Down Pipe Fabrication
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Fleet Mechanic
Joined: Mar 2007
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From: Meigs County Ohio
Down Pipe Fabrication
Hello Yall
I have a down pipe, that was give to me.... of a school bus (I still have 4 inch exhaust pipe if people want it!) But I would like a downpipe other then the stock cobra head. So I want to know willa 3 and 1/2 inch exhaust fit between the engine and the firewall? Now note I will be fabricating this all, so I am not worried about length, I got a ton of pipe. But can it be done? If so I will probably get to working on it soon. I can get my hands on a saws all, and I will be on my way.
I have a down pipe, that was give to me.... of a school bus (I still have 4 inch exhaust pipe if people want it!) But I would like a downpipe other then the stock cobra head. So I want to know willa 3 and 1/2 inch exhaust fit between the engine and the firewall? Now note I will be fabricating this all, so I am not worried about length, I got a ton of pipe. But can it be done? If so I will probably get to working on it soon. I can get my hands on a saws all, and I will be on my way.
Senior User
Joined: Sep 2006
Posts: 247
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From: NC
It may take a little firwall modification, but it will fit. Most go with a 3 inch downpipe because it only take bending the pinch mold down for it to fit. Anything bigger and you have to bend the firewall in to get it to fit. I hope this helps.
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Joined: Sep 2003
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From: North East Ohio
From teh ones I ahve worked on even a 3" dp involves more than just a pinch weld mod, the firewall always need caved in.
Anything larger and you're looking at either a 3" body lift and/or cutting a section of firewall out and weld in a section of 5" pipe.
Int he end, a 3" dp will support 500hp+, but if the pipe was given to you you need to consider the cost of what it takes to actually use it.
Anything larger and you're looking at either a 3" body lift and/or cutting a section of firewall out and weld in a section of 5" pipe.
Int he end, a 3" dp will support 500hp+, but if the pipe was given to you you need to consider the cost of what it takes to actually use it.
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Thread Starter
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Fleet Mechanic
Joined: Mar 2007
Posts: 1,808
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From: Meigs County Ohio
I got a ton of wire, I got the MIG and Plasma cutter, and I got all the tools i need. But, if my job pans out, I will just buy a DP. Man there goes my idea of a custom DP lol. Thanks Guys.
More Turbo
Joined: Dec 2002
Posts: 664
Likes: 129
From: Columbia, MD
Assembly tolerances affect downpipe fit?
What will fit without major mods probably is determined by assembly tolerances on your truck. Engine rear / cab forward is less clearance between engine and firewall, cab rear / engine front is more clearance.
From reading posts over time, most folks are lucky and get a 3" pipe in with no more than slitting and bending the weld flange at the bottom of the firewall. On mine, for 3", I had to move the firewall to get clearance. Not too bad once I figured out the tool... short little air hammer with a short, blunt chisel and peen a dent in the firewall. 3.5" pipe would have been another 1/2" of firewall movement... and a lot of pain.
From my experience, once you have the stock downpipe out, you can get an idea of how much firewall effort it will take by trying to drop your pipe down between the engine and firewall. But with the stock pipe cut out, you're kind of committed already.
From reading posts over time, most folks are lucky and get a 3" pipe in with no more than slitting and bending the weld flange at the bottom of the firewall. On mine, for 3", I had to move the firewall to get clearance. Not too bad once I figured out the tool... short little air hammer with a short, blunt chisel and peen a dent in the firewall. 3.5" pipe would have been another 1/2" of firewall movement... and a lot of pain.
From my experience, once you have the stock downpipe out, you can get an idea of how much firewall effort it will take by trying to drop your pipe down between the engine and firewall. But with the stock pipe cut out, you're kind of committed already.
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Fleet Mechanic
Joined: Mar 2007
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From: Meigs County Ohio
My stock pipe is still in there lol I am not going to play the game till I know allt he rules, but thanks guys. I appreciate it, probably cut it out in spring and go from there.
Postmaster
Joined: Jan 2004
Posts: 3,207
Likes: 0
From: Kearney, Nebr
I have a custom fabed downpipe because I'm running a holset charger. The firewall needs to be pushed in. I'm still trying to figure out the best way to do this because my dp hits the firewall on occasions. It's annoying.....
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Hotshot
Joined: Jun 2007
Posts: 10,008
Likes: 7
From: Austin, TX
use a port a power and push the firewall in a fuzz more wherever its hitting. gotta love port a powers. LOL
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