I wouldnt advance a car with the factory mid pipe mine had audible detonation on tip in with the Predator canned tune no adjustments. When I installed the O/R mid pipe which came with the a/f bung already welded in I installed a/f data logging equipment. Set my a/f and then adjusted the timing I went up 2 % at a time till all the way and didnt experience any audible detonation. I installed new plugs NGK TR55IX plugs and looked over my old ones no visable detonation evidence. Data logged with Predator no real amount of timing being pulled. The only conclusion that makes any sense to me is that the exhaust gas temps where alot higher with the cats and pre cats and the aluminium heads picked up that heat from the cast iron exhaust mainfolds which could have caused the detonation issue with the canned tune from Diablo with the factory mid pipe. Sorry for the long post but I put alot of thought into adjusting my car having little experience in tuning with this kind of equipment. It is not like the old days of shooting down the 1/4 and trying to pull you plugs to read them without burning your hand on the headers. I still recommend getting a wideband and some kinda data logger it is always better to be safe than sorry imho.