Motorcycle Fuel Mapping Explained
Fuelling is where most of the difference between a stock bike and a well-tuned one comes from. This is how a motorcycle ECU decides how much fuel to inject, and what changes when you remap it.
The fuel map
A fuel map is a table. One axis is engine speed (rpm). The other is load, measured either by throttle opening or by air pressure in the intake. Each cell holds a value the ECU uses to work out how long to open the injectors. Between cells, the ECU interpolates.
Alpha-N and speed-density
- Alpha-N uses throttle position and rpm. It is common on motorcycles because the intake pressure on big singles and twins is too unsteady to measure well at low rpm.
- Speed-density uses intake pressure (MAP) and rpm, with the air temperature, to estimate how much air is going in.
- Many bikes blend the two: Alpha-N at small throttle openings and speed-density further up, or the other way round.
Knowing which strategy your ECU uses tells you which tables to edit.
Corrections
The base map is adjusted by correction tables: air temperature, coolant temperature, barometric pressure, battery voltage (for injector opening time), warm-up enrichment, and acceleration enrichment when the throttle is opened quickly.
Closed loop and the oxygen sensor
At cruise and small throttle openings most bikes run in closed loop: the oxygen sensor tells the ECU whether the mixture is rich or lean, and the ECU trims the fuel to hold a ratio of about 14.7:1 (stoichiometric). That is ideal for the catalytic converter but lean for smooth running, and it is the main reason stock bikes feel snatchy at small throttle openings and run hot in traffic.
Under heavier load the ECU switches to open loop and uses the map values directly, usually richer, around 12.5 to 13.2:1 at full throttle, for power and cooling.
What a remap changes
- Moves the closed-loop area or richens the mixture in it, for a smoother pick-up.
- Corrects the base fuel after an exhaust or intake change.
- Adjusts acceleration enrichment for a cleaner throttle opening.
- Sets sensible full-throttle targets.
Measure, don’t guess
Any change to the fuelling should be checked with a wideband oxygen sensor, on the road or on a dyno. Lean at full load is the most dangerous mistake in tuning. Our TunerPro tuning guide covers the full workflow, and how to tune your ECU yourself covers reading and writing.
Frequently asked questions
What AFR should a motorcycle run?
About 14.7:1 in closed loop at cruise, and usually 12.5 to 13.2:1 at full throttle. The right target depends on the engine; always measure with a wideband sensor.
Why does my stock bike feel jerky at small throttle openings?
Mostly because of lean closed-loop fuelling around 14.7:1, written to pass emission tests. Richening that area is one of the most common remap changes.
Do I need to change the fuel map after fitting an exhaust?
A full system or a free-flowing intake changes how the engine breathes, so the fuel maps should be corrected. A slip-on often makes a smaller difference.
