4" Downpipe Install
Thread Starter
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More Turbo
Joined: Mar 2016
Posts: 643
Likes: 6
Well too late for me, I already bought the 4". My goal is to get EGT as low as possible towing because on my H&S tunes it seems to be dumping A LOT of fuel in there and running some high temps. I'll be interested to see the difference in spool up time, especially on the hotter tunes when I'm not towing. I just might attempt to install it myself this weekend and see what kind of disaster I can create :-)
Fleet Mechanic
Joined: Jul 2013
Posts: 1,330
Likes: 65
From: 'Pitlock, Maine
I installed a 4 in. Down pipe along with the rest of a 4in exhaust and my stock engine runs/tows Better than ever, I also think you'll be fine.
4wd Low
Joined: Dec 2019
Posts: 10
Likes: 0
From: New Jersey
hi im new to the 6.0 ive read all the threads for ideas. ihave a 4in exhaust from the turbo all the way to the 5in stacks no cat and muffler so with everyones answers it good to run 4in from turbo back
4wd Low
Joined: Dec 2019
Posts: 10
Likes: 0
From: New Jersey
replaced from the turbo back 4in to 5in stacks no muffler and cat sounds nice just had to bang alittle on firewall to clear exhaust pipe from turbo
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I had the serious lag our friend here is talking about. I had a full 4inch Flo-Pro exhaust and had two problems with it:
1. Lag was absolutely dreadful (Simon Cowell)
2. It constantly vibrated against the shackle for the front leaf spring.
The lag even with the Atlas 40 was the worst part of it. It would think....think....think some more, then BAM! Take off like a raped ape. No matter how they adjusted my tunes or even backdated me to something earlier, nothing fixed it.
When I went to a 3.5 inch downpipe, both those problems went away and my mileage even got better. Keep in mind I ditched the Atlas 40, went back to my stock FICM and still have better spool times as well as mileage than before.
Of course it doesn't make sense, you're thinking about it backwards...
What takes longer to completely fill with air?
A 4 inch cylinder? OR A 3.5 inch cylinder?
The correct answer is the smaller of the two or 3.5 inch cylinder.
When Ford originally designed the exhaust for the 6.0, they designed it with a 3 inch down pipe and 3.5 inches after the cat. This helps spool the turbo, but helps the truck breathe some as opposed to having 3 inches from downpipe to exhaust tip.
Also, contrary to what's been previously posted, the 6.0 does indeed require a reasonable amount of backpressure to run properly. Again, this is why the exhaust was designed the way it was:
1. To help spool the turbo and allow it to respond reasonably quickly.
2. Maintain the required back pressure to spool the turbo.
3. Provide the good low end torque you paid for.
When you add a 4 inch downpipe, yes you're eliminating back pressure, but you're also increasing spool time because the truck has to dump more fuel than it usually did to get the truck moving and now the torque you paid for, is now at a higher RPM. Which is exactly why I was told by the Engineer who wrote the programming for the 6.0, that : "Unless you plan on racing the truck, leave the exhaust alone".
Keep in mind that if you were running 175, 190, or 225 cc injectors, I could see a 4 inch exhaust making sense because those injectors would dump more than enough fuel for it to make sense. Otherwise, if you feel you have to upgrade with stock injectors, stick with the 3.5 inch downpipe.
1. Lag was absolutely dreadful (Simon Cowell)
2. It constantly vibrated against the shackle for the front leaf spring.
The lag even with the Atlas 40 was the worst part of it. It would think....think....think some more, then BAM! Take off like a raped ape. No matter how they adjusted my tunes or even backdated me to something earlier, nothing fixed it.
When I went to a 3.5 inch downpipe, both those problems went away and my mileage even got better. Keep in mind I ditched the Atlas 40, went back to my stock FICM and still have better spool times as well as mileage than before.
Of course it doesn't make sense, you're thinking about it backwards...
What takes longer to completely fill with air?
A 4 inch cylinder? OR A 3.5 inch cylinder?
The correct answer is the smaller of the two or 3.5 inch cylinder.
When Ford originally designed the exhaust for the 6.0, they designed it with a 3 inch down pipe and 3.5 inches after the cat. This helps spool the turbo, but helps the truck breathe some as opposed to having 3 inches from downpipe to exhaust tip.
Also, contrary to what's been previously posted, the 6.0 does indeed require a reasonable amount of backpressure to run properly. Again, this is why the exhaust was designed the way it was:
1. To help spool the turbo and allow it to respond reasonably quickly.
2. Maintain the required back pressure to spool the turbo.
3. Provide the good low end torque you paid for.
When you add a 4 inch downpipe, yes you're eliminating back pressure, but you're also increasing spool time because the truck has to dump more fuel than it usually did to get the truck moving and now the torque you paid for, is now at a higher RPM. Which is exactly why I was told by the Engineer who wrote the programming for the 6.0, that : "Unless you plan on racing the truck, leave the exhaust alone".
Keep in mind that if you were running 175, 190, or 225 cc injectors, I could see a 4 inch exhaust making sense because those injectors would dump more than enough fuel for it to make sense. Otherwise, if you feel you have to upgrade with stock injectors, stick with the 3.5 inch downpipe.
Fleet Mechanic
Joined: Jul 2013
Posts: 1,402
Likes: 114
From: Tuscaloosa, AL
Unless I am really BADLY mistaken, the 6.0 uses a 3" down pipe. I put a 3.5" down pipe on mine and it was significantly larger than the stock one I took off.
Laughing Gas
Joined: Aug 2010
Posts: 1,049
Likes: 230
From: Amarillo, TX
I've always thought mine was kind of slow off the line (4"DP into 5" exhaust). On the plus side, I've never hit 1250 on my egt's, even hammered down pulling the 5th wheel. Hardest part of the install for me was getting the stock down pipe out.
Fleet Mechanic
Joined: Jul 2013
Posts: 1,330
Likes: 65
From: 'Pitlock, Maine
Here's my thoughts on the down pipe, if you slow the wind on a fan, it slows down. If that isn't the case with a turbo why not? I'm not trying to start a pissin contest, just trying to understand how this all works with my turbo?
Cargo Master
Joined: Dec 2021
Posts: 2,359
Likes: 991
From: Chambersburg, PA
I did the MBRP exhaust. Isn’t the outlet of the turbo 3.5”? I don’t see why you would want to fight the 4” on the down pipe. Not sure you gain much until the first hard turn at the outlet of the down pipe. On the MBRP kit, it transitions to 4” at the down pipe.