Ok here we go.

With Ford modular engines, they aren't pushing 300 horses at the rear wheels. These modern day engines to not produce enough power to require true duals. They're all small blocks and put usually 240 at the rear wheels if anything. With a single exhaust say a 3" single with a SI/DO muffler in 3" in/ 2.5" dual out configuration the velocity pressure remains strong and exhales through the muffler very healthy. True duals the scavenging is thrown off unless you go with a real small size ( 2.25" most people swear by ) the off set length of the exhaust is what really screws ya. Due to the fuel tank the drivers side bank needs guided over to the passeger side and ran along side it until it passes the axle. The drivers side bank is averagerly 3' to 3'5 longer than the passenger bank which is another torque killing issue with true duals. Cross overs can help elliminate alot of drag, but the fact of the matter is in the beginning that left bank being guided over to the passenger bank still knocks down performance. The right bank hits the crossflow harder than the drivers side will, but better balances scavenging.

If this makes sense to you then hopefully you will have a better understanding, and if you must have true duals, possibly keep it at 2.25" if you want the best performance you can get.

 

darren knows what hes talking about! my 08 f150 5.4L has 2.25" true dual straights with an x-pipe, and i felt little if any power loss on low end.