtime baro - a dedicated second pressure sensor is used to monitor ambient pressure. Required if your vehicle will go on journeys at varying elevations.

Required Fuel – (Req_Fuel) 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.

RPM: Revs Per Minute. The term used to describe engine speed.

SAW - Spark Advance Word is the 'returning' signal to a Ford EDIS ignition unit from the ECU that sets the amount of ignition advance requested. It is in the form of a 5 volt square wave.

Semi-sequential - Fuelling system where injection events are timed to each cylinder (like spark is) but occur twice per cycle. Offers some of the benefits of full sequential without needing a cam wheel input.

(Fully) Sequential - Fuelling system where injection events are timed to each cylinder (like spark is) and occur once per cycle. Do not confuse with direct injection. Sequential uses normal injectors of a normal size, but their timing can give slight power and/or economy increases

Stim (MegaStimulator) - the original stimulator is a small board which plugs into the connector of the MegaSquirt® controller. It simulates all the sensor the inputs the MegaSquirt® controller would normally see (but the rpm signal is only suitable for use with distributor or EDIS configurations) and provides power to the MegaSquirt® controller. The Stimulator also allows you to monitor the MegaSquirt® controller's injection pulses [actual], fuel pump relay operation, and fast idle solenoid output with four LEDs. The JimStim is an enhanced version that simulates many wheel patterns and has more LEDs for multiple spark outputs.

Speed Density - Fuel control algorithm that calculates fuelling based primarily on pressure, air temperature and volumetric efficiency.
PW = dead time + (ReqFuel * MAP * VE(rpm,map) * GammaE)
All the corrections are included within GammaE.

Stoichiometric - the chemically ideal air:fuel mixture. Expressed as 1.0 Lambda for all fuels or 14.7 for petroleum/gasoline.

SPOUT - Spark Out is the spark advance signal sent from MS-II™ to the Ford TFI module to set the timing advance.

Table blending - A firmware feature that allows the user to combine two tables, usually using different algorithms. e.g. one table could be a regular speed-density table and the other alpha-n. This could be useful on an individual throttle body install or as a way of extending the table size.

Table switching - A firmware feature that allows the user to swap to a different fuel, spark, etc table. Can be used in conjunction with dual fuels or nitrous or as a way of extending the table size.

TBI - Throttle Body Injection is a form of injection is which the fuel is injected above the throttle(s). It was typically used on older engines since it can be a simpler system, but is also found on some very high output racing engines because the vaporization time is longer than with port injection.

TPI - Tuned Port Injection is General Motors bank-fire port fuel injection system. It was widely used on 305 and 350 cid V8 in the mid to late 1980's.

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

TPSdot - rate of change of TPS value. (dTPS/dt) Used mainly to trigger Accel enrichment.

Transistor A solid-state (no moving parts) electronic device that uses a small voltage (on the base pin) to control a larger current (across the emitter and collector pins). There are two basic types of transistors, PNP and NPN. A PNP transistor allows current to flow if there is no signal on the base, and restricts the current as the base current increases. An NPN transistor allows current to flow in proportion to the signal on the base, and restricts the current as the base current decreases.

TS (TunerStudioMS) Phil Tobin's tuner software for tuning MegaSquirt EFI controllers. It is is 100% compatible with MegaTune INI and MSQ file's. TunerStudio is required for Megasquirt-3 and is recommended for all new installs. Get it here

AFM - Air Flow meter. Often a vane or flapper type device used to measure air-flow into an engine.

Volatile Organic Compounds (VOC): in an automotive context, generally refers to organic components or precursors to smog formation. Organic compounds are all chemical compounds containing carbon-hydrogen (C-H) bonds of covalent character. The U.S. Environmental Protection Agency defines a VOC as any organic compound that participates in a photoreaction. An example of a VOC is PAN (peroxyacetyl nitrate), CH3COOONO2.

Voltage divider A simple circuit that uses two resistors in series to reduce the output voltage Vo proportionally to the input voltage Vi. Two resistors, R1 and R2, are arranged in series, with one end connected to the external signal, and the other to ground. The signal is taken from between the two resistors. From Ohm's Law we can calculate that the total current will be I = Vi/(R1+R2). The same current flows in each resistor because they are in series. The voltage drop across each resistor is: V1 = I*R1 and V2 = I*R2 = Vo. V2 is output voltage Vo, which we can write as:

Vo = Vi* R2/(R1+R2)

V2.2 - The first mainstream circuit board used within Megasquirt. While still available for sale, it is largely obsolete.

V3.0 - The current circuit board used within Megasquirt (accurate Nov 2010). This superceded the V2.2 adding better support for low-z injectors and a circuit for VR conditioning.

V3.57 - A surface mount circuit board, largely the same as the V3.0 board.

VB921 - a particular IGBT designed for use with automotive ignition coils. Now superceded by BIP373.

VE - Volumetric Efficiency. The actual amount of air being pumped by the engine as compared to its theoretical maximum. A 2000 cc engine will theoretically move 2000 cc of air in one cycle at 100% efficiency. If the engine is actually running at 75% VE, then it will move 1500 cc of air on each cycle.

Vref - a 5 Volt supply used to power the TPS sensor (and sometimes other external components needing a 5 Volt supply, like ignition modules or pull-ups).

- variable reluctor sensor is an induction type sensor, it is "passive", i.e. it does not require a power source, and has a small magnet built in. Needs to use a conditioning circuit - as provided on the V3.0 and V3.57 mainboard. Cannot be used directly on the V2.2 mainboard.

Wasted-COP - a method where individual coils are fitted per spark plug, but the coils are fired in a 'wasted' manner twice per cycle. This is used during startup in some wheel modes and could also be used temporarily by a user before full coil-on-plug is installed.

Wasted Spark - A method of firing spark plugs in which one 'double-ended' coil simultaneously fires two coils on different cylinders. One of the cylinders is the intended 'target', and is near TDC on its compression stroke, the other cylinder is offset by 360° in the firing order (of the 720° 4-stroke cycle), so it is near TDC on its exhaust stroke. The second spark is said to be 'wasted' because it does not ignite a mixture. On the other hand, the hot ionized exhaust gases require little energy to create a spark, so nearly all the energy goes to the 'target' cylinder. An example of a wasted spark system, is Ford's EDIS. The advantage is that while a missing tooth crank wheel is required, no cam sync signal (and the corresponding wheel and sensor) are needed.

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. Allows for lean or rich tuning.

WOT - Wide open throttle.

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

.XLS - file extension used by the MegaSquirt® fuel injection controller for storing datalogs (.MSL might also be used for datalogs with more recent versions of tuning software).

Zener diode A diode that allows current to flow in one direction, unless the reverse voltage is higher than its rated avalanche voltage, in which case it will flow in the reverse direction. Often used to limit excessive voltage to certain circuits by shunting it either to ground or the power supply.

MegaSquirt® and MicroSquirt® controllers are experimental devices intended for educational purposes.
MegaSquirt® and MicroSquirt® controllers are not for sale or use on pollution controlled vehicles. Check the laws that apply in your locality to determine if using a MegaSquirt® or MicroSquirt® controller is legal for your application.
Derived from ©2004, 2008 Bruce Bowling and Al Grippo. All rights reserved. MegaSquirt® and MicroSquirt® are registered trademarks. (C)2010 James Murray.