New exhaust for my 8.1L

ShortCrew · July 12, 2003

I've been searching for several weeks for an exhaust system for my 2003 2500HD 8.1L. I could not find one to suit exactly what I wanted. Some were way too expensive like the Corsa and Borla. Others were too loud like the Flowmaster. Others used smaller pipes than stock. There were not nearly as many choices for the big block as there are for the small block, sooooooo.....

I decided to have my muffler guy fab me up something custom. We started with a Magnaflow (p/n 12594) straight through muffler. Dual 3" in, single 4" out. We were able to cut the dual factory 3" pipes right in front of the stock muffler and slide the Magnaflow right on. We then used a 4" to 3.5" reducer after the muffler and a 3.5" mandrel u-bend to connect to the factory tail pipe.

I'm going to look around for a factory Duramax tail pipe (4") and install that later. I don't think I'll gain anything, but it looks .

Here are a couple of picture:

We used all factory hanger locations. The sound is REALLY DEEP with no droning at all. Overall it is very quiet and when crusing you can barely hear it if the windows are up. I also noticed a nice improvement in throttle response. Combining this with the UPD (NSV) intake I installed last week and this truck hauls ass.

The best part was the whole thing only cost me $265 for everything.

greyghost03 · July 12, 2003

Looks good!

Glad you're happy w/the results!

BTW- the UPD (NSV) works well w/my Flowmaster 2!

R Dogg 56 · July 12, 2003

....well, you get the idea

96Z71asskicker · July 13, 2003

grndually · August 19, 2003

Is it ok to run the muffler backwards or is that model good for both directions?

Thanks, Andy

chevy38fan · August 19, 2003

i would never run a muffler backwards.. ROFL... mufflers are designed to flow in one direction. you'd probably run into some exhaust leaks if you did or tried that.

that muffler was designed to flow with two inlets and one outlet, its not put on "backwards"

ShortCrew · August 20, 2003

I've had Magnaflows on several of my past vehicles and I've never seen one that was directional. Flomaster and some other companies make directional flow mufflers and I would certainly not put those on backwards.

Tjwong · August 27, 2003

Magnaflows are all uni-directional. I had the same setup on my 02 8.1L Suburban, but the interior resonance was extremely annoying. The passengers in the rear compartment couldn't talk to one another at certain speeds and from stop light to stop light it was real obnoxious.

I ended up removing the muffler and replacing it was a high flow Banks SS muffler. It had dual 2.75" (same as stock tubing) inlets and a single 3.5" outlet. While not near as free flowing as the Magnaflow it far outflows the GM unit. And the noise level is now acceptable and it still has an authorative bark, especially at WOT and at full boost of the 8PSI Whipple.

I sold the muffler to my friends son that used it on his 1T 454 dually. In that truck it didn't have the annoying interior drone, most likely because the muffler was way out back and he had no full interior compartment as with my Suburban.

CMNTMXR81 · August 27, 2003

If you can see straight through it, it ain't directional.

Once you start getting into 3, 4+ chamber mufflers, those can be directional and I wouldn't put those in backwards.

I have Magnaflow's and like TJ's friend, I have resonance, but it isn't obtrusive because the mufflers are mostly under the bed. But it sounds awesome.

Although I may try something a tad different just for change.