My muffler shop installed the dual 3" in, single 4" out magnaflow part # 12594 on my HD, and utilized the stock 3.5" tailpipe. It sounded fine at idle, but when I took it for a drive, the resonance between 1500 and 2000 rpms was quite bad. This is the same muffler that the engineer at Magnaflow told me would have no resonance on this HD 6.0 I immediately called him up, explained to him that the decibels were not a problem, the exhaust was plenty quiet at idle, and even at full throttle, but just cruising around town between 1500 and 2000 rpms, was enough to drive a person crazy pretty quick. He said I could either try putting another 4" inline muffler/resonator part# 12770 right behind the main muffler and see if that took care of the problem, or put it back to stock and get a refund for the muffler, but still have to pay the labor charges. My concern was though, that if installing the second muffler/resonator didn't take care of the problem, I'd be stuck with paying for that resonator, and all the hours of labor the shop had spent working on this, and then have to put it back to stock if this didn't work. They couldn't guarantee me that it would work, so I basically decided to take the chance, and had the shop move the main muffler further forward, and installed the secondary muffler behind it. $385 dollars worth of mufflers, parts, and labor later, it does sound pretty good now, and a majority of the resonance is gone. Enough that I can live with it at least. I was more than a little unhappy with the engineer at magnaflow, that they are releasing this muffler as part of their catback system for these trucks, without a little more thorough testing. According to him, they're going to go back to the drawing board, and see about extending the muffler out to 30" instead of 22", and see if that helps the resonance problem. I wouldn't be suprised if the system ends up being sold like it is now, and there will be many unhappy customers. I would not recommend going the route I went, even though the end product isn't so bad now. I would recommend waiting till they actually come out with a muffler that really does work, and let them do the testing, instead of making the customers be the guinnea pigs, like I feel I was today. On a different note, I feel that the 8.1's may do just fine with just the magnaflow #12594 dual 3"in, and single 4" out, as they more than likely send out different exhaust frequencies than the 6.0's do. I'll most likely be going to the dragstrip this saturday again, and see how much the exhaust system really did help, if any.

Jason

 

I'm very happy with my Magnaflow mufflers. I listed the details of what I did in the other Magnaflow thread. If you go to www.chevy4x4trucks.com/CMNTMXR81/ and listen to a few of my MPG's you can hear what it sounds like on the 8.1. I don't have any resonance problems either.

The only thing I'm going to do in the future is physically split them (do away with the factory 2:1 3.5" tailpipe) and do two 3" tips out each side.

 

Thank you for the info Jason. Sorry that the muffler didn't work out for you as expected.

CMNTMXR57, this was my favorite video. <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />
http://www.chevy4x4trucks.com/CMNTMXR81/MVC-018V.MPG

 

I plead the 5th! <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />