2006 Hummer H3 Original Owner. Seeking advise on Exhaust Manifold replacement issue

[The True Aquaman]

Hello everyone,
Been in possession of my 2006 Hummer H3 since 2007 and she currently has just over 200,000 miles on her. Ive done a lot of upgrades to her over the years but in the last few months she has had a string of issues. I believe about 15 years ago, I upgraded the exhaust manifold to a Magnaflow system and recently it broke, not cracked. One minute it was normal and then the next it sounded like I was driving a Mack Dump Truck. Up to that point I had fairly good acceleration and she ran at about 19-21 MPG with the help of a K&N air intake, computer tuning and a few other modifications. I had the manifold replaced with what the shop could find and now she runs like i have a governor on the engine. Would a new exhaust manifold and catalytic converter do that? Any input would be greatly appreciated.

Rick ~Aquaman~

 

[alrock]

I think the question is - what did the shop "find" and install? Did your old headers not include a cat? If you now have a cast iron manifold vs. headers, and the computer turning perhaps accounted for the manifold, I could see a change in performance happening.

 

[crwalkerasla]

When you said that you upgraded the exhaust, was it the manifold or CAT back system or both. And you said it lasted 15-years?

Mine had a Magnaflow CAT Back on it when I bought it and I have just added the Magnaflow exhaust manifold. It does run quieter and better than before... (and no more 2K drone).

My thoughts are, if it broke, and possibly dropped, what else did it take with it that may not have been put back correctly. Vacuum leaks would do that, but not sure about on this engine.