As with previous discussions, the major exhaust flow increase, due to his different X/Y pipe configuration is most likely leaning out the motor. As it has already been stated by DemonSVT in this thread, it's the AF ratio.
One observation we have noticed is that the elimination of the 180 bend seems to be causing a lot of V6 Contours to lose torque. Most of the Contours we have seen tested in our area, with stock Y-pipes or MSDS y-pipes, show TQ levels closer to or over 160 lbsft. Contours with the rear 180 bend removed from the Y-pipe have been seen to drop to the low 150's or under. I'm not saying that this is always the case, but just an observation. However, when the 180 bend is removed and a fuel computer or tuned chip is added, then the TQ levels are very high, generally over the 160lbsft mark. That is another obersvation.
