nice basic summary data (and nice hand written HTML!) from safrtool.com
TARGETING AFR VALUES
What does Air/Fuel Ratio (AFR) mean?
AFR is the mass ratio of air to fuel in an internal combustion engine. The AFR is an important measure for performance-tuning reasons. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture which is defined as an 14.6 AFR ratio. The lower the AFR the richer the mixture. The higher the AFR the leaner the mixture.
Recommended AFR Values for Naturally Aspirated 4 Stroke Engines
IDLE / LIGHT CRUISE: 13.5 AFR
When you start your vehicle we refer to this as being in IDLE condition. When you shift into gear and drive at a low mph we refer to this as being a LIGHT CRUISE driving condition. With 14.6 being the industry standard stoich AFR value, we typically like to see these conditions run slightly richer. We are looking to fine tune an engine to get the smoothest operation.
CRUISING: 14 AFR
Most of the time you are driving your vehicle you will be within the state of CRUISING. This condition is the most wide open for customer preference based on two factors. If you prefer better GAS MILEAGE then run a leaner mixture. Most stock to slightly modified engines will run just fine at an AFR of 15 and yield better MPG. If you prefer better PERFORMANCE then we suggest to run an AFR of 14 or even richer to around 13.
WIDE OPEN THROTTLE: 12.5 AFR
Driving full out within a gear will bring you into a WIDE OPEN THROTTLE driving condition. This condition is where the peak horsepower value is obtained. We typcially like to run a richer AFR here to get the best performance. Best performance for most vehicles is in the 12.5 to 13 AFR range. Going too rich will have the opposite affect and decrease your performance.
ACCELERATION: 12 AFR
Getting up to cruising speed or getting ready to pass someone, you usually are looking for the performance to get up and go. We refer to any fast turn of the throttle as being in an ACCELERATION driving condition. Again, we typcially like to run a richer AFR here to get the best performance. Best performance for most vehicles is around 12 AFR. Going too rich will cause a sluggish roll-on to get up to speed.
NOTE: OPTIMAL AFR VALUES WILL CHANGE BASED ON FORCED AIR INDUCTION VS NATURALLY ASPIRATED AND FUEL TYPES