I am looking for for information on downtubes for the Duramax. It is my understanding due to the tight fit at the firewall there is no space for anything larger than the factory oval tube? Are there any aftermarket exhaust companies offering a replacement? I already have 4" but seems to be choked at the fitting between the downtube and the exhaust. Can one be custom fabed? I heard you can get 1-2lbs of Boost by increasing the diameter of the tube?

I have a Banks Exhaust and I have the same problem you do, Ive been looking at downpipes for a while now. I think Magnaflow carries one, and Borla also say they carry one. You could try theirs. hope that helps

becareful....i think theres one out for a zf tranny but not the allison

Thats alot of work for 1-2 lbs.. not worth it, take it from a guy thats been there

Try this

http://http://www.dieselplace.com/forum/showthread.php?t=24970

Hey dmaxalliTech (member.php?u=127),
So are you saying the gains in doing this are not worth it? I heard from others in the world of turbos, a 1 to 2 lb boost gain is good for about 30-50hp based on the size of the turbo on the dmax?? I have a local shop who has offered to build one for me for free as an expeiriment...sort of a sponsorship spread the word about them kind of deal. This is really what drove me to ask for information on doing this. So I take it you have experience in doing this? What are the harmful effects to the turbo if I do this?

Hey dmaxalliTech (http://member.php/?u=127),
So are you saying the gains in doing this are not worth it? I heard from others in the world of turbos, a 1 to 2 lb boost gain is good for about 30-50hp based on the size of the turbo on the dmax?? I have a local shop who has offered to build one for me for free as an expeiriment...sort of a sponsorship spread the word about them kind of deal. This is really what drove me to ask for information on doing this. So I take it you have experience in doing this? What are the harmful effects to the turbo if I do this?If your DMX makes 242RWHP @ 19lbs boost (Factory) The assumption made by most people is that for every pound of boost pressure you make, you get 12.7368 HP
The above statement is correct for a stock truck under load at max stock fuelling.

If you do not change your fuelling, and plug your wastegate (not recommended) you might net 5 additional HP.

Our stock turbo runs out of air to combust the available fuel somewhere in the 475-530HP range. That being said, on an observed boost level of 39ish pounds at a recorded 550.5 RWHP level the figure is as follows: 550.5/39= 14.1153 HP per LB of boost.

Pressure is not the be all and end all in the turbocharger world. Pressure Ratios, Adiabatic efficiency(Air Temp Delta T/VS Pressure@EFF Mapping), Map Width, and drive pressure are all carefully balanced to give the turbo it's response charecteristics, Flow potentials /inducer /exducer and RPM capabilities.

I have NUKED, BLOWN, Destroyed, Shattered, Turbos from ignoring these parameters. The 550RWHP run cost me a turbocharger.

FWIW, Even at 450 RWHP, the turbo is running out of capacity to sustain airflow at a reasonable EGT level. A Banks Wastegate helps, but the root problem then becomes the 'cork in the bottle' of the exducer side of the turbo and the resulting drive pressure. Plus the design of the inducer wheel while good from the factory, is something that runs out of breath when needing more air.

Is a Downpipe worth it? For a guy wanting to stay at or below 450 RWHP, sure it is. But if you intend on going further with HP, you would be better off saving the $$$$ and Going with a bigger charger (less drive pressure) and a bigger downpipe in the kit.
BD-Power, ATS, Are both releasing their Single Charger Setups. I would be surprised if Someone in Cali wasn't working on one as well.
DTT is building Twin Turbos for the DMX as we speak.

I'll get off the soapbox now.... before i write a novel!):h

Map out where you want to be power wise, and ask yourself if you tow, dragrace or sledpull... then decide on what is best for you

Great information, thanks for sharing your knowlege. I do plan on keeping it under 450hp...of course I say that now but if I had a money tree...sky's the limit. Anyway, this is my daily drive and I also pull a 50 ft trailer to haul toys and cars so I need to keep it reliable. So with a 4" down tube I would net close to 30hp? Sounds like a good and inexpensive performance add on to me, why are they so hard to find then? You guy's need a ZF-6 test subject let me know! You should try to come down to Rocky Mountain Truck Fest at Bandimere Speedway http://www.bandimere.com/events/event_detail.php?eventID=246