im upset to see everyone talking about afr's and no one is talking about fuel octane... when i was tuning a 91 fc with a ported cosmo, i went all the way down to 9.0 afr under heavy load 6k or so, 10 psi, and still got detonation, but this was with good old Cali 91 octane. iu put some 104 in the tank to dilute the cali crap and leaned it out to 11.0 with no problems... so remember, having a low afr # dosent mean its safe, just means its rich, wich means not all the fuel is burning, not that your car isint going to detonate or ping....

 

You must have had a lot of other issues than the a/f ratio. There are MANY FD owners running over 350 rwhp on 91 octane gas just fine.

 

Yeah, you probably have a timing issue. The higher octane is harder to ignite so it's combusting a bit later. Retard your timing some and see if it helps.

 

im at 8deg. adavnce under full load

 

okay, retard it and see if it helps

 

Well that might be a problem using 91 octane. With 0 deg. advance, I can run 89 octane at 10 psi without any issues.