Anyhow, when you lean it enough, the combustion gets colder again. Ï've mapped my engine to around AFR = 16:1 and beyond up till around 80 kPa. Then it jumps to stoich and goes gradually to 12:1 on full boost. It works really well and could be even better if the standard CVH hemi head design wasn't so crap as far as squish zones, lean burning and not detonating are considered.