Why is desired AFR 12.5

Why is desired AFR 12.5 instead of Stoichiometric AFR? and how did you get the number? Thanks

hello the principle of afr is to use a safe afr for the engine too lean it will melt to rich it will bore wash use excessive fuel and generally be down in power the following table shows a general afr table for most street engines with an rpm over map the same map will apply to non-turbo engines if you note the map reading as 100kpa (approx) at atmospheric condition or engine not running

Great, thank you!