lean, rich, a/f ratio, stoichiometric.
what does it actually mean ?

Fuel and air, a balancing act

The combustion engine process burns a mixture of air and fuel. The ratio of this mixture is referred to as the a/f ratio. As the relative amount of fuel in the mixture decreases (a leaner mixture) the a/f ratio value becomes larger. As the proportion of fuel becomes greater (a richer mixture) the a/f ratio value becomes smaller.

The a/f ratio of an engine may be measured in many ways, but the most representative and accurate methods use highly specialized exhaust gas analyzers. The a/f ratio information is the key to establishing an appropriate fuel metering calibration for a given engine combustion. Anyone attempting to optimize an engine a/f ratio should purchase and utilize an a/f ratio mixture meter. Whether you are tuning a carburettor, fuel injection or forced induction and using unleaded fuel, there are many to choose from, the "wide band" version is the preferred version. Do not discount the "narrow band" version, either version is far more accurate than "i think it's lean" or "it feels rich", without accurate information you are wasting time, money and possibly the engine, this is especially true with air-cooled engines where the increase in heat due to lean running can very quickly lead to catastrophic failure. At RACE SHACK we run a wide band sensor which is linked in to our data logger and for which data is captured on every Dyno run.

A fully warmed up engine with an a/f ratio as rich as 6.0-1 and as lean as 22.0-1, these are the general rich and lean combustion limits, but during actual driving situations the actual a/f ratio needed during various operating conditions will be very close to the mid-point of these extremes.

A/f ratio Effect

Generally engines of different basic designs have the same a/f ratio requirements. These calibration needs are typically a function of operation mode, engine temperature, engine speed, and load. Generally a production based high performance engine will have a/f ratio values in a range from 12.0:1 to 16.0:1. Water cooled engines are able to better cope with the increased engine temperatures generated by leaner running and will operate in a leaner zone than their air cooled predecessors.