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I stopped by a shop today to have my driveshaft ballanced. I told them about my project. While there I asked about bending 4" exhaust. There are 2 late model Dodge diesels at the shop. They said that they have removed the 5" exhaust that was added to these trucks because it killed the low end torque. They recomended that I not use 4" and stay with the 3" exhaust. This wasn't done to try to get my exhaust bussiness. It turns out they don't do exhaust. With my gearing I will do all of my driving below 2000 rpm. Since I am using the TH400 transmission there is a big step between gears. I will need all the bottom end that them motor can make just to make it up to highway speed.
What are everyones thoughts on this. I have a little time to research but don't know where to find the information other than here. This is the largest group of 4bt enthusiasts in the world.
Thanks.
 

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If you look in a Walker Dynomax catalog, here are some of the results that caught my eyes. A #17216 (2-1/4") Welded Ultra Flow muffler will not become restrictive until 323hp. A #17218 (2-1/2") Welded Ultra Flow muffler will not become restrictive until 515hp. A #17220 (3") Welded Ultra Flow muffler will not become restrictive until 1000hp. If I am reading this right, a 2-1/2" pipe should flow at least 515hp. I won't have a problem with that! I believe that muffler restriction is far more important than pipe size. My plan is to run 2-1/2" mandrel bent pipe (minimum) and use an appropriate muffler for noise/restriction. But the 4", 5", & 6" have a higher "bling, bling" factor! The tailpipe can always be adapted up for that or a smaller pipe can be hidden inside a larger one! Two other reasons for my choice of 2-1/2" are cost and exhaust temperature for the scavenging effect (and soot build-up).
 

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Personally I wouldn't go with a 2 1/2" exhaust. Cummins put a whole bunch of research and diesel experience into these 4BT's and their installations. The turbo itself has a 3" outlet. It cannot do anything but hurt performancce to restrict that. Also diesels don't play by gasser rules. On a gas engine built for torque you do use a slighly smaller header, like 1 5/8" headers on a torque-built smallblock, 1 3/4' on a high rpm hp built engine.

I want top efficiency out of my turbo. 3" is the minimum size without choking off the turbo, and I'll use 4" for free flowing from turbo back. From what I've learned on various diesel websites you want the turbo to work as efficeintly and freely as possible if you want power, bottom end or top end.

Regarding bottom end power, from what I understand that's more of a function of injection pump timing than anything, but other factors do come into play. Others have more exerience to share on that than me.
 

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I bought several Wonder bread truck last year. The strongest runner had a 2,25in straight pipe. I would go with no larger than 3in.
 

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If you look in a Walker Dynomax catalog, here are some of the results that caught my eyes. A #17216 (2-1/4") Welded Ultra Flow muffler will not become restrictive until 323hp. A #17218 (2-1/2") Welded Ultra Flow muffler will not become restrictive until 515hp. A #17220 (3") Welded Ultra Flow muffler will not become restrictive until 1000hp. If I am reading this right, a 2-1/2" pipe should flow at least 515hp. I won't have a QUOTE]

I think you would be compareing apples to oranges if you apply these figures to Diesel. All of those hp rateings are for gas motors. Diesel's run unrestricted air to the intake, gas motors dont.
 

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Personally I wouldn't go with a 2 1/2" exhaust. Cummins put a whole bunch of research and diesel experience into these 4BT's and their installations. The turbo itself has a 3" outlet. It cannot do anything but hurt performancce to restrict that.
What is the outlet size of the exhaust turbine wheel?
I doubt it'd be bigger than 2.5 inches.

The way to know if you're restricting performance is to measure backpressure. But no-one seems to do that.

There's a rule of thumb for petrol engine air consumption of 10hp per 1lb/min of air flow. So a muffler suitable for a 300hp petrol engine can pass about 30lb/min of airflow.
The air consumption of your diesel will vary, but I put the figure between 1.5 and 2 times the air consumption of equivalent petrol power.

BUT.
Diesel exhaust is colder than petrol, so the volume flow is lower. Running a max EGT of about 600 deg C for most people compared to about 900 deg C in a petrol.

When you adjust the flow figures to compensate for that temp difference, we're down to maybe 30% more airflow than from an equivalent power petrol engine.
 

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My exhaust outlet is 2.5 inches. Everyone else seems to be using 3 inch v-clamps, do I have an oddball? If you consider the 16cm2 exhaust housing has about a 1.75 hole at the turbine, 2.5 inches is really big!
 

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I'd have to say that my recommendation is based on information I've gotten from other diesel sites, primarily Dodge CTD sites. From what I read in performance threads one of the #1 upgrades suggested is BIGGER EXHAUST. many statements like, "You won't believe how it wakes it up..." and that kind of thing.

Me, I don't have any idea from personal experience. Wish I knew for sure before I go to the trouble and expense of swapping mine out from head to tail! I plan on using correct tuned length, but that again is from gasser specs & calculations.

We need some feedback here like, "I have done it and this is what happened..." Can anybody describe their personal experience of performance changes, or no performance changes, JUST FROM EXHAUST?
 

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This is what I'm thinking, and I've heard some technical eplanations from diesel experts with the right words that back it up 100%.

The turbine has a huge effect on diesel performance, possibly more than any other single component [assuming decent injectors]. The turbine is spun by exhaust gases/heat thermal expansion. It must fight against drawing air through filter and piping to intake manifold. Up to a point, more fuel fed to cylinder increases the expansion of gasses at combustion. More fuel, more pressure. That expanding wave then goes out the exhaust manifold and passes over the exducer side, spins the wheel, and out the downpipe, exhaust/muffler and into atmosphere.

Where an engine is allowed free flowing exhaust by larger pipe diameters the exhaust gases can more easily transmit their power to impeller. An exhaust brake works against this principle and by restricting exhaust it greatly slows the engine's revolutions. A smaller exhaust housing on the turbo transmits more of this power directly to impeller. A larger volume compressor side, with correctly sized impeller, forces more air during the process for a given amount of exducer's energy exchange. After that the key is to allow these gases to get out of the way as quickly as possible to allow the incoming exhaust gas to do its work with the least resistance i.e. back pressure.

Turbo diesels do not want exhaust back pressure because they're not petrol engines, and because they use an exhaust driven turbo. Exhaust back pressure benefits in a gasser engine have to do with exhaust wave exiting exhaust pipe and the relationship of positive/negative pressure wave inversion in pipe, which can cause a beneficial extraction effect at correct diameter and length. The same inversion wave theory applies to diesel, but the restriction by back pressure gives no benefit, but instead slows down the exhaust/turbo process.

If this is not so, can somebody please explain how restricting a turbo diesel exhaust can improve performance? How can slowing down the gases working in and exiting the turbo help the engine to make more power? Why would it help on bottom end when the turbo hasn't even spooled up yet? Thanks,

JimmieD
 

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If this is not so, can somebody please explain how restricting a turbo diesel exhaust can improve performance? How can slowing down the gases working in and exiting the turbo help the engine to make more power? Why would it help on bottom end when the turbo hasn't even spooled up yet? Thanks,

JimmieD
It can't.
This is another myth that gets carried over form people who are used to non turbo petrol engines.
Much like the "diesel running too lean" myth.

On any engine your goal is to get the gas out as well as possible. Backpressure is always the enemy, most people get confused with backpressure and scavenging.
Scavenging is using the low pressure zone after one exhaust pulse to help pull the pulse from the next cylinder. This requires decent gas velocity to work, which some people confuse with backpressure.

E.G. Putting an oversize header on a small non turbo engine can kill the scavenging effect, engine doesn't perform as well and some people think it's because of a lack of backpressure.


BUT
On a turbo engine, the exhaust manifold has the job of directing the exhaust pulses in an orderly fashion to the turbo. This is where any exhaust tuning needs to be done.
The exhaust from the turbo outlet is simply a drainpipe. It needs to get the gas away with as little backpressure as possible.

To do this, bigger and more freeflowing is better, but like anything it's easy to get carried away. Until someone actually takes some backpressure measurements, we won't know what these engines actually need and we will all continue to speculate.
 

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To do this, bigger and more freeflowing is better, but like anything it's easy to get carried away. Until someone actually takes some backpressure measurements, we won't know what these engines actually need and we will all continue to speculate.
Would you know how to measure backpressure, with what kind of devce?
 

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Would you know how to measure backpressure, with what kind of devce?
Simply plumb a boost gauge into the exhaust straight after the turbo.
But make sure you use metal tube for the first foot or so after the exhaust. After that far the tube will be cool enough for plastic to survive.

Technically there are all sorts of considerations with the geometry of probe required that must protrude into the exhaust stream, but none of it matters for the level of accuracy we're looking at. Simply welding a patch the to dump pipe and threaded a hose fitting in there will do the job fine.
 
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