Glossary of Tuning Terms

The following are basic terms used when tuning. These will help you to understand discussion about tuning in general, refer to these when posting questions or when reading posts about fuel injection.

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 on motorcycles 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. Power Commander has a Accelerator pump feature for Fuel injection as well as throttle sensitivity.

Advance – See Ignition Advance/Retard

AFR Air Fuel Ratio – refers to the ratio of air to fuel in the intake mixture going into the cylinder. It is always air mass:fuel mass ratio, and is typically between 11:1 and 17:1 (the volume ratio is closer to 9000:1 air:fuel.)

ATDC After Top Dead Center – used for ignition advance timing, it refers to the crankshaft position (in degrees) After Top Dead Center on the power stroke.

Boost – This refers to the artificial increase in manifold pressure above the barometric pressure based on some sort of mechanical compressor or pump. Typically this is a turbocharger.

BTDC Before Top Dead Center – used for ignition advance timing, it refers to the crankshaft position (in degrees) Before Top Dead Center on the compression stroke.

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

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.

Detonation – The sharp, metallic sound produced when two pressure, or flame fronts collide in the combustion chamber. Normally, the combustion starts at the spark plug and spread smoothly from there. If the combustion starts in a second location within the combustion chamber, then each raise the pressure in the cylinder, possibly to destructive levels.

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

DC Duty Cycle – 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

ECU – ECM Computer – Many modern motorcycles have a central computer called an engine control unit (ECU) or module (ECM)

EGO – refer to exhaust gas oxygen. The amount of oxygen remaining in the exhaust can be a good indication of the air/fuel ratio of the intake mixture.

EGT – refers to exhaust gas temperature.

IAC Idle Air Control – Typically a stepper motor.

IAT Intake Air Temperature – The refers to the intake air temperature, or the temperature of the air entering the throttle body. Sometimes the same as IAT but may be refered to seperatly paticualrly on air cooled motorcycles. Sometimes the difference between intake and manifold temperatures is used to change values for the ECU as pertaining to warmup.

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

Ignition Advance/Retard – The advancing or retarding (in crank degrees) of ignition spark relative to the piston location in the cylinder.

Knock – aka detonation or ping. See Detonation

kPa (kiloPascals) – the measurement of air pressure used in some ECU computations. Average pressure at sea level is 101.3 kPa. In EFI applications, it is typically used to refers to measurements of intake manifold vacuum, boost, or barometric pressure.

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

MAP Manifold Absolute Pressure – 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. (Somtimes people refer to a fuel map for Power Commanders and the like or a base map for ECU’s that is not what MAP is refering to.)

MAPdot – The rate at which the MAP sensor output changes (and thus the rate at which the MAP itself changes).

millisecond – 1/1000th of one second.

MAT Manifold Air Temperature – The refers to the air temperature entering in the manifold. Sometimes the same as IAT but may be refered to seperatly paticualrly on air cooled motorcycles. Sometimes the difference between intake and manifold temperatures is used to change values for the ECU as pertaining to warmup.

NB-EGO NB-O2 – narrow band exhaust gas oxygen sensor.

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

Ping – aka. detonation or knock. See Detonation

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.

Retard – Ignition Advance/Retard

rpm – revolutions per minute – a measure of the rotational speed of the engine at any time.

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).

Switch point – the voltage at which a narrow band sensor goes from a low voltage to a high voltage, indicating a stoichiometric mixture.

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

TPSdot – The rate at which the TPS sensor output changes and thus the rate at which the throttle position itself changes.

Vacuum – The measurement of vacuum starts with 0 at atmospheric pressure, and measures the pressure below that as vacuum usually in inHg – inches of mercury.

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

WB-EGO WB-O2 – Wide band Oxegen Sensor An exhaust gas oxygen sensor that signals the intake mixture air to fuel ratio based on the content of the resulting exhaust gases. These sensors require a sophisticated controller to work.

WOT – Wide open throttle (Woo Hoo).

WUE Warm Up Enrichment – The enriched mixture applied when the coolant or intake air temperature is low.

X Tau Enrichment – a form of acceleration enrichment that models the changes in the fuel film on the port walls to estimate the effect on fuel entering the cylinder.

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