Engines burning air and fuel to create heat, exhaust, and power. For gasoline engines, the chemistry of this reaction calls for a 14.7 ratio of air to fuel by mass, meaning 14.7 grams of air for every one gram of gasoline. The perfectly balanced ratio is called the stoichiometric ratio, or stoich for short.
Air/fuel ratios typically range from 10.0 (a “rich” mixture, leaving unburned gasoline in the exhaust) to 15.0 (a “lean” mixture, leaving unburned oxygen in the exhaust).
AFR may also be expressed as Lambda?, Lambda is the ratio between the actual AFR and the stoichiometric ratio, so a lambda of 1.0 corresponds to a stoichiometric mixture, lower values are richer, and higher values are leaner.
In practice, the 14.7 air/ratio is rarely used above idle. Engines make the most power around 12.75, but they also produce a lot of heat, are more prone to detonation. Ratios around 11.5 are typical in the powerband of a turbocharged engine, because the increased fuel content helps to reduce heat and detonation.
TODO: add a typical dyno chart showing torque, power, and AFR across the RPM range.