Does anyone know which size is producing more power on the new 5.4? I heard the if you change the size of the exhaust to a 3" you change the backpressure and the computer has problems? Is this true?

 

It's primarily a factor on your tailpipe(s). All systems are 3" before the muffler, usually the tailpipe is 3" if single, or dual 2.5". Dual 3" will make the backpressure too low...

Larger pipe reduces back pressure. Here's the caveat: decrease the back pressure too much, and atmospheric air will push back into the pipes actually increasing back pressure. The sweet spot is just enough back pressure to keep the outside air from pushing back in. That's why I suggest sticking to total aftermarket systems as opposed to custom bends. The kits are engineered, dyno tested, etc. to get the best performance for your specific application. Most custom jobs are just guessing...