[69gmcc10]

According to the exhaust program I have, if your engine is a stock LQ4 and you have Doug Thorleys on it your exhaust diameter should be 2.75" to 3", you should have an H in the system 19.4" behind the end of the 2nd primary pipes and a muffler 38.7" back form the 2nd primary into chambered mufflers approximately 370 cubic inches in dimension. After the muffler it doesn't really matter what you do as long as you keep the diameter at least the same size as goes in it. I would personally go with 2.75" pipes because you will reach volumetric efficiency sooner and start creating power lower in the rpm range.