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When I get to work I want to check the inside angle of that bend. Anyone know the cell count on stock cats vs. Milltek?

based on the email addy (.se)

With the stock 2nd oxygen sensor in front of the main cat, it appears that the main cat is not involved whatsoever in ECU feedback regulation, only the pre-cats. Compare this to the Milltek where you have only one cat, and a pre and post cat sensor as one would expect.

The second O2 sensor needs to read post-cat levels or the CEL will light up.

This would seem to imply that in the stock configuration, only the first cat is involved in the ECU feedback mechanism, and that one could dump the second cat entirely from the stock exhaust without triggering a CEL?

But your comment about dumping the second cat without triggering the CEL is correct. Having said that, why would you want to retain the front cat that is so close to the turbine outlet? Better to get rid of the first cat and move the second O2 sensor behind the second cat.

It would not be easy to move an O2 sensor in a stock pipe, since that would require reinforcement, drilling and tapping of a new hole.

He's talking about somehow removing the 2nd cat to increase flow. However, that would be lots of fun too. Lots of cutting and welding.

down - It expects the second sensor to have a pretty much steady signal.

If the second sensor starts to track the first sensor then it triggers a CEL.

Most tuners will turn off this comparison switch in the software when they sell downpipes.