AFR, lambda, and VE explained
Separate the mixture the ECU asks for, the mixture a sensor reports, and the air the ECU estimates.
Target and measured mixture
Air–fuel ratio (AFR) describes the mass of air relative to the mass of fuel. Lambda expresses mixture relative to the fuel’s stoichiometric point, the ratio at which the fuel and oxygen can react completely. Lambda 1 is stoichiometric, below 1 is richer, and above 1 is leaner.
When two tools display AFR using different fuel assumptions, the same lambda can appear as two different AFR numbers. Compare lambda when you can, and confirm whether a log channel is a target, an estimate, or a sensor measurement.
Where VE fits
Volumetric efficiency (VE) describes how well a cylinder fills relative to its displacement. Some ECUs use VE tables as part of their airflow model, then use that estimated airflow to calculate fuel delivery. Other ECU strategies use different table names and load models.
Changing a mixture target changes what you ask the ECU to deliver. Correcting an airflow model changes how it estimates the fuel needed to get there. A log with target and measured mixture can show a difference, but it cannot by itself tell you which table to change.
Read the measurement in context
- Check the sensor type and operating status. A stock narrowband oxygen sensor is primarily useful near stoichiometric, while a wideband system can report across a wider range.
- Compare stable periods at similar RPM and load. Throttle transitions, exhaust leaks, and sensor placement can affect the reading.
- Find out when the ECU is in closed loop and what corrections the logger exposes. A correction is feedback the ECU is applying, not a measured power gain.