Computerized Engine Controls - Google Books
What air fuel ratio burns valves?
What air fuel ratio causes the most NOx to form?
Looks like just short of 16:1 is your peak for NOx production and likely the peak for combustion flame temperature.
But once you pass 16:1 NOx levels start to drop quickly. 16.5:1 looks about as NOx-ious as 14.7:1.