I'd like to open up the exhaust on my '05 duramax, but don't want the issues of setting codes. Does anybody have real data on improvments with just a cat back system vs stock, or removing the kitty but leaving the rest of the system alone?
The gains I would be looking for would be mileage, performance, and lower EGT's. Thanks.

Sledman: I think the general consensus here will be to go to a 4 inch turbo back system. Eliminating the cat is illegal but most of us use our trucks off road anyways. I would not buy one for mileage gains. I think Kennedy has shown that a free flowing exhaust can be good for about 20 extra hp. The best thing you will see is lower egt's which will allow you to add future performance enhancing products.

Eric

You are going to set off a code when you eliminate the cat.

You are going to set off a code when you eliminate the cat.One thing to consider, is that we do have to have our diesels emissions checked yearly in Idaho, and they do a visual. Also, other than setting a code, what would going kittyless hurt? Does the code prompt the computer to alter anything?

I don't think it will hurt anything. Diesels run better with less back pressure. If they do visuals you can hollow out the cat and run a cat back exhaust.

Guys, removing the "kitty" does not throw codes. The 4" exhaust with the "kitty" gone does. I have checked mine with all the modifications. Preditor diagnostics and Tech 2.
check out the info I have learned here
http://dieselplace.com/forum/showthread.php?t=24221

I don't think it will hurt anything. Diesels run better with less back pressure. If they do visuals you can hollow out the cat and run a cat back exhaust.
I tried that with my old Ford, that had a 460 in it. Sounded terrible. Almost sounded like an exhaust leak, but that wasn't it. That large chamber (after being emptied of the honeycomb) created an echo chamber of sorts. Also, the shell on the cat was a thinner SS, which probably made it worse. If I was going to do something like that, I would probably try to drill a 4" hole through the center of the honeycomb. I think that would give the best of all worlds. Maybe.....

I wonder if anybody has tried that yet, with success?????

I think there are a few people here that have drilled out the cat. Try to run a search on it.

Rick I saw you ran stock with no Kitty and no codes, then you ran straight piped. Did that set any codes?

I Run The Mbrp Cool Dual System With The Replacement Big Diameter Turbo Down Pipe. No Problems No Engine Light And Exh Temps Down 200 Degrees With A Load And My Foot In It With A Preadator Tuner. Bubba:)

The reason codes are thrown is because the EGR does not have enough back pressure to work properly. If you cut the Cat, the stock muffler still provides enough back pressure to keep the EGR happy. If you get a Cat back system, same result. Cat gives it enough back pressure to work properly. Now, open them both up and you WILL get codes. Silent or SES light, they are there. Hope this helps.

The reason codes are thrown is because the EGR does not have enough back pressure to work properly. If you cut the Cat, the stock muffler still provides enough back pressure to keep the EGR happy. If you get a Cat back system, same result. Cat gives it enough back pressure to work properly. Now, open them both up and you WILL get codes. Silent or SES light, they are there. Hope this helps.
So what do you recomend? Cut out the cat and run with just the muffler or vice versa. Which would sound better?

Guys, removing the "kitty" does not throw codes. The 4" exhaust with the "kitty" gone does. I have checked mine with all the modifications. Preditor diagnostics and Tech 2.
check out the info I have learned here
http://dieselplace.com/forum/showthread.php?t=24221
That's not necessarily true, Rick....For whatever reason, on my truck, I get no codes...The cat is gone, full 4' exhaust, and no codes...SES light is not on, and no codes present when scanning either....This is with the dynomax system with their muffler....Is the muffler the reason for no codes? I have no idea....It does seem to me that the LLY trucks have a mind of their own when it comes to exhaust changes and egr changes....It seems some guys (like me) can remove the converter and no codes appear, yet some others have the check engine light issue....Just like some can unplug the egr with no light, and others (myself included) unplug it and the light comes on within minutes (without the finger stick, of course)
It would be nice to know what the differences are....

hmmmm, very strange. I am not familiar with the dynomax systems, is the muffler a straight through design? I have heard of a few isolated cases of no egr codes, but chalked it up to coincidence or restrictive muffler.

The reason codes are thrown is because the EGR does not have enough back pressure to work properly. If you cut the Cat, the stock muffler still provides enough back pressure to keep the EGR happy. If you get a Cat back system, same result. Cat gives it enough back pressure to work properly. Now, open them both up and you WILL get codes. Silent or SES light, they are there. Hope this helps.
So, which is better? Stock exhaust with no cat, or cat with MBRP cat back system?

And Will I trully see lower EGT's with either combination?

I would say leave the Cat on and get Cat back system. I believe the muffler is still the biggest restriction in the stock system. This is easier for dealer visits, emissions, etc. You will see a temp drop with the Cat back system. I see ~100 deg. drop depending on outside temperature, some claim more. If you later feel that you want/need a bigger drop in temps, get the optional front-pipe. Easy install in and out.

I think the Full MBRP system sounds bestw/Cat removed. Gives it a low growl like a big block. If you leave the Cat in place, the sound is very close to stock in the cab. Outside sounds like a jet.

I don't know why we can't duplicate the same problems from truck to truck. I have had people tell me they did something and how great it worked, and I try it and get completely differant results. All I know is what I have been able to do. I have 1 LLY running no cat and a very small flowmaster with the 3 1/2" exhaust, and 1 LLY with no cat and no muffler with 3 1/2" exhaust. Neither have thrown any codes yet. I tried the 4" with no cat and got codes. I wonder if GM has more than 1 ECM or ECM programing they are putting in our trucks. I have had both trucks reflashed, if that might make a differance. DSTRBD, my egt is at 850 degrees with an ambient temp of 65. This is from a dead stop to the top of 4th gear full throttle with the Predator on 100 hp. Does this sound right? Some of the people here say that might be a little low. It is a new Autometer pro-comp gauge with the probe in the exhaust manifold.

Perhaps weather, length of exhaust system, variation in manufacturing on the turbo.

Mine never threw a code with the cat gone and stock muffler in place. When the muffler vanished (must have fallen off) the codes happened immediately.

hmmmm, very strange. I am not familiar with the dynomax systems, is the muffler a straight through design? I have heard of a few isolated cases of no egr codes, but chalked it up to coincidence or restrictive muffler.Yes, the muffler is straight through....I've installed 1 on my truck and one on a friend of mine's truck as well...No codes in either truck...:confused: I would have to think that the muffler design is the key here, but weather considerations (humidity, barometric pressure, etc) might play a role here too....
.....and so the mystery continues.....

Had the stock system with cat removed, no muffler and have had no codes or light s in a month now. LB7 BTW