I have seen many dyno charts done with an A/F ratio graph. Most have some sort of horizontal line at A/F=12.8-13.0. Why is that and does that apply to all fuels, including race fuels?

 The major component in pump gas is 2,2,4-TMP and other iso-octanes, which comes from the fractionation of crude oil at a refinery. There are hundreds of other minor components in this mix. The refiner then blends in isomerates, alkylates and oxygenates to achieve the various octane numbers and vapor pressures of the final pump gas. Government regulations also require seasonal changes in the blends.

 My point is this:

The stoichiometric number for iso-octane is 12.5, that is, 12.5 molecules of oxygen are needed to completely burn one molecule of iso-octane. I guess the 12.8-13.0 number is ideal to provide enough excess oxygen for a more complete burn. That would be true if iso-octane were the only ingredient but the blend contains other compounds with grossly different stoichiometric numbers, ie. iso-pentane=8, iso-butane=7.5, ethanol=3 and methanol=2. If I filled up with gas containing 20% ethanol then my stoichiometric number becomes only 10.6. How does the ECM know that my target A/F ratio should now be 10.6? My guess is that it will reduce the fuel feed to achieve its mapped target of 12.8, thereby reducing power and torque for this blend.