What are the advantages to having a 3.5" over a 4" downpipe?? Will I see more power/torque with the 3.5 or the 4"? and how if you have a 3.5" downpipe what would you need to bolt it up to your 4" exhaust? thanks

 

What are your planned mods? If you are staying anywhere near stock, there is no reason to go larger than 3.5". Your stock exhaust is 3.5" not 4", but in either case, who ever sells you your down pipe will also have the appropriate connector.

 

i have a stinger plus kit from banks, and my down pipe and tail pipe is 3 1/2", plenty of power.

i will tell you though it will be very difficult to fit a 4" down pipe between the cylinder head and firewall. i had a hard time w/ the 3 1/2"

 

It's not so bad. Just gotta get it twisted the right way.

 

getting it in there is not the issue, if you got an air hammer and a large bar to bend the pinch welds.
but not to get any noise or drone is the issue.

 

I have installed both a 3.5 and a 4" down pipe on my 99. I liked the 3.5" for everyday driving cause the turbo spools faster and has some more bottom end torque. I like my 4" downpipe for highway driving and towing. Drops the exhaust temp while towing with the 4" downpipe another 50-100 degrees. With the 4" exhaust the truck just cruises at 80 mph but with the same amount of push on the pedal the 3.5" downpipe only does 75mph. You can do 80 easily but needs for more push on the pedal to get it up there. So if your towing or allot of highway driving go 4" downpipe.

 

Hmm, I just put a banks power elbow and banks 4" down-pipe on and welded it to my Flowmaster American THunder 4" eaxhuast and I have to say...it was simple to install. I laid under the truck and had my buddy lay over the engine with the clamp on the power elbow. I pushed it up and held it in place while he worked the clamp over it. About 10-15 minuts and it was done and I was getting the welder dragged over.

Now putting a power-elbow on is a real treat! Remove turbo, get screwed by the dealer for the turbo R&R kit for $70 bucks so that you can change the O-rings, put turbo assembly on the bench and bust all your 5/16" sockets and 2 ratchets trying to get the factory outlet valve off....end up welding sockets to the now stripped 12 point bolts and finally get it apart. Go find new bolts that will fit, had to goto a specialty store...get the elbow on and put the turbo back on the exhaust Y and clamp it.....clamping this joint is a nightmare, took an hour....then realize the turbo is 1" up and .5" back from where the dowel pins are. Get 2 ratchet straps and a pry bar, ratchet strap from the turbo to a garage post that is at the right fender, another to a truck in front of this one, and use pry bar to do the fine movements while your 240 lb friend lays on the turbo to provide down pressure. Finally get the turbo on the dowels and your over the hump! Nightmare, I'll have cold-sweat dreams over that evening for a long time!

That being said...I would think you would get a faster spool up from a larger exhaust. A turbo works on air flow and diferential pressure. A larger pipe makes for a lower exit pressure, ...inlet pressures are uneffected. So a lower outlet pressure caused by a larger cross sectional outlet pipe and exhaust system compared to the same inlet pressure should lower EGT's and increase turbo efficiancy and shorten reaction or spool-up time.

 

Apologies....just realized you have the OBS truck. The 4 incher fits the late model trucks without issue.

 

now you see what i meant!! thanks for reading further
what does OBS mean anyways?

 

Old Body Style

 

old body style??? dah but actually you have one too. since the '05 SD is the new one right????

 

haha...I guess you're right....though it's difficult to differentiate the two from the rear or side. They all look like Superduties. I suppose we should add MBS for middle body style and NBS for the new.

 

lol, yah but at least we have the best looking trucks around...
ODS or MBS or NBS!!!! later

 

I won't even talk about the 05 GMC Sierra my buddy drove to work in today

 

ugly,way too much cash, and wait till the allison gives up..have heard stries about them trans going bad w/ little miles on them.
hey they are all junk when broke.lol