5" Down Pipe

[RichardBush]

Does anyone make a 5" down pipe for the 2003 5. 9 cummins?

 

[macdaddy]

I believe Banks power does, however i believe it's their exhaust brake elbow. So you would have to buy the brake to get it.



Mac

 

[retirednak]

I bought mine from BD. It came with my turbo back system. You can buy just the down pipe also.

 

[cerberusiam]

A 5" DP is a bad idea anyway. Run the 4" DP into a 5" at the bottom where it levels out you will be much better off.

 

[retirednak]

Just out of curiosity,why is a 5" down pipe a bad idea? The only problem I had with my system is the flange where the pipe mounts to the manifold cracked and was replaced by BD 3 yrs later for free.

 

[cerberusiam]

Just out of curiosity,why is a 5" down pipe a bad idea?

Short answer, exhaust flow. Diesels don't really like back pressure in the exhaust system after the turbo, that part of the exhaust system needs to have as little restriction as possible to promote spool and limit the amout of DP.



A larger than normal down pipe allows the exhaust gasses to expand and cool to fast. The cooler they are the less velocity they have and it takes more pressure to move them out the tail pipe. This ends causing slower turbo spool and more drive pressure to move the exhaust. The 3rd gens are already limited in this area with the small turbo housing and the emissions valve timing, they don't need another imbalance to enhance that effect.



A 4" exhaust is good for around 700 hp before its a restriction, the early trucks had a 3. 5" down pipe for a reason. Even with a 4" turbo back wrapping the turbo and down pipe produces gains by keep that exhaust flowing fast and hot until its part way the tail pipe. A 5" sounds good but its leaving something on the table unless your really cranking the power up.

 

[Joseph Donnelly]

I disagree. A 5" down pipe is fine if you can fit it. After the turbo, the gases should freely expand and smaller tubing does not help velocity, it is just that more velocity is needed to move the volume of gas that has to exit.

 

[cerberusiam]

After the turbo, the gases should freely expand and smaller tubing does not help velocity, it is just that more velocity is needed to move the volume of gas that has to exit.

Thats the problem, the more expansion room the more velocity is lost as the gasses cool faster on expansion. Cooler is heavier so velocity slows because the volume remains constant. To maintain the same velocity one would need to increase the volume proportianately, ie more fuel. Not what you want in most cases.



Garden hose concept, the smaller the orfice the higher the velocity at the same flow. Over distance the higher velocity translates to more flow at the end becuase of parasitic loss to friction.



In the old 12V's a turbo boot and\or DP wrap were good for 10-20 HP on simple flow properties, applies even more to the emissions engines with in-cylinder EGR.

 

[Hawk]

Cerberusiam,

You stated that the 4" is good for up to 700 hp. What is the formula to get those numbers? I am getting ready to purchase new duals for my new truck and was going to get the 5" Magnaflow and replace the cat with a hiflow unit. My 03 had 4" all the way out and no muffler or cat. It sounded like a tractor but I didn't mind one bit. Any theories would be greatly appreciated.

 

[cerberusiam]

Formula? The math is complicated and esoteric to prove at what HP level a 4" is inadequate, so much so you can easily get lost in the calculations. Kinda like figuring turbo maps and looking for a break off point, its gnarly. :-laf



Its just a general rule that has held realtively true over the years from experiences. Nothing scientific about it at all.



Again, until you get up over 500-600 HP the stock CAT really is not a problem. The COST of a high flow unit to meet the needs will scare you and put a huge dent in the pocket book. Loose it if you want or just leave the exhaust 4" to the CATand 5" from there.



Thats what I'd do but to each his own.

 

[Hawk]

The CAT will get changed due to my bad habits of making sure I squeeze every bit of efficiency out of my cars. All our turbo cars got a DP that matched the output of the exhaust housing. I have read alot of theories about it all including one that suggested a exspansion chamber like a 2 stroke has.



The truck is getting a set of ATS compound turbos, twin CP3's, cams and all the support stuff that goes along with it. I'm not building a race/sled truck but I do want a strong tow rig.



The big thing about the 5" was the CAT and muffler vs the 4" that was open. I will never see 700HP with my truck so if the theory stands then I will be getting a good 4" system that splits for my duals appearance.



Oh and all the hot parts to include the DP down to the CAT will be coted at HPC. Great coting that is worth every penny in my book.



Thanks for the info. I'm sure we'll have more of these in the future.

 

[SShafranko]

Be careful with the magnaflow dual kit. My friend put his on two years ago advertised 5 inch turbo back and it wasn't. It was 5 until it split by the rear, from the rear on it is 4 inch. The fit is very tight with duals not sure if you can do 5 inch without rubbing issues. Maybe they changed it now I'm not really sure. His kit was also advertized as stainless which it was but the cheaper grade. His exaust is rusted all over. I don't think it is anything to worry about because it is just surface rust and souldnt rust all the way through anytime soon but for 1200 dollars he. Thought he was getting 5 turbo to tip and the 304 stainless if that's the better of the two.



All I'm saying is do your homework before you purfchase any kit. Good luck



Sasha

 

[Hawk]

Be careful with the magnaflow dual kit. My friend put his on two years ago advertised 5 inch turbo back and it wasn't. It was 5 until it split by the rear, from the rear on it is 4 inch. The fit is very tight with duals not sure if you can do 5 inch without rubbing issues. Maybe they changed it now I'm not really sure. His kit was also advertized as stainless which it was but the cheaper grade. His exaust is rusted all over. I don't think it is anything to worry about because it is just surface rust and souldnt rust all the way through anytime soon but for 1200 dollars he. Thought he was getting 5 turbo to tip and the 304 stainless if that's the better of the two.



All I'm saying is do your homework before you purfchase any kit. Good luck



Sasha

I don't want 5" all the way out. I would actually like to have the Banks set up instead. It's only 3. 5" out the back. If I decide the 4" is the way to go I will prob get it. Once I get some time to search some info I will make up my mind. I'm sure the 4 in going to be ok, but I would hate to have to buy new exhaust later down the road.

 

[Cumminsduallyguy04]

heres 3 actual comparisomes on three trucks 3 completely different setups



My 04. 5 dually 48re(at time): stock cp3, 90hp injectors, afe stage 2, stock turbo, ats manifold, banks intercooler kit with intake. banks 6 gun/speedloader/PPE HOT stack (hate me idc) i went from 4" straight pipe addition on otherwise factory exhuast to a 5" post downpipe to 5" stack inlet to 7" stacks. it made a difference. I went back and adde

d a 5"down pipe from rpm outlet (last time i checked unavailable). the only gain was more rachet and i loss a bit of low end grunt



The 04. 5 SRW SB grey truck 48re: Had FASS 150, dual cp3, 120 injectors, banks intercooler with highram intake again, Smarty/ts mp-8, Afe stage 2, full billet gorend trans. I added a 4" test pipe to this truck 2 weeks after i bought the truck and it came with the goerend. after went all out with the fueling I added rpm outlet 5" downpipe, 5" striaght section of pipe right into the 5" stack inlet to 7" stacks. i gained alot more rachet, faster spool up, upset neighbors, and too much neg. attention. (can you look at my stacks and keep your car in your lane thanks) later i added Junky PDR stg 4 twins. blew the ht3b, then went with a SB66 which was incorrectly mapped for my application (no boost) then switched to a SB64 and was happily ever after...



the 06 F650 Tow truck with 5. 9 cummins and allision: engine is stock with exception to exhaust (just getting started lol). The OEM exhuast had a adapter plate (looked like restricter plate or wall) to adapt from smaller than 04. 5 dodge turbo flange to actual 2. 5". the 2. 5" pipe. I reengineered the exhaust, fuel lines, pto lines, so that the exhuast comes straight off turbo makes two 45degree bends and dumps via turn down. when i did I tried to use a 04. 5 dodge downpipe flange but it was too big for turbo. so i torched the "wall" out of the exhuast adapter and run 4" pipe. the truck lost alot of low end grunt. I later added 1 foot to that exhuast plus 3. 5 feet of muffler and I alot of power over both prior setups on the truck.



you kinda have to play the exhuast it til you have it just right in my case i needed to add length and a muffler to make it perform. as i have found there is such thing as too big and too small of exhuast pipe... . so wheres the sweet spot on intercooler plumbing?

 

[Cumminsduallyguy04]

Oh and exhuast wrap helps make up for the difference when you use too big of a pipe. it also acts as a muffler when you put it from turbo to just behind front doors.





and another thing. wheelbase actually has alot to do with proper exhuast size. the longer the length the bigger the pipe can be.





another angle is on this would be your egts and volume your trying to flow post turbo... sorry im long winded