best AFR for Power??

NA motors usually peak around 12.8 to 13.0. It depends on RPM, Load, Octane etc. A dyno session is around 500.00 usually and "they/dyno #s" will tell you what AFR work on your setup. Look at my recent engine dyno results with AFR's listed per 100 rpm increments. 91 octane, 180 deg engine temp, 34 deg timing, 10.2 to 1 compression.