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Discussion Starter #1
I looked at the exhaust system for the regular normally aspirated 2.3 in the Mazda6 and it seems to have either the same or very similar cat setup to my Speed6, where there is a primary cat right at the manifold, followed by O2 sensor, then secondary cat.

I'm going to be getting a custom 3" exhaust done up for my Speed6, and was originally thinking of doing it from the cat(s) back. After looking at the layout again, the space the second cat is occupying could be better used for a good, straight thru muffler. The Aeroturbine I'm planning to use is 6" in diameter and 12" long (just the body). This frees up the space at the back by the spare tire to better route the piping and put in a small resonator. I was planning on the reverse at first (muffler at back, resonator towards front just after 2nd cat) but it looks a bit awkward to get the Aeroturbine in that rear spot.

Since the 2nd cat is behind the O2 sensor, there shouldn't be any problems with CEL. My questions are, if anybody has done this:
1. Did it serioulsy affect the emissions (ie. any risk of failing AirCare without 2nd cat)?
2. Any difference in performance (ie. is the 2nd cat quite restrictive, and removing it takes out one of the bottlenecks)?
3. What was the sound level difference (if you removed only the second cat and put in a bypass pipe, but didn't change anything else)?

I'm trying to determine if there is enough performance benefit vs risk of failing AirCare to warrant taking out the 2nd cat.
 

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Nobody has done this yet? I guess the next question will be negative too, but I might as well ask. Anybody with the 2.3L engine replace their complete exhaust system and have the 2nd cat lying around and wants to sell it? I don't care if the cat itself works or not, I just want the front flange from it (the one that has a semi ball joint to provide some flex).
 
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