Exhaust Questions

KJMedic · **Posted:** Sun Jan 13, 2008 12:45 am

I wast to improve my Libby. If I go with at 3 inch exhaust from the Turbo back, what would be the advantage of having the Cat. on. This is for Off Road only :twisted:

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warp2diesel · **Posted:** Sun Jan 13, 2008 2:22 am

3" on a 2.8L CRD would be like 4.25" on a 5.9L Cummins. I have seen 5" exhaust advertised for Cummins 5.9L that are hopped up to 1000 HP. Our member gmctd may be the guy to have an opinion on how effective the mod will be.
My only comment is that a Turbo diesel does not need any back pressure to hold it together. In fact even a big 400+ CID tractor engine running with less than half the boost our CRDs run at, could have the rod caps removed (if you could do it with the engine running) with the engine running under full load and never have the rods lift off the crank. Therefore, 3" exhaust will not harm the engine. I suspect the ECM will cut back on the fuel rate to prevent any over boosting by the turbo that could happen with a larger exhaust.

Steve :wink:

KJMedic · **Posted:** Tue Jan 15, 2008 9:43 am

I have been reading about the 4" and 5" exhaust on the 3/4 ton trucks. Most say that there is no better proformance in the 5" over the 4". I am looking for a 3" down pipe and other stuff now . I don't know if it will work or not but somebody has to try it first. :twisted:

And I am about 3800 miles from 70,000 so the useless warr. will expire.

dgeist · **Posted:** Tue Jan 15, 2008 3:50 pm

KJMedic wrote:

I have been reading about the 4" and 5" exhaust on the 3/4 ton trucks. Most say that there is no better proformance in the 5" over the 4". I am looking for a 3" down pipe and other stuff now . I don't know if it will work or not but somebody has to try it first. :twisted:

And I am about 3800 miles from 70,000 so the useless warr. will expire.

Seems like the diameter is restricted at the exit from the turbo to 2.5 anyway. one would think you'd get a similar benefit by just removing anything narrower than 2.5" (like the stock muffler).

Weegie5 · **Posted:** Tue Jan 15, 2008 4:48 pm

I've been thinking about doing this myself, but going with a 3" catalytic converter and 3" muffler to keep it street legal and reasonably quiet. It'd have a 2½" to 3" concentric transition immediately out of the turbo and 3" from there on out. The cat seems to be the most expensive part, from what I've seen.

DarbyWalters · **Posted:** Tue Jan 15, 2008 5:23 pm

The 2.5" pipe from the turbo to the "Cat" is pretty good size stock...the problem comes after that. The exhaust necks down and then enter the Muffler then out. I removed catback and made a straight 3" pipe all the way out...no muffler to bend off road or replace down the road. Sounds and works good:

Extended the new tip down a bit:

dgeist · **Posted:** Tue Jan 15, 2008 5:34 pm

DarbyWalters wrote:

The 2.5" pipe from the turbo to the "Cat" is pretty good size stock...the problem comes after that. The exhaust necks down and then enter the Muffler then out. I removed catback and made a straight 3" pipe all the way out...no muffler to bend off road or replace down the road. Sounds and works good:

Darby, what was your cost (if you don't mind) to have the 3" cat-back bent (I assume it's stainless and mandrel-bent) and does that have a significant benefit over 2.5" considering the CAT and Turbo are already that diameter?

gmctd · **Posted:** Wed Jan 16, 2008 3:27 pm

The only problem with upgrading to turbo-back exhaust system, eliminating the oxidation convertor, is Federally-mandated state emissions inspections: strict 2008-2010 requirements will put each state in the Kalifornia mode, with smog cops carrying long-reach inspection mirrors for casual under-vehicle walk-by inspections - missing items result in a ticket logged in the state computer, flagged at inspection time = new sticker denied - all states will be forced to comply by congressional mandate, where currently it is only to qualify for highly lucrative Federal road and highway funding - if you also installed the 3" or 4" oxidation convertor from a GM or Ford or Dodge Diesel vehicle, your new system would satisfy the certified equipment specs for the Jeep - only other consideration is available real estate in the lower engine bay from the turbine exhaust housing back.

Other than that little glitch, a 3" system would help, particularly when towing - eliminating that restrictive muffler with a new 2.5" system would help daily driving by removing a source of back pressure

Keeping the soot-trap blown out also helps

FYI: an installed EGT guage would quickly prove all this out, even better than the ole buns dyno...............

DarbyWalters · **Posted:** Wed Jan 16, 2008 8:02 pm

I used Flowmaster Mandrel Bent Tubing and cut to fit. I think you could get someone to do the whole thing for about $200.00 including materials. I wish I had an EGT to see the effects...but I am planning a dyno day in the near future...just for curiosity sake.

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