I have a 2002 Excursion with a 5.4 v8 and would like to know what size the pipe diameter is so I can get a custom cat-back exhaust put on at my local shop.

 

true duals or just a single pipe?

for true duals the best thing is to run 2.25" pipe..for a single pipe system you should run 3" from the factory Y pipe back into muffler of your choice

 

I would run 3" on the 5.4L. It will do ok with 2.5" but there will not be much savings with the smaller size, and the 3" will be deeper sounding, and will give you more pull in the top end without sacrificing the bottom end.