In N/A cars the best AFR's appear to be around 12.5 to 13.1:1 AFR or 0.85 to 0.90 Lambda. The AFR of about 1 Lambda is about the perfect balance (stoichiometric) but is unlikely to where the car is mapped to as standard. Also if you were to go to about 1.5 lambda that is around 22.0 AFR which is way too lean for a car IMO.