FYI: Raw fuel in the exhaust will not show as rich on a lambda sensor. If you want to verify this, richen it up until it misfires and see what the sensor outputs. ;) Lambda sensors are essentially telling you the composition of the gas... the pipe can be full of raw fuel and it will still read a lean gas if it wasn't in the combustion process.
With that being said, I've personally witnessed better power all the way down to 12.5:1 (ish) on N/A bikes. On my 07 I ran a dry shot that was tuned to 12:5, and spraying it was a steady 13:1. I ran out of bottle one day at the strip, and just left the 12:5:1 tune in it to see how it behaved, and low and behold it picked up consistent MPH over the 13:1 tune up on the back-up map. Even with forced induction you see more and more power up to that magical 12.5:1 mark, so long as your fuel can support it.
What will mess with your 02 reading are things like pulse reversion in short pipes, exhaust leaks, misfires, or if you have some really wacky cam timing/supercharging that is allowing you to flush "clean" air from the intake into the exhaust stroke event. Those sensors are also pressure sensitive, and the error under pressure grows the further from stoich the reading is. Changes in altitude can effect this, as well as restrictive exhaust designs. On top of that, sensors do age/wear... especially if you like leaded fuels. Sensor calibration is critical for repeat or comparative results.
Even if all engines were equal, every motor would likely have a slightly different 'happy afr' do to the myriad of variables in sensors, controllers, and installation. I feel like this accounts for most of the difference in what you see bike to bike, tuner to tuner. Jake is right about AFR only being one piece of the puzzle. EGT is a brilliant tool, and so is a 4 gas analyzer. At the end of the day, an accurate measurement of power (dyno, trap speed) VS destruction observation (knock sensor, plug reading) is the real measurement of what's best.... all the sensors are just there to help get you close and make it repeatable when you get to that last couple %. :) Just my opinion, of course.
Attack life... it's going to kill you anyway.
Last edited by SpazOnaZX; 11-08-2018 at 08:40 AM.