3.5" or 4" Downpipe
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Senior User
Joined: Jun 2003
Posts: 100
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3.5" or 4" Downpipe
I just traded my '03 F350 (V10)CC 4X4 for a '04 F250 (6.0 diesel) SC 4X4. I am real pleased with the performance of the 6.0 diesel. I would like to change to a 4" exhaust. Should I stick with the factory downpipe or change to the 4" downpipe? I would also like to get a tuner. I don't know which one to get, the SCMT or the Diablo Predator?
Thanks for any input!
Thanks for any input!
Hotshot
Joined: Dec 2003
Posts: 15,368
Likes: 121
From: Stuttgart, Ar
The question of the year!!! 
That's hard and will draw lots of answers. Lots will say 4" down pipe--some are of the opinion 3.5 is fine. I personally have a 3.5 downpipe with 4" magnaflow exhaust. Running the predator on 65hp all the time and 100 occassionally (but that's too fast for my ole lady ). Each has it's own advantages. I picked the predator for it's updateability online. I picked the magnaflow because it dynoed more HP with the 3.5" downpipe than the 4" (according to magnaflow). Whichever you pick --the Predator or the SCMT-- you'll enjoy! 
That's hard and will draw lots of answers. Lots will say 4" down pipe--some are of the opinion 3.5 is fine. I personally have a 3.5 downpipe with 4" magnaflow exhaust. Running the predator on 65hp all the time and 100 occassionally (but that's too fast for my ole lady ). Each has it's own advantages. I picked the predator for it's updateability online. I picked the magnaflow because it dynoed more HP with the 3.5" downpipe than the 4" (according to magnaflow). Whichever you pick --the Predator or the SCMT-- you'll enjoy!
Freshman User
Joined: Aug 2004
Posts: 41
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From: New Jersey
I have the banks turbo back and it is a 4 inch from the turbo all the way to the 5 inch tip.. I also run the edge with attitude and love it.. what a difference in power plus the change settings on the fly and all my gauges in one place.. imo it is awsome and worth the money..
Elder User
Joined: Jun 2005
Posts: 710
Likes: 0
From: Fallon,NV
This is a blast from the past!
npccpartsman, I was glad to see that the 3.5" DP dynoed more than the 4" DP. I am thinking of getting a 4" turboback exhaust and I've been debating what downpipe to go with. Also, I heard that the 4" DP raises the rpm of the torque curve which I wouldn't like when towing. Thanks for the info.
npccpartsman, I was glad to see that the 3.5" DP dynoed more than the 4" DP. I am thinking of getting a 4" turboback exhaust and I've been debating what downpipe to go with. Also, I heard that the 4" DP raises the rpm of the torque curve which I wouldn't like when towing. Thanks for the info.
Fleet Mechanic
Joined: Jan 2007
Posts: 1,708
Likes: 7
From: Circle City
the left bank up pipe to the turbo is 2"
the right bank up pipe to the turbo is 2"
this exhaust pressure needs to go somewhere, quickly.
The exhaust manifolds are bad enough with velocity restrictions . I vote 4" after turbo.
the right bank up pipe to the turbo is 2"
this exhaust pressure needs to go somewhere, quickly.
The exhaust manifolds are bad enough with velocity restrictions . I vote 4" after turbo.
Hotshot
Joined: Dec 2003
Posts: 15,368
Likes: 121
From: Stuttgart, Ar
You can't add 2+2 and say 4 in this case. IIRC one 3" pipe will outflow two 2 1/2" pipes. It has to do with the area of the circle, etc. I'm still running the same 3.5" downpipe and dynoed 517 rwhp just last Saturday. Depending on how much towing you're doing I think you'd be absolutely fine with a 3.5" downpipe, especially with a stock turbo. A 4" will require a little more "fabrication" to make it fit without rattling.
Junior User
Joined: Jan 2006
Posts: 67
Likes: 0
From: Prince George
I used the 4" magnaflow on mine, no installation problems good fit easy job just had to cut the little stud away from the fire wall half way up and removal went easy with the sawsall! I went 4" to get the lowesr EGT's which I thought was important to the health of the motor, and the 4"s/s was a very good price.
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Elder User
Joined: Dec 2007
Posts: 971
Likes: 1
From: IA MN SD
The 4" tube has 30% more capacity than the 3.5" tube.
IMO, 30% is a pretty significant amount.
You just need to decide if the 30% is worth the small amount of extra effort (and extra $).
IMO, 30% is a pretty significant amount.
You just need to decide if the 30% is worth the small amount of extra effort (and extra $).
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Elder User
Joined: Jun 2005
Posts: 710
Likes: 0
From: Fallon,NV
Well that's the funny thing: the 4" downpipe is a lot cheaper than going with the 3.5" downpipe! Probably due to the extra fabrication for the transition piping?
I think that if the 4" DP will fit without rattling and if it will help lower EGT's, then that's what I'll be going with.
I think that if the 4" DP will fit without rattling and if it will help lower EGT's, then that's what I'll be going with.
Cargo Master
Joined: Mar 2007
Posts: 3,103
Likes: 32
From: AB/BC
My ex has a 4" turbo back with the kitty still in there for some reason(PO did it).My sig truck has straight stock pipe that runs cooler and sounds better.So if you are spending the money on an exhaust ditch the cat or just stay stock and ditch the cat and muffeler.
Cargo Master
Joined: May 2002
Posts: 3,271
Likes: 229
From: Gulf Coast, FL
Do you use a fire hose on your house water spigot?
Elder User
Joined: Dec 2007
Posts: 971
Likes: 1
From: IA MN SD
However, a larger pipe decreases head loss and backpressure, which lowers egts. To use your analogy.... 10 gallons per minute will flow through a fire hose with much less head loss than through a garden hose.
A larger pipe will also increase the capability of the system to handle increased flowrates, such as when a larger turbo or a tune were installed.
I'm not advocating a 4" pipe on our trucks. I will probably use a 3.5" if I ever replace mine. But you can't argue that a 4" pipe would provide for better performance than a 3.5" pipe.
Best performance would be with no downpipe at all (an infinitely large downpipe
)