If your engine had a Compression Ratio of 11.5:1 it would be non-existent by now!!!

The AFR is the 'Air/Fuel Ratio' and is a term to describe exactly that, the ratio of Air to Fuel. The higher the number, the leaner the mixture. So, for example, 12.5:1 is leaner than 11.5:1.

11.5:1 is safe.