All I know is my last turbo'd vehicle was always running at 13.8, and that was a 2013 Nissan with a turbo'd L4, and it annoyed the piss out of me. Pretty much guaranteed only getting 26 MPG at highway speeds. This was despite claims in the manual saying the AFR was fuel octane dependent & would automatically vary (which I found out through experimentation was full of shit). It just stayed pinned to 13.8 whether you ran 87 or 91.