Stock exhaust VS 4"

[DBT]

Can any body tell if a 4" exhaust will really make a difference over the stock 3" ??

[madmatt]

I don't think size matters nearly as much as the restriction amount of the muffler, cat, or the degree or number of bends in the system. I've found little to no backpressure in the stock exhaust past the muffler. remove it and it should be just as good if not better then a aftermarket 4" cat back system w/ muffler.

[DmaxMaverick]

Welcome to the forums DBT!

Your OEM exhaust, if we are talking about a Duramax, is 3 1/2", downpipe back.

It all depends on your reason for needing upgraded exhaust. If you never intend to install mod's, the OEM is more than adequate. Larger exhaust will help to keep EGT's in check if you start pouring in more fuel, like with a box or programmer. It also depends on the engine you have, LLY or LB7. Some of the LLY engines have had DTC/SES issues with larger exhaust systems, or systems that have had the cat or muffler removed.

[Kennedy]

What is critical is the back psi near the turbo. Dyno results in my gallery here:

http://www.kennedydiesel.com/photogal/pages/dynoplots.htm

[rjschoolcraft]

Kind of takes the speculation right out of it, doesn't it?

[silverback]

I have a 4" exhaust, a programmer and the (2004) LB7. And I am astonished at the differences between the two engines in JK's plots. The torque curve is flat over the powerband in the LB7. I'm glad I bought when I did (of course if I'd had a clue the LLY was coming down the road I might have waited...).
-Ken

[Snowbear]

Ive been thinking of going to 4" mbrp 409 single sometime prolly later this summer if it happens. I got no boxes/programmers, and not planning on any anytime in future. however i do like the benefit of lower egt temps when i pull...or even just driving around for that matter...Other reason was for sound. Im not lookin to break the sound barrier, just want a better tune...maybe i should just go w/muffler...hmmm

[Kennedy]

Originally posted by Snowbear:
Ive been thinking of going to 4" mbrp 409 single sometime prolly later this summer if it happens. I got no boxes/programmers, and not planning on any anytime in future. however i do like the benefit of lower egt temps when i pull...or even just driving around for that matter...Other reason was for sound. Im not lookin to break the sound barrier, just want a better tune...maybe i should just go w/muffler...hmmm Your current system is aluminized. If you want to see what a 409 system looks like after a NY winter, look at a 2004 system in front of the muffler...