I have been trying to decide which setup will be better so I don't lose low-end torque. I have a set of LT's waiting to be installed, but I cannot decide if I would be better off with reducing them at the collector to 2.5" and keeping it that size until it reaches the y-pipe, or if I should go with the 3" off-road y-pipe assembly. The exhaust from the y-pipe will be a single 3" through a Spintech muffler to the rear bumper. I will be keeping the cats, but I just cannot find any dyno results from either setup.

My truck will never be at the track and it's a daily driver that occasionally tows an enclosed trailer for work. Most of my "fun" driving is around town, so faster launches will be far more desirable for me than "WOT down-the-highway fun".

1) Has anybody kept the pre-y exhaust pipe at 2.5" and kept their cats with their LT's? If so, how did it change the power throughout the rpm range and how satisfied are you with the power changes overall?

2) For those who have gone with the 3" off-road y-pipe and kept their cats, was there a loss in low-end torque by keeping everything at 3" and how satisfied are you with the power changes overall?

I only want to spend the money to do this once, so I would like any / all input I can get.
Hey Hammer, I believe you kept the OEM cats and used 3" for everything. What's your opinion on the low-end torque with that setup?

 

It really depends on how you plan on modifying your engine. 2.5 is ideal if you're staying under 400BHP, if you think you'd like to build a stout motor and hit 500+ in the future then go with 3".

 

I don't have any plans for making my truck that much fun due to both a lack of funds and patience from my wife.
Thanks for the input. I'm leaning toward the 2.5" because I believe from what I've read in the past, my engine won't be producing enough to need or even benefit from 3" before the y-pipe.

 

Honestly, with 4.56 gear should you have ANY worries about low end torque?

 

Do a 3" before the Y and a single 3" pipe, could get away with a single 4" from the y back honestly IMO. Heck even older GM 454 trucks with less power get 2.75" pipe before the muffler, the. 3" from there back.


With the gears you have I can't see torque being any issue.

 

The 2.5" is fine IMO unless you're planning a cam that's ground for low end torque. You could get away with 3" because of the 4.56 gears, but I think you'll be happier with the low end with 2.5"...

 

Thank you guys for the input. Hammer, I think you're right. I've been looking into this a bit more and believe I'll be happiest with 2.5" before the y-pipe. It will already be slightly larger than the 2.25" on there now, and by replacing the factory y-pipe with a better one will open things up a bit as well. The LT's will be allowing the exhaust to flow better too, so since I have no plans for a better cam or forced induction, I'll stick with the 2.5" pipe. I've seen a few charts showing optimal sizing of exhaust based on engine HP and actually I could go to 2.25" for the amount of power the HEMI is putting out, but 2.5" is what I'm most comfortable with.