Idle fuel mixture setting

[DIRK BROER]

General EFI question:

On the mustang list - the question that came up is how the EFI sets the 
mixture at idle.  For emission reasons I suspected that the EFI uses the O2 
sensor to set the idle mixture at 14.7:1  Does anyone know if this is a 
hard and fast rule?  I know some engines with radical cams like seeing more 
fuel at idle - but I always thought it was due to the exhaust contamination 
- you basically had to add more fuel to keep the ratio the same.  The final 
ratio at the exhaust would still appear to be 14.7:1 by the O2 sensor.

Can anyone shed some light on this?

Dirk