Tuning glossary
The terms you meet when tuning an ECU, in a sentence or two each. Most link to a guide that covers them in depth.
- AFR (air-fuel ratio)
- The mass of air per mass of fuel in the mixture. Lower is richer. Gasoline is stoichiometric at about 14.7:1. Read the guide
- Lambda
- AFR divided by the fuel's stoichiometric AFR. 1.0 is stoichiometric on any fuel, below 1.0 is rich, above is lean. Read the guide
- Stoichiometric
- The mixture where all the fuel and all the oxygen can burn completely: about 14.7:1 for gasoline and 9.8:1 for E85.
- VE (volumetric efficiency)
- How well the engine fills its cylinders compared with its displacement. The VE table corrects the ECU's air estimate so each cell gets the right fuel. Read the guide
- Speed density
- A fuelling method that estimates airflow from manifold pressure, RPM, air temperature and the VE table. Read the guide
- Alpha-N
- A fuelling method that uses throttle position as load instead of manifold pressure. Common on individual throttle bodies. Read the guide
- MAP (manifold absolute pressure)
- Pressure in the intake manifold, measured from a vacuum. About 100 kPa at sea level with the engine off; higher than that is boost.
- TPS (throttle position sensor)
- Reports how far the throttle is open, from 0 to 100 %.
- Required Fuel
- On Speeduino and MegaSquirt, the injector pulse width that gives a stoichiometric mixture at 100 % VE. Calculated from displacement, cylinders, injector flow and the fuel's stoichiometric AFR, so it changes when you switch fuels. Read the guide
- Injector dead time
- The time an injector takes to open, during which it delivers no fuel. It gets longer as battery voltage falls. Also called latency or open time. Read the guide
- Injector duty cycle
- The share of each engine cycle an injector is open. Plan for no more than about 85 % at peak power. Read the guide
- BSFC (brake specific fuel consumption)
- Fuel used per unit of power per hour, in lb/hp/hr. Used to size injectors and fuel pumps. Read the guide
- Ignition advance
- How many crank degrees before top dead centre the spark fires. Set by the spark table against RPM and load. Read the guide
- MBT (minimum advance for best torque)
- The least ignition advance that gives the most torque. Adding timing beyond it adds no power. Read the guide
- Knock (detonation)
- The last of the mixture igniting on its own after the spark, causing a sharp pressure spike. Repeated knock damages pistons and head gaskets. Read the guide
- Wastegate
- A valve that lets exhaust bypass the turbine to limit boost. Its spring sets the lowest boost an electronic controller can hold. Read the guide
- Boost control solenoid
- A pulsed valve that bleeds pressure from the wastegate actuator. More duty keeps the wastegate shut longer and raises boost. Read the guide
- Closed loop (EGO correction)
- The ECU trimming fuel from the oxygen sensor reading to hit the target mixture. A steady correction means the VE table is off. Read the guide
- Wideband O2 sensor
- An oxygen sensor that measures AFR or lambda across the whole range. Needed to tune full load and boost. Read the guide
- Write and burn
- Write sends tune changes to the ECU's RAM, where they work until power is lost. Burn saves them to permanent memory. In efi2000
- INI (ECU definition)
- The file that describes an ECU firmware's settings, tables, menus, gauges and live channels. TunerStudio and efi2000 both build their screens from it. Read the guide
- MSQ (tune file)
- An XML file holding a tune's settings and table values. Read the guide
- MLG (datalog file)
- The binary datalog format TunerStudio records and MegaLogViewer reads. efi2000 records and reads it too. Read the guide
- Base map
- A starting tune that runs the engine well enough to begin tuning, or a reference tune kept to go back to. In efi2000
- Autotune
- Software that corrects fuel tables from wideband readings. TunerStudio's is VE Analyze Live; efi2000 has its own autotune that works from the same INI settings. In efi2000
- MCP (Model Context Protocol)
- An open standard for connecting AI assistants to apps. efi2000's MCP server gives assistants read-only access to your projects. Read the guide