AFR vs Lambda in a Datalog: Which Is Which, and How to Compare Them
AFR and lambda are two ways of writing the same thing. Different loggers use different ones, and mixing them up is an easy way to misread a log.
The two scales
Air/fuel ratio (AFR) is the mass of air to the mass of fuel, for example 14.7:1. Lambda is that ratio divided by the fuel's stoichiometric ratio, so a perfect burn is 1.00 whatever the fuel.
For gasoline, stoichiometric is about 14.7, so lambda = AFR ÷ 14.7. 12.5 AFR is about 0.85 lambda.
Which logger uses which
COBB Accessport logs normally record AFR on a gasoline scale. Many standalone ECUs, including Emtron, record lambda. APDataview reads the values to decide which scale a channel is in: values around 1 look like lambda, values around 10 to 20 look like AFR. If a column's label and its values disagree, it uses the values and tells you.
Why other fuels need care
Stoichiometric changes with fuel. Pure ethanol is about 9.0 and E85 is roughly 9.8 to 9.9, so the same lambda is a very different AFR. Lambda is the clean way to compare across fuels. If you log AFR on a flex-fuel car, keep the fuel in mind when you read it.
Reading it in practice
- Measured versus commanded is the useful comparison: does the car hit the mixture the ECU asked for?
- Under load, a mixture leaner than commanded is the one to watch. APDataview's default rule flags about 0.05 lambda leaner than commanded and treats about 0.08 as serious.
- At idle and light load, a sustained gap between measured and commanded means the ECU could not reach its target.
Those are APDataview's settings, and a tuner can change them. Related guides: fuel trim and knock.
Check your own log
Upload a COBB Accessport log to APDataview and see whether this shows up in it, with the exact stretch of the log behind every finding. It is free, and the same log always gives the same result.
Check my log free →Common questions
What lambda is 12.5 AFR?
About 0.85 on gasoline (12.5 ÷ 14.7).
Is lambda 1.0 always 14.7 AFR?
Only for gasoline. For E85 lambda 1.0 is roughly 9.8 AFR.
Can I compare an Accessport log with an Emtron log?
Yes, once both are on the same scale. Convert AFR to lambda or the other way, and keep the fuel in mind.
Related guides
General information only. APDataview reads what the ECU recorded and cannot see your engine, fuel or hardware. It is another tool in your toolbox, not a substitute for professional diagnosis or tuning.