What does a down pipe back 4 inch exhaust really add. Do I really see much improvement in power, exhaust temp, fuel economy? If so what are some real work numbers?

On a stock truck probably no gain at all, with a 100+ hp tune you may see a little gain. The setup that worked the best for me on my 03 was a 4" cat back system. You keep the stock 3 1/2" front pipe and a cat back system will bolt right up to it. The 03 already has no cat. My 03 spooled better with the stock front pipe than it did with a 4" front pipe, I dont understand why, it just did.

On an LB7 that had no cat from the factory the difference isn't as noticable as on cat equipped trucks. You will lower EGT's though and the truck should spool faster whan the factory mufflers gone, as it can be restrictive. HP wise there's not really much gain. You will make the truck sound a whole lot cooler though

i defiantly spools faster.

and my 5yr old thinks it's cool in town as it echos of the buildings,,,

Also will look much better. After driving a truck with a large pipe the stockers look like drinking straws. But just remember to buy just a cat back and save some money, the stock front pipe wont begin to hurt power until your in the built engine catagory JMO.

This summer I installed the BD exhaust manifold (Drivers Side) MBRP Downpipe, Gutted the Kitty, and a 4" exhaust with a Corsa SS muffler. I have much better throttle response then before. I can also hear the turbo spool up which I really didn't hear before. Since I did all the mods at once it's impossible which made the most difference, but my sons who use the truck occasionally noticed the boost in performance. They had thought I had finally broke down and bought the PPE programmer or EFI that's on my list....Another benefit that I noticed when towing to Havasu/Parker in 100 + temps I have very little fan activity.

Go dyno your truck and then do the changes and go back to dyno. You will see it on the printout and feel it in the drive. It just helps show you where the rpms have the power related to the tune. EFI is the ultimate I messed with the canned ones and should have just done EFI.

feb 2007 edition of dieselpower compared stock to 4" cat back to 4" turbo back exh. used afe. max power on 2003 duramax went from 257 hp to 270 hp at about 3200 rpm. max torque went from 425 to 452lb/ft at 3100 rpm. not bad unless you look at the middle of the graph where torque jumped from 280 to 391 lb/ft at 2568 rpm. hp went from 137 to 191 hp at same rpm. stock exh had catalytic converter on it. not much difference between cat back and turbo back exh throughout the dyno test. bearing in mind this is just one test but i know what my first mod on my 03 is going to be.:D

I have dyno tested before/after on my LB7, LLY, and LBZ systems. I have posted the results for the LB7 and LLY on my site. The LB7 and LLY gained the most. LBZ was less. All were tested with stock tuning. To be fair, the LB7 and LLY systems were 4-5" systems and the LBZ was 4". The LBZ gained about 1/2 what the others did. I attribute this primarily to the way that Bosch is controlling TQ.

What does a down pipe back 4 inch exhaust really add. Do I really see much improvement in power, exhaust temp, fuel economy? If so what are some real work numbers?

You will notice cooler EGTs and some additional power with a full system installed. 4" is all you really need, 5" is more for looks and sound.