At a stoich afr of 14.7, you are most efficient. So at most normal driving operations, you want to target a lambda of 1, aka afr of 14.7. During WOT maneuvers or heavy tip-ins, you enrich to reach peak power as well as cool the cylinders, as running rich provides a cooling effect. Like the post above me, you want to see afr below 12 during WOTs. With a turbo, staying at stoich during a WOT would likely result in you sprinkling metal bits all over your engine bay.