I tried to do a search but didnt come up with a good answer.

What is a good size pipe to run for a 6 liter LQ9 with 11:1 compression. Im gonna run the Dynomax Ultraflow welded mufflers. Also, Im looking to keep it a little on the quiet side so I was thinking about using the 20 inch mufflers. Any ideas would be appreciated. These will also be true duals. Also, should I put in an X pipe?

 

IMO if you plan to run the pipes all the way out back or at least over the axle I would do 3.0 off the headers, into a X or H pipe the into a set of 3.0 in/out mufflers, then 2.5 pipe after the mufflers... By the time the exhaust gets to the back of the muffler is is cooler and does not need the volume like it does right out of the headers, and the 2.5 will make the exhaust tone/cam lope more defined but not as loud/deep....... Just my .02....

 

6.0 you cant go wrong with 3inch duals with x you got the cubes then you never have to change it for future i gained 40lbs of tq over my old 2.5 inch cat back

 

I've read so much on this debate. I still don't know if 2.5" will retain low end over a 3" dual system. and even if it does will it be noticeable and will it choke the motor too much up top