6.2 exhaust: 3.5" vs 3"

Roll Bama Roll · November 26, 2014

On a stock 6.2 motor with a Magnaflow muffler already installed, what are your thoughts on downsizing the last section of pipe, from the back of the muffler to exit, from stock 3.5" to 3" for normal daily driving?

SouthernSilveradoGuy85 · November 26, 2014

On a small block V8 you don't really need anything bigger than a 3" pipe. Even 2 1/2" would be fine especially if you run true duals.

ATMINF · November 26, 2014

With today's LS series small blocks their ability to flow large amounts of air and thus making great power is a change from the GEN 1&2 small blocks of old. With that being said, the factory had dual 3 in pipes then flowing into a muffler which then exited with a single 3.5in pipe for a reason, and it was because the engineers believed it was required for the setup. This is in regard to my truck which is a 2006, 2500HD with the 6.0l gasser motor.

SouthernSilveradoGuy85 · November 26, 2014

With today's LS series small blocks their ability to flow large amounts of air and thus making great power is a change from the GEN 1&2 small blocks of old. With that being said, the factory had dual 3 in pipes then flowing into a muffler which then exited with a single 3.5in pipe for a reason, and it was because the engineers believed it was required for the setup. This is in regard to my truck which is a 2006, 2500HD with the 6.0l gasser motor.

My '14 just had a single 2 1/2" pipe the whole way (stock) and I doubt a 6.2 flows that much more air. My old 98 the stock setup was true duals but only 2" pipe which seemed way too small, even for duals. And I'm pretty sure my current 5.3 probably flows more than that old 5.7. Of course trucks need performance, especially on the low end, but just because it comes from the factory a certain way doesn't mean it's the best. Another example is the air intake system, always seems to be constrictive fuel economy. Now something like the Corvette, Camaro SS, Z28 or ZL1 and the Cadi CTS-V ya they will make those systems as good as they can for performance. I could be wrong, but ya it's good to open the exhaust up but you still need some back pressure for the low end. So you can have too much exhaust. I could be wrong on that part but it's what I've understood.

Roll Bama Roll · November 26, 2014

The reason I ask is I lost some low end after a magnaflow swap which sometimes happens. Although the magnaflow reduced restriction, I think it also allowed the exhaust to cool a little sooner, causing it to slow down a bit and possibly develop some turbulence before exiting. I began wondering if, instead of changing mufflers to get my low end back for daily driving, if increasing exhaust velocity after the magnaflow would make a difference. I started thinking that maybe the 3.5" stock pipe, which could handle 460-470 hp, wasn't flowing well when pushing anywhere from say 200 to 375 hp in normal daily driving.

I figured the easiest and cheapest way to test the increased velocity theory was to swap out the back pipe section with one from a smaller Vortec motor. It would have the same bends and hangers but just narrower in diameter. If the swap made a difference on low end I'd know I'd be on to something. If it hurt top then I could always research and get a custom section made that somehow increased velocity without hurting top end. My exhaust guy had a 2.75" and I think a 3" section in his shop. I had him install the 2.75" section to test.

This was were my best acceleration was before the change:

And after the change:

The mod made a huge difference in daily driving. I've done similar mods on V-twin motorcycles to shift the torque curve but not on a truck. To say it woke the truck up is a little understating it. A few objective results I noticed were take off power, a lot less downshifting on hills, better city and hwy mpg, and the exhaust is quieter despite losing the stock resonator. For top end, when manual shifting and flooring it, it seems the power starts to drop off about 5300 rpm. For me that's no biggie cause I never go over that in normal daily driving. I'm a pretty laid back driver. I've never tried to spin the wheels cause tires cost big money. After the mod I noticed the tires giving a little here and ther on normal take off until I adjusted my driving. When the first rain came after the mod, it surprised me when the first light turned green, I eased on the gas like normal and the tires spun for a few seconds.

I am in no way saying this is a mod for anyone else to try. Heck, it may cause restriction for anything other than a straight thru muffler. I'm just sharing how it worked on my truck and my magnaflow. Also, I think other truck motors, like then 6.0, 5.3 and 4.8 have stock exhaust diameters closer to optimum for daily driving. On my previous Yukon 5.3 a Magnaflow had much less impact on low end. I honestly think the 6.2 would be better balanced with a stock 3.25" pipe but that would have been too expensive since it's not an industry standard.

I've been running it for about 6 weeks and maybe 1200 miles. I ran it for a week, reset the ECU just to see if it would make any difference (didn't seem to have) and here, another 5 weeks later, the transmission's adaptive learning hasn't made any changes that diminished the performamce change. It still runs like the day I installed it. The question that is bugging me a little, and the reason for my initial post, is not knowing how a 3" pipe would perform and if it was even worth trying out. I can tell myself the 2.75" is better but, like many of us, I love to know for sure. But I'd hate to go through the trouble of a 3" pipe only to discover I liked the 2.75" better.