I asked my mechanic today about putting a flowmaster dual exhaust on, and he said i would experience back pressure problems. I've heard of this before. Anyone face anything similar to this.

 

There are some mechanics who believe that engines must have back pressure in order to run right. The restriction from the normal bends in the exhaust will cause enough to keep from burning the valves, after that the lower amount the better the engine will run.

 

Depends what size of pipe you use and how much air your engine is going to need to exhaust:
http://www.flowmastermufflers.com/index.php?page=54

I would say with your 5.4L, running 2.5" duals would be appropriate, as that's what the dual catbacks have. I wouldn't run any bigger than that or you will lose low end ft/lb.
http://www.flowmastermufflers.com/sh...4hn26m7p7m9ke3

 

2.5 inch true uals will be fine, the cats provide plenty of back preasure.

 

Ok thanks