Complicated question! BSFC is a measure of how effecient the engine is in making hp. The lower the bsfc, the less fuel it takes to make 1 hp. A/F numbers tell you the ratio of air to fuel. As an engine tuner you want to make the most hp at the lowest bsfc without detonation. The type of fuel used has a huge effect on this. A richer mixture will reduce detonation. A good rule of thumb is to run an A/F of 11.5 to 12.0 on pump gas below 20 psi, and 12.0 to 12.5 on race gas. We have run as high as 12.7 to 12.9 when everything was on the line and we were going for broke.