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Discussion Starter #1
I understand that the manifolds themselves are very restrictive. My observation is that the y-pipe is also extreamly restrictive, especially on the 06 models with the T-intersection and the forward pipes are flattened.

has anyone tried replacing the y-pipe with something more free flowing?
my idea is to replace the y-pipe starting at the manifold flanges and run to the stock cat (sliced off at the end) or a high flow cat so a decent sized pipe can be fitted and run back.
I didn't see one but does anyone know if there's an o2 sensor after the last cat?
 

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The manifolds include the pre-cats and many of the bends and restrictions in the system. From the diagrams I have seen, I don't recall an O2 sensor after the main cat. If you could get some custom mandrel pipes leading from the manifolds into a dual inlet main cat with a 2.5" or better single exit pipe (or maybe duals) all the way back to the mufflers, I think it would make a big improvement. maybe something like this...


High-flow dual inlet cat rated for 400HP on a 2.5L Duratec
 

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Discussion Starter #3
that was the plan. I got under the car earlier this week with my exhaust guy. the stock setup is horrible.
unfortunately I can only install a cat back type exhaust if I want to remain in the stock or STS class for auto-x. guess my car will have to remain quiet after all.
 
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