grand prix GTP: AFR need some help

[fst100]

I have a custom setup on my 98 pontiac grand prix gtp. It has the eaton m90 series 2 roots-type blower. I have a throttle-body spacer that is tapped with an M3 nozzle(~3.15gph) with a progressive controller. I have a smaller pulley that allows the car to run ~11psi of boost. The stock compression ratio on the GTP is 8.5:1.

performance wise i have alot of bolt-ons, stock bottom-end. my controller runs via the MAP sensor and turns on at ~3PSI.


Now my question. i have a PLX WBo2 sensor and i need to know what is the optimal AFR with a supercharger with WI? im running -20* straight washer fluid. currently my tune is dialed in at ~10.5. i read turboed cars can and should run a 12.5 AFR, alot leaner than 10.5. i tried running leaner as much as 11.0 but i'll start knocking. this is done by either advanced wide open throttle timing or adding more boost vua smaller pulley. im currently running 18* of timing. some ppl told me im running too rich and should run 11.8 and leaner but i dont think i should for various reasons. please let me know of your inputs. thanks!

btw the car ran a best 13.1@105.4mph :-)