Air/Fuel Ratio's and What They Mean

Air/Fuel Ratio Basics

EFI tuned vehicles determine how efficiently the engine is running by measuring the exhaust gasses that are emitted.  Using an O2 meter, the ecu measures the gas exhausted by the engine checking for extra oxygen content and unburnt hydrocarbons, correcting the amount of fuel delivered to each cylinder in an attempt to keep the air to fuel ratio at 14.64 parts air to 1 part fuel.  This mixture of 14.64:1 is called the Stoichiometric AFR, it is the ratio at which the engine completely expends all of the air and fuel.  Running the engine lean (14.8+ AFR), or having less fuel in comparison to the amount of air, on one end conserves gas but on the other makes the engine run more hot. 

Extremely lean mixtures can be dangerous for the engine because they can make the cylinder very hot causing the piston to possibly melt.  Rich air to fuel mixtures have a cooling affect on the engine and allow it to gain the most power.  Rich mixtures are needed when the engine is running at higher loads to prevent damage from the cylinder getting too hot. 

When running wide open throttle, on a naturally aspirated engine a good air/fuel ratio target is around 12.5 to 13.2.  One must be careful messing with leaner air/fuel ratios because things could go downhill very quickly!  Richer air/fuel mixtures are a little safer to work with because for the most part you will only bog down the engine with too much fuel.