I have noticed a drop in fuel mileage. I have a '04 F150 with the 5.4L. It has 92000 miles, I bought it at 60000 and I got about 15 MPG, which I thought was low. I am now getting about 13 MPG. I cut the exhaust off at the Y pipe and installed a 3 inch pipe with a Flowmaster muffler, still no change. Also the truck doesn't seem to have the power it did when I first got it. I changed the plugs at 80000 miles. Has anyone had this problem? Any ideas?

 

I'm not sure what you expected the exhaust modification to do since you still left the "Y" bottleneck. A muffler just muffles sound. Well some are more restrictive. A strait through or X through would be less restrictive then chambered ones.

fuel filter, clean MAF, check for codes. not sure what else. low power could be a sign of clogged cat's. You can have a pressure test done or the muffler place told me he drills in to it, but i'm not sure if I like that idea.

 

Pretty much the muffler was just for sound, and I was informed (by the guy at the muffler shop) that if I went with 3 inch after the Y-pipe would increase fuel mileage and power. I understand that with no other mods it will not help a lot, but every little bit counts. I am now looking into high-flow cats. Thanks for the pointers I will have to try them this weekend.

 

Well you have volume and volocity. The exhaust is hot as it exits and then cools down the pathway to the tail pipe. Since cooler air contracts and hot air expands you could go smaller towards the tail pipe and maintain velocity. The restriction is at the merge of the "y" section. If it merged to a 2.5" section and he cut that off and then welded on 3" to what is left which looks like a V, then that would help. If he welded on adapter reducer/increaser peice then that doesn't make any sense to me as the exaust is cooling and needs less space to maintain velocity. Sorry, i'm not sure if i'm wording it right. You can do a dual exit on the muffler but that is strictly for looks.

I removed the Y section and run dual 2.5" pipes all the way to the tips. My muffler guy kept telling me i'd get more power from a single. After I did my custom dual exaust I was able to spin my tires. Beforehand, I could not. I lost alot of power going to 37's though. Anyhow, he just wanted to sell me a new exhaust at full retail markup instead of getting paid to weld in what I brought in and purchased wholesale.

The best place I found to purchase metalic hi flow cat's is performance peddler. Do the best offer and let them email you a price and discount code.