Rigorus testing since 2014 on Boosted LT1 has reveiled that MBT is achieved at .82 which is a 12.05 AFR for reference.
Where LS Port injected engines MBT is .78 or 11.46.
Fuel is a piston coolant as well, leaner mixtures put undue heat stress on components that over long term will drastically accelerate wear and can lead to detonation.
All this is verified via an ECM 1500 Lambda meter.
https://www.bmotorsports.com/shop/pr...fX4j6V_TWIMBYQ
There will be some error in other brand/pricepoint wide bands.