Internal combustion engines use fuel and air (oxygen) to produce energy through combustion. To optimize the combustion process, certain quantities of fuel and air need to be supplied in the combustion chamber.

Air-Fuel ratio or AFR is the ratio between the mass of air and mass fuel used by the engine when running. We measure this with an exhaust probe called an O2 sensor. This sensor measures free oxygen in the exhaust pipe and we can back solve to the AFR ratio which we are running.

The stoichiometric ratio is the exact ratio between air and gas at which complete combustion takes place. If you have too little fuel during combustion you will have excess oxygen in the exhaust charge and we will have a less than complete burn. For pump gas this ratio of air to fuel is 14.7:1. This means for every 14.7 grams of air we ingest in the motor we will need to add 1 gram of fuel.

In short, the ratio of fuel we add to the air to reach a complete burn is called the stoichiometric ratio.

When the air-fuel ratio is higher than the stoichiometric ratio, the air-fuel mixture is called lean. When the air-fuel ratio is lower than the stoichiometric ratio, the air-fuel mixture is called rich. For example, for a gasoline engine, an AFR of 16:1 is lean and 13:1 is rich.

How does AFR affect performance?

It might sound like running 14.7:1 AFR is always the goal. That isnt the case. Maximum power is usually found with a slightly rich mixture. Below we have generated a graph of what generally is the goal afr by fuel.

Youll notice that at idle and in higher vacuum zones we run a target of 14.7:1 afr or stoich. When we get into heavy throttle 65%+ we begin to taper into more rich mixtures. Generally for peak power in a non-boosted application will target between 12.7 and 13.2 afr. Once we transition into boost we want to see AFR richer than these non-boosted applications. Moving into medium boost 10+ psi we want to see mixtures more rich than 12.2 afr and moving into high boost we start to move to a target of 11.76 afr.

Every setup is different, every O2 sensor location is in a different position, and at a different level of health so seeing slightly different afrs than above is normal.