Seth,
I also have a 4.7 and I am considering the same system (single in dual out).
I don't see why there is a need to go so much larger on the 4.7. A 2.5 inch
inlet tube into a good high flow muffler such as the flowmaster should be
large enough. This is especially true if all the factory crimps and crappy
bends are taken out of the system from the cat (the large one) back. From
the muffler back, there is even less reason to go larger, in my opinion, for
a couple of reasons. First, by the time the exhaust gasses get that far down
the system the have cooled and slowed dramatically. This means that to
maintain exhaust gas velocity, you should probably not increase the size of
the pipe, at least in theory. Consider that the y-pipe on our 4.7's is 2
inches in diameter. This is right after the exhaust mainfolds where the
gasses are much hotter than after the muffler. Admittedly this is about one
of the crappiest y-pipe executions I have seen and if it were remade using
mandrel bent tube of the same diameter as the factory tube it would likely
have some positive effect on the flow of exhaust gas while maintaining
velocity, but it works fairly well given the size of the motor. Here, where
the gasses are under greater "pressure," achieving smooth efficient gas flow
to maximize the extraction effect is very important. The second reason is
that you are using two outlet tubes. This already increases flow capacity
even if you use only a 2 inch tube.
The point I am trying to articulate is that if the y-pipe is only 2 inch
tubing, then going to a larger tube(s) farther down the system where the
gasses have cooled is proabably going to slow the gasses down even more.
This is not what you want for a street driven vehicle the needs to make most
of its power in the low to mid RPM range.
This is long and I have barely scratched the surface of exhaust systems and
all that goes on with them. I have researched and talked to knowlegeble
people to learn all I can. In my efforts I spoke with the tech people at
Flowmaster (amongst others) and they recomended Gene's muffler shop in
Downey. They do a lot of custom work and will work with mandrel tubing.
Their number is (562) 923-4814.
Good luck, and keep us posted
Andrew
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