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1st Generation Dodge Cummins 89-93 Discussion of 12 Valve 5.9 Liter Dodge Cummins Diesels with Rotary Injection Pumps

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Ok so I have a hybrid pickup it's a chevy with a 1st gen motor. I went to the muffler shop today to see what my options were. I learned some things one they can't bend 4" pipe so it's going to be completely custom. ... JOIN NOW TO REMOVE TRACER

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  #1  
Old 10-13-2011, 10:58 PM
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Default 3 1/2" - 4 or 5" Exhaust which way to go?

Ok so I have a hybrid pickup it's a chevy with a 1st gen motor. I went to the muffler shop today to see what my options were. I learned some things one they can't bend 4" pipe so it's going to be completely custom. That normally means $$$. What the shop thinks is that they would have to order the downpipe from cummins and the pipe from Chevy. As we talked he thought that 4" would be better than 5" so as not to loose to much backpressure. The other thought was just to delete the muffler.

My questions are would I gain any power or mileage just deleted the muffler and going with 3 1/2" straight pipe? What size would I get the best performance and milage? Since the turbo is still 3" wouldn't that be the point of restriction? Do they make a 3-4" or 3-5" adaptive downpipe that would open up the flow? Hopefully that make since thanks
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Old 10-14-2011, 12:01 AM
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with a turbo diesel you dont need back pressure as it goes against the turbo with = more turbo lag. i straight piped my 1993 and it spools much faster

jdbigblue12v's Sig:1993 ram w250 ext cab long bed 4x4 208,000mi 4'' straight piped to 6 inch tip behind tire, pump tweaked,homebrew fuel pin and intake, fresh .20" marine HG, new head bolts, timing advanced, hx35w, 33x13.50 on 18 inch billet wheels. soon to come low pressure lift pump, maybe compound with an hx55 and 5x.012 sticks by summer.
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Old 10-14-2011, 02:28 AM
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FYI: THIS IS DIESEL!!!! Can I get the guy from the movie 300 to say this???? Less restriction to flow means more power.

901stgen's Sig: Daily Driver~2000~24 Valve~Ext. Cab~Long Bed~Sticks~Turbo~Fass 150~4 Inch Exhaust
My Project~1990 non i/c 12v~Maxed Pump~3.2k spring~Timing bumped one gear tooth~Denny T Stage 2 Fuel Pin~5x.012's from LBD Performance.
Dads Rig~2011 6.7 Cummins~4x4~QC~SB... Stock... For Now

Last edited by 901stgen; 10-14-2011 at 02:34 AM..
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Old 10-14-2011, 02:20 PM
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Ok I guess I am a bit corned-fused. If the turbo outlet is 3" and you go larger isn't your restriction still at the turbo? Any thoughts on only doing a muffler delete? I get the less restriction the faster the spool up on the turbo however if I'm not going to be pushing more than the 3 1/2" pipe can handle trying to justify the cost. Thanks
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Old 10-14-2011, 03:22 PM
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if ur gunna do 3.5 just delete your muffler. if not 4in will work great. better spool up and probly a little lower egts too. but ya ur turbo is wat creates the back pressure.

Rusty89Cummins's Sig:'89 Cummins W-250, '01 intercooler, tweeked pump, time advanced, M-4 fuel pin, custom intake, throttle linkage adjusted, 366 spring, s362/74/14 turbo, 4" back exhaust, SDX 5x.014 injectors, holley black fuel pump, 3 in 1 guage, tach, head gasket thanks to BBD, A518 tranny swap, restoration complete
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Old 10-14-2011, 04:10 PM
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It may not make complete since but ya you're right it is 3 inch than has to go bigger but still helps tons to go bigger right after. You could easily feel it when I did a 4 inch downpipe to 5 inch straight on my old 24 valve. I will never run below 4 inch on a diesel again although I think 5 inch is just overkill but sounds great. On my conversion just get the downpipe and then figure out the rest ended up just sticking out the side in the middle behind the cab welded my own stuff up with nineties etc.

luke46's Sig:98 24v 4x4 ext cab 5 speed 6 inch long arm 35s flat bed ats manifold/intake edge juice w/att rv275s he351 south bend dual disk afe 4 inch straight
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Old 10-14-2011, 04:45 PM
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Luke did you do this on your 78? What kind of mileage are you getting with the '78? Looks like your setup is like my 89. Thanks
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Old 10-14-2011, 05:19 PM
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The closer to zero backpressure you can a make diesel exhaust system the better for performance and fuel milage. The bigger within reason the better IMO.Rocky

1arock's Sig:"If you're gonna err, err on the stout side", my grandad. 1arock
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Old 10-14-2011, 05:58 PM
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78 is drivable right now but under knife don't know exact mileage just started project a little while back has a hundred gallon fuel cell and no speedometer so never knew wasn't too bad tho had stock exhaust ya not hard to do but exhaust is off now too I'm building a hood stack instead it's my toy Just welded my own exhaust on my 07 too my chev truck started leaking from pump so it's waiting on parts new governor spring and I got a denny t for it

Last edited by luke46; 10-14-2011 at 06:01 PM..
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Old 10-17-2011, 01:42 PM
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Ok so Friday I went to the muffler shop and deleted the muffler on my beast. Instantly I saw some changes in which I'm still baffled (no pun intended) about.

1) Instant difference in power, and pickup remember this was just deleting and still using the 3 1/2" straight pipe. Quick responce.
2) The obvious sound difference
3) pulling the same load I pull and on the highway my RPM's at 65mph went from 1700 to 2100. This to me doesn't make sense and I was able to achieve 70mph much easier as before it was acting like a had an anchor just getting to 65.
4) My egt ran at or a little below what it did before. I expected this.

My question would be is the difference in RPM's make sense? I suspected that the muffler was either clogged or was just that restrictive. I did have to pull the trailer 160miles which sucked 3/4 of a tank but I will fill up tonight to see if there really was a difference in mileage.

I still want to take it in and have the mods done but just can't justify the $1400.00 unless I knew for sure that I would be able to save on fuel. What gearing would be typical and what rpm at 70 should I be in for a 12v? Thanks
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