to answer a few questions i saw...
when i said downpipe/ y pipe i wasn't aware of the turbocharged 3.5 upgrade, i was referring to the Y pipe that goes under the subframe. its squashed into a crappy square and then goes to a 2.25" flex pipe that was then mated to my 2.5" magnaflow exhaust. I had a local custom exhaust guy cut the pipe after each manifold and put in 2- 2" pipes that run under the subframe instead and then go into a 2.5" collector to remove the 2.25" restriction... the 2" pipes are not mandrel bent unfortunately (no one around here does that, and if they did it would cost a fortune) so there is one bend that takes it to around 1.8" X 2" on one of the pipes, but even if both pipes had that restriction, together they would still flow more air than the 2.5" exhaust they are going into, and the o2 sensors sticking into the pipe are probably more of a restriction so i don't think if they were mandrel i would see any more gains.
the 80mm is off a mustang GT, but the info on how to mate it to a v6 upper intake is somewhere on the v6 mustang forums, that is where i got the idea, since the engines are basically the same, that is also where i found out about porting the lower intake manifold... lone wolf is the company that does them but I'm just doing it myself, doesn't seem that complicated.
also no one makes headers for our 3.5 NA explorers, but magnaflow does make an aftermarket manifold/ higher flow cats, that looks better designed than the stock manifolds, and i did some porting to them on the top and smoothed out the bungs after the cats to provide more flow there. I'm not expecting header like gains but replacing the cats on my 3.6 impala lfx netted around 20whp/20wtq, in parts of the dyno curve, which was almost unbelievable and my tuner needed to add more fuel to the trims. They were metallic core i.e. higher flowing than what is available for the explorer, but i still hope for 10-15, especially with the better design of the manifold. That being said I don't have a before dyno, ill just be relying on the old butt dyno and weather or not more fuel is needed in the tune to make up for the better airflow. Also the magnaflow cats are $$$$ but i was able to get a really good deal on them through
www.hottexhaust.com they may not be worth the $ only time will tell...