I forgot to say, you also need to weld the lambda bosses into the secondaries on the downpipe, you need to get them at the right angle to the gasflow so they'll read properly.
If you wanted to avoid the cutting/welding of the two downpipes you'd need a CF2 centre cat and the small silencer that comes after it, this would then bolt on to the rest of your exhaust system with the CF2 manifold & downpipe. But you'd need to bung the lambda boss on the back half of the CF2 downpipe, weld another lambda boss behind the CF2 centre cat and extend your 3rd lambda sensor wiring to reach it otherwise you would get cat efficiency errors. You would still also need to weld lambda bosses into the CF2 downpipe for your two CF3 primary lambda sensors.
Using the CF2 centre cat & small silencer would make the car quite a bit quieter than using the standard CF3 centre cat.
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