Electronic Solutions for Motorsport

Air Fuel Ratio – What is it? What is too Lean, Best for Power & Idle?

2 min read
Engine ECU datalog

Air fuel ratio is the ratio of fuel-to-air in the combustion chamber before the mixture is ignited. The air fuel ratio is measured by how much oxygen is in the exhaust gas (after the combustion process).

We are measuring what went into the combustion chamber by measuring what came out.

Measuring Oxygen

The less oxygen in the exhaust gas (measured by the lambda sensor), the more fuel there was in the mixture before the mixture was burnt in the combustion chamber.

And vice versa, the more oxygen in the exhaust gas, the leaner the mixture was in the combustion chamber.

Misleading AFRs

However, if we have a misfire, the measurments can be misleading.

A misfire being a condition where the spark did not ignite the mixture or a condition where there was no spark on the ignition stroke.

In these conditions, the air fuel ratio may have been perfect but because the fuel did not ignite and burn, there will be a lot of oxygen in the exhaust gas which will cause the lambda sensor to read lean (when actually the mixture could have been perfect, or even rich, or even actually lean).

In short, if we have a misfire, lambda reading will be lean regardless of what the mixture was before the spark. If there is a misfire, we will have no idea of air fuel mixture ratio.

Misfires can potentially lead us astray when tuning an engine or when troubleshooting a problem.

Best Air Fuel Ratios – Power, Idle

Will depend on the engine and setup (compression ratio, piston design, boost, cam etc etc etc)

Ball Park Numbers

For cars with a cat (catalytic converter), 14.7:1 is the optimum AFR for idle. This is the mixture at which the cat works best.

Same thing is true for cruise conditions.

For high load conditions, a normally aspirated engine 12.5-13.5 is a good place to start. For turbo engines 11.5 to 12.5 a good place to start.

More article on AFRs here.

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