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Next week My project will be ready for exhaust and now wondering what size I may use for the Burb'. I know 3 inch exhaust systems are the norm for a stock 105 horse 4BT but will a 4 inch exhaust improve over all performance and lower EGT's?

My 4BT has the H1C turbo with the small compressor housing so I don't know if that would be better off with a 4" or just leave it at 3"???
 

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The 1st Gen CTD 6BT used a stock 4" exhaust which was fine for some very high power levels. The 3" 4BT exhaust is equivalent to a 4.5" exhaust on a 6BT. Seems to me it should allow for some pretty exotic power levels as-is.
 

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The 1st Gen CTD 6BT used a stock 4" exhaust which was fine for some very high power levels. The 3" 4BT exhaust is equivalent to a 4.5" exhaust on a 6BT. Seems to me it should allow for some pretty exotic power levels as-is.
I'm not sure this is right.
The area of a 3" pipe is 7.07 sq. in.
The area of a 4" pipe is 12.56 sq. in.

So if you calculate that a 6bt has 6 cylinders / 12.56 area (4" pipe) = 2.09 sq. in. per cylinder.
A 4bt with 4 cylinders / 7.07 area (3" pipe) = 1.76 sq. in. per cylinder.

You would need a 3.25" pipe on the 4bt to be the same effect as the 4" on the 6bt.

Correct me if I am wrong.
 

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Welcome to the only place on the internet where a group of men will discuss whether or not 4" is "big enough":smile:


RW
 

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I'm not sure this is right.
The area of a 3" pipe is 7.07 sq. in.
The area of a 4" pipe is 12.56 sq. in.

So if you calculate that a 6bt has 6 cylinders / 12.56 area (4" pipe) = 2.09 sq. in. per cylinder.
A 4bt with 4 cylinders / 7.07 area (3" pipe) = 1.76 sq. in. per cylinder.

You would need a 3.25" pipe on the 4bt to be the same effect as the 4" on the 6bt.

Correct me if I am wrong.
I ain't no math whiz, but what I did was divide the 6BT 4" diamter by 6, and got 0.6666667. Then I divided the 4BT's 3" by 4 and got 0.75. If I take the 4" divided by 6, and multiply times 4 for 2 less cylinders it comes out to 2.667" which is smaller than 3". If I take the 4Bt's 0.75" and multiply times 6 it comes to 4.5", which is larger than a 6BT stock exhaust.

I could be off base 'cause I'm not real bright on mathematics.
 

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Too large will only give you more noise and fitting headaches.

Has anyone measured exhaust backpressure?
 

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If you plan on moddin' just go 4" you're good to go and it sounds much better; when we ran our 4bt on the stand we had 4" piping routed out of the shop and it sounded no different than a Dodge w/ a 5.9l and an exhaust, it sounded down right mean... With our 3" downpipe now it just sounds like a tractor...
 

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I ain't no math whiz, but what I did was divide the 6BT 4" diamter by 6, and got 0.6666667. Then I divided the 4BT's 3" by 4 and got 0.75. If I take the 4" divided by 6, and multiply times 4 for 2 less cylinders it comes out to 2.667" which is smaller than 3". If I take the 4Bt's 0.75" and multiply times 6 it comes to 4.5", which is larger than a 6BT stock exhaust.

I could be off base 'cause I'm not real bright on mathematics.
The issue with your math is that there isn't a linear relationship between 3" and 4" exhaust.

when it comes to cross-sectional area of the tube, the formula is pi*r^2. so you have to compare the area/cylinders.

hopefully that explains it. I don't know for sure that the 2/3rd rule works in this case, but even if it's not 100% true, it's probably close.

i came up with the same results as Kurt (biobusgroup)
 

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If you plan on moddin' just go 4" you're good to go and it sounds much better; when we ran our 4bt on the stand we had 4" piping routed out of the shop and it sounded no different than a Dodge w/ a 5.9l and an exhaust, it sounded down right mean... With our 3" downpipe now it just sounds like a tractor...
Is there a way to go 4" out of the Turbo? I currently have a 3" downpipe and then it will go to 4".
 

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Yea, that makes sense, even to me....
 

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www.htturbo.com sells an adapter for non-wastegated exhaust housings for 98-02 dodge application downpipes. There are several companies that have 4 inch downpipes for those model years.
 

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good deal.

Now i just have to figure out routing. tailpipe is easy, it's the intermediate stuff going around 2 crossmembers that may be tricky. I may end up running it below a crossmember. not something i really want to do, but my truck will only be 2wd and won't see much dirt, let along rocks. that's what the jeep's for.
 
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