Is there any chance of leaning out or going to rich using a 12:1fmu at 5-6psi?

 

yes and yes
hence why fmus arent used so much anymore

 

ONly a wideband can tell you...

 

i had mine at 8psi and it was fairly even across the board at 12ish. it leaned out a bit at higher rpms but still ran like a piece of ---- lol.

 

i plugged the wideband into my friend's t3/t4 z6 (huge yeah i know) which was running like ---- and turned out he was leaning out to 16:1 in boost
fmu's are a good way to blow your engine real quick. after seeing that i convinced him to let me plug some 450s in and chipped it with uberdata and it runs like a demon now