for your 02 I would get a set of gauges first...

 

When I cut out the factory exhaust I was quite suprised to see how narrow it got at the bend going up over the rear axle. Since I work with pipe a lot in my trade I am pretty sure with the bend in it it was under 3" by a bit so I rounded down!

 

A good 4" system is way easier/cheaper to buy complete TBE instead of trying to fix the factory setup. A nice straight 4" is better for flow no doubt and is probably good for a 50°F drop in cruising EGT's with less than 100hp goodies added. The main restriction is the turbo, especially if you have the HY35. The turbo outlet is tiny so swelling to 4" right there has some benefit, but not like getting a turbo with a 4" outlet like any of the aftermarket S300 variants.

 

If you need to replace the exhaust system then I'd go to a 4 inch. Otherwise just drop the muffler and add a piece of straight pipe. You will get some drone around 2000 rpm. 4.10 gears that's about 60 mph. That is also the optimum speed for towing so just get used to it as we say..

My 99 I just dropped the muffler and the resonator and added straight pipe. There is nothing noisier than a 24 valve with straight pipe. Even stock. I have a Smarty that I use for towing a 12k GN hotrod trailer fully loaded. I set it on SW 3 for towing and it works just great. It really barks when going up a grade or going thru the gears with the trailer.

As for power added due to losing the resonator and muffler.....0 no mileage gain either. I carefully documented this years ago. The ULS fuel cost a couple mpg that I was able to get back with the Smarty.

 

I didn't care to hijack the thread but I might as well.....

I learned that the reason a straight piped Cummins acquires the dreaded drone at around 2000 RPM is simply because the exhaust vibrates like a guitar string which basically rings a tone.....the drone. So by changing the thickness or the length of the "string" (the exhaust), you can alter the pitch (the drone).
What I did was add a piece of metal to the exhaust pipe. I hose clamp strapped a 5 foot T post to the same spot the muffler and resonator were located. All done cheap and simple for testing purposes but it worked so good that I didn't change a thing. I still have a mild drone which is because the exhaust is so darn loud but its nothing like it was before. I may even be able to improve on it more but I was so happy with how it worked that I didn't care to mess. Food for thought.

 

Now that's pretty cool. I think I have a chunk of 1/4 x 2" angle at the shop. sounds like a Sat project.

 

Yankee engine-nuity at its finest!!!

 

Yea, yea..... But how many things have you done to your truck that didnt cost more than three hose clamps and a T post laying behind the barn?

 

The previous owner of my truck put on the 4" turbo back exhaust with the resonator removed. It also had a K&N FIPK filter set up with the cone filter. I Changed out the K&N filter for a BHAF, using the K&N intake tube, and it actually made my exhaust temps a lot worse. I put the K&N filter back on, and put an adjustable boost elbow on. The boost pressure maxes out at 34 psi, and that drastically improved the exhaust temps. I guess more boost equals more air, and better temps. I can tow my 38' GN race trailer fully loaded at 19K lbs and rarely get the temps over 1200* I just wish there was a gear between 3rd and 4th, haha.

 

Its amazing how many dont understand this. Boost isn't there just to make power, its there to push out all the hot air from the previous combustion too.