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Did several searches and could not find the answer if both the headers and cats can be swapped going from 3.6l to a 3.4l. For thread completeness, there several threads noting that you can put 3.6 mufflers on a 3.4, connecting to 3.4l cats. I use bypass pipes so mufflers are not part of my equation.


My goal is to buy a used second set of catalytic converters to be modified with new 200 cell guts, but keep the stock parts just in case I want to revert back to fully stock in the future or have trouble at visual inspection. There seem to be more 2002+ exhaust parts available and came across a 2003 set but I have a '99.


1. Do 3.6l cats bolt onto 3.4 headers without any mods required (no need for 3.6l headers)?


2. If I replace both the headers and cats from a 3.6l onto my 3.4l, will they bolt right on and all brackets fit without any mods?


3. If I have to do a swap back to the original 3.4l cats with 3.6l headers on, would they fit on the headers, or would I have to put the original 3.4l headers back on to get them to fit?



For reference, I pulled part numbers for both cats and headers:



Catalytic Converters:


3.4l  [M96.01/.02/.04] ('98-'99 / -'99  ->  '00  ->  '01-)


3.4l LEFT

996-113-021-53  /    996-113-021-56   ->   996-113-031-01  ->   996-113-031-02


3.4l RIGHT

996-113-022-53   /   996-113-022-56   ->   996-113-032-01  ->   996-113-032-02


3.6l  [M96.03] (2002-2005)


3.6l LEFT



3.6l RIGHT




Exhaust Manifold/Headers:


  3.4l  [M96.01/.02/.04] ('98-'01)


       3.4l LEFT (1998-2001)       3.4l RIGHT (1998-2001)

       996-111-101-52                            996-111-102-52



  3.6l  [M96.03] (2002-2005)

       3.6l LEFT                            3.6l RIGHT

       996-111-101-53                  996-111-102-53



     3.6l LEFT                                3.6l RIGHT

     996-111-101-40                     996-111-102-40




edit: Simplified part # section and added color

Edited by xmac
simplified part numbers
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While they will likely bolt up fine - I have no idea if it will pass smog tests as the later style cats got different O2 sensors.

If you decide to try it let us know if you can pass CA smog.


Wow, O2 sensors - I hadn't even thought of that angle for possible issues. Thanks for passing that along Loren.  Are the bung sizes are the same?  I may roll the dice on the set of cats considering the price.

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