04 F350 6.0 How to decide what size exhaust to get. 4 or 5 inch? Turbo back or cat back?

 

Given the specs on that 65 F100 in your profile, I"d recommend custom 6in turbo back system cat and muffler delete.

Seriously though, what are you planning for the truck as far as use and other mods?

 

It is my bad weather driver. 2 wheels is my main transpo. So pretty much a daily driver. Do have a plan to get a 5th wheel toy hauler probly next year and haul my 65 in the 24ft enclosed. Plan on getting the SCT after that. That's about it. As far as HP upgrades. I do have a 4in turbo back dual setup for less than $600 lined up that keeps the cat

 

4 inch should be just fine. I think you start having clearance issues with a 5 inch with regards to the down pipe. I kept the down pipe stock and went to a 4 inch with the CAT deleted. I pulled a 13k fifth with it no problems with a mild tune.

 

5 inch is too large to see any gains (you may even see losses due to decreased flow speed of exhaust) unless you are running major power.
Personally I think a 4 inch turbo back cat delete with a high flow muffler sounds awesome! Quite enough to not get harassed and loud enough to enjoy!
I chose to delete the cat since it's a restriction and I would plug it with smoke being tuned anyways.
Turbo back is the only way to go otherwise you don't really get much improvement because the flow is still restricted right from the beginning.
As far as brands they pretty much all sound good from what I've read.
Dual tip vs single tip: dual has more throaty V8 rumble and leas whistle, single has more whistle and less rumble, but has an incredibly unique sound at higher rpm to it that I love!

 

Also I picked up my 4" turbo back single AFE system for $480 brand new and simply bought the mbrp tip (because their tip looks the best)
Many have gone with diamond eye or others for even cheaper