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i want to purchase some doug thorley headers and i was wondering are these direct bolt on to your cat back exhaust? and how hard is the install.
 

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The DT long tubes replace your oem exhaust manifolds (and the OEM primary cats integrated with the manifolds) and will bolt up to your OEM B-pipes. The aftermarket cat back exhausts bolt on to the aft ends of your B-pipes.

Since they eliminate the primary cats, you may need to install bigger cats in your B-pipes or buy B-pipes w/ cats (e.g. Berk) in order to pass emissions and run w/o codes. You will likely need Uprev to correct AFR ratio.

The install is a major PITA, but a little less so on a 2wd cuz you don't have to work around the front diff.

OEM Exhaust Diagrams: (The downstream end of the DTs bolts up to the flange near the designation 20691+A The cat-back exhausts bolt up to the flange near 20692M. The enlarged sections of pipe at 20010 and 20020 are the secondary cats)

200 Exhaust Tube & Muffler :: Exhaust & Cooling :: Genuine Nissan Parts :: Titan Parts (A60) 2004-2011 :: Nissan Parts, NISMO and Nissan Accessories - Courtesyparts.com

Note: I think on 2004 2wd you have resonators instead of secondary cats. Thus you would have no cats upon replacing the exhaust manifolds with DTs. So to pass sniffer emissions, I think you would need to replace the resos with MF 94035 cats and put the rear sensors in the bungs on the aft ends of the cats. This may require extension of one of the 02 sensor wire harnesses. I think the Berk pipes will bolt up and allow plug in of the sensors on a 2wd, but they are currently revising them to fit a 4x4 so that the sensor clears the transfer case.

DT has an available stainless version of their header if you call them.
 
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