04.5 LLY Dmax, Superchips Cortex, K&N FIPK2 what next?

[Gearstix]

My dad has a 2004.5 Silverado 2500HD crew with the LLY.
We had a "Tri Before You Buy" 72 hour trial program from Bully dog in it, and man it was a rocket.
However, dad did not want to spend the $1200 on the bully dog programmer, so he bought the Superchips Cortex. We have not programmed it yet however.
He has a K&N FIPK2 in it, which he recently bought and installed.
The rest of the truck is stock.
He wants to do an exhaust, and I have convinced him to do a full turbo back exhaust.
Would it be possible for us to make our own exhaust?? He was talking about going off the turbo with a 5" downpipe, 5" piping into a 5" truck muffler like they use on semi-trucks, the ones you can buy from napa, and then have it turn out before the rear wheel. Would be better to just buy a pre-made exhaust?

He says that the Bully Dog programer programed in some kinda safeties or something, so if say your EGT hit a certain temperature it would throttle itself back or something like that. He wants to know if the Cortex has it.
If it doesn't, he wants to put a gauge pod pillar on it, and put in EGT and some other vital gauges(what do you guys use?)

Are there any simpler mods we can do to it? Any free ones?

[DmaxMaverick]

The exhaust should have been a first thought, not a convenient afterthought, IMO.

If you want a 5" downpipe, you'll have to run it through the cab (I don't recommend this). Not enough room for a 5" between the turbo, engine and firewall w/o some serious modification.

Other than that, you can use whatever fits. 5" is fine, but overkill. The "semi truck" muffler would be a bad idea. You won't flow enough to take advantage of it. It will probably enhance the unbearable drone you are already working on.

Sorry to sound so pessimistic, but that's the reality of it.

[Gearstix]

The exhaust should have been a first thought, not a convenient afterthought, IMO.

If you want a 5" downpipe, you'll have to run it through the cab (I don't recommend this). Not enough room for a 5" between the turbo, engine and firewall w/o some serious modification.

Other than that, you can use whatever fits. 5" is fine, but overkill. The "semi truck" muffler would be a bad idea. You won't flow enough to take advantage of it. It will probably enhance the unbearable drone you are already working on.

Sorry to sound so pessimistic, but that's the reality of it.

So how are aftermarket exhausts done then? Are most just cat backs?
I think we can get a 4" turbo back.. not sure.

[DmaxMaverick]

Aftermarket Duramax systems are "downpipe back", or cat back. No "turbo back". Meaning, the system starts after the downpipe, or after the cat, depending on the emissions requirements. An advertised "turbo back" system will be downpipe back. I don't know of any aftermarket system that doesn't retain the OEM downpipe.