Hello,
Sometime back I saw somewhere here there is an aftermarket pipe for the 5.4 that connects the manifolds to the rest of the exhaust system. I noticed how restrictive the factory y pipe is when I was replacing the passenger side manifold on my 01 SD. I did a search but didn't find it. I'm looking to gain some torque and hopefully some MPG, this seems like a good start. Thanks.

 

You noticed how restrictive it is by measuring the restriction? Its really not as bad as some let on, don't get me wrong, it can be improved. But are you going to notice any improvement and is it going to be worth your while? Tough questions to answer.

As far as the y-pipe itself, the best improvement you can have is just replacing the y-portion, the actual part where the two pipes come together. The rest of the design is far more free flowing than most would give it credit for. Despite the fact that it has 4 cats instead of 2, the setup allows you to produce 5 more hp per liter than the Chevy equivalent, which only has two cats and by appearance, a freer flowing y-pipe. Go figure. The Chevy 5.3L from 2001 produces about 50 hp per liter, while the 5.4L produces 55 hp per liter. if you feel really ambitious, you could replace the cats with true high flow cats, which will make the truck mean sounding, and you could pick up some power. MPG with any of these mods will be almost unaffected.

Your best place to start is with the muffler, followed by headers. Y-pipe mods will produce little measurable effect unless those restrictions are dealt with first. After that, a better flowing y-pipe section and freer flowing cats would complete the system, but are more expensive for the gains. The stock pipe is more than adequate, no need to replace it, in fact anything larger would likely cost you torque and fuel economy.

 

SPD performance has what you're looking for.

00-04 Ford F250-F350 6.8L V10 Performance Y Pipe | eBay


Khan, the superdutys use a different Y than the F150s. No cats until after the Y then there is only 1 huge cat.