Sorry this is kind of long.. it is from my webpage and I thought you may be interested...
Some key points about Air/Fuel ratio (AFR): During easy cruising our stock ECU uses the stock Oxygen sensor to adjust the AFR to a target of 14.7/1. The reason is like this. At 14.7/1 (this means 14.7 pounds of air to 1 pound of fuel) the amount of air will combine with all of the available fuel. This is the best condition for the catalytic converter to do its job thus providing for a better environment.
For performance most tuners will agree the peak horsepower is achieved with an AFR of around 12.5/1, So .. compare 12.5/1 to 14.7/1 you can see there is less air, which means the mix has got more fuel = Rich.
As mentioned above, our stock ECU tries to get the mix at 14.7/1 during easy cruising. So ... what happens when we mash it to the floor? Well .. it is not so easy to explain in detail but in general this is what happens when we mash it: The stock ECU disregards the oxygen sensor readings and uses a fuel map to determine how to work the fuel injectors.
The fuel map is just a table of numbers that an engineer has established to define the proper injector pulsing for a set of parameters. The parameters considered include: Boost, Intake air temp, Intake air flow, RPM, Throttle position and a few more. The stock ECU adjusts the injector pulsing according to the data in the fuel map. A simple example may be expressed like this:
The fuel map tells the ECU to pulse the injectors 10 times when ever the boost is 1, RPM is 1000, the air temp is 70, the throttle position is 25% open and the mass air sensor voltage is 1.25v.
I have more information on my webpage: http://n8rws.com
you will note that most of the AFR and tuning information is pertaining to my Supercharged TOYOTA 4Runner but the same principles apply to our WRX / STI's