Originally Posted by dyn085
^^^This. Plus he is also running a header. Even his mild lean isn't overly dangerous because it is a DD and he probably isn't running WOT constantly.
A tune will eventually come out to get back in the 14.1 range, a mild deviation is ok if you aren't racing everywhere.
13.x:1 is not running lean.
A lean air fuel mixture is higher than the stoichiometric air-fuel ratio, which for gasoline is aproximately 14.7:1. A lean condition is when you have an excess quantity of air not an excess quantity of fuel. Running at 12-13.x:1 parts of air per part of fuel is running rich as you have an excess of fuel and incomplete combustion will occur.
Unless you're running fuel with a greater quantity of ethanol, at E10 perhaps 14.1:1 is the stoichiometrically correct air fuel ratio which is still higher than what was mentioned. A lower numerical air/fuel ratio is therefore running slightly richer than stoichiometric, and a higher numerical air/fuel ratio is leaner. Lean burn doesn't occur below the stoichiometric air fuel ratio of the fuel.
E85 contains even more oxygen so the stoichiometric air fuel ratio is nearer to 10-11:1, but that would really be the only way you would be running lean at 13.x:1 at the present time as there would be an excess of air and an insufficient quantity of fuel at that point in time.
If there is an insufficient quantity of air complete combustion won't occur, and the cylinder temperature and pressure are reduced from one in which all or nearly all of the fuel is harnessed.
Running lean can burn up pistons, valves etc, where as running rich can dump raw fuel into the catalysts causing them to clog and fail, as well as getting poorer fuel economy and less power.