this is my first post as i am a new member. i have a 2000 expedition xlt 4x4 -4.6l...i have installed a K&N FIPK & now i am getting 13.5 mpg city(up from just over 10!!!)& it definitely has a little more "pep". i want to increase horsepower but NOT at the cost of gas mileage! i am thinking about a flowmaster big block 2 "70 series" .(i want the same sound as a harley f-150) I have read on this site about the stock "Y" pipe being very restrictive, but i am concerned about lowering backpressure too much will hurt torque & mileage? do i need to stick to the stock "Y" or get an aftermarket "Y" & finish up with a SI/DO muffler?how about "true" duals with an "X" pipe & a DI/DO?(either system will still have cats). i have read about swapping the fan is a good mod? is the power increase noticeable? i know a superchip gives alot of performance, but having to run 93 octane(due to advanced timing)is defeating the quest for better mileage! lastly...how about pulleys? pros & cons? remember , i dont want to go backwards in mileage OR horsepower/torque. ANY advice would be GREATLY APPRECIATED!!! SORRY about the LONG post!

 

To increase gas mileage, get electric fans, intake, and exhaust. Possibly superchip as well, but you already counted that out.

I would replace the factory y-pipe with a 90 degree elbow and a dual 2.5" in to single 3" out Y-pipe, then the Magnaflow 3" single exhaust system. Dual out rear doesn't look right IMHO on an Expedition and for performance/efficiency you'd need to stay at dual 2.25" or smaller for the 4.6L. All aftermarket DOR systems I have seen are 2.5" A single 3" high flowing system is ideal for you.

 

I had a 97 Expedition with a 5.4. I added a K&N filter and Flowmaster kit 17138 and got about 16mpg with no other mods. The kit has SI/SO 3" pipe from the cat back. Cost is about $225.