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· Premium Member
17,244 Posts
I think the cats close to the manifolds are intended to decrease emissions right after startup. Not sure, but I don't think it matters after you are warmed up, cuz the downstream cats get plenty hot. A little extra cold start emissions seems like a good trade for not getting cat grit back sucked into the engine.

"Many vehicles have a pre-catalyst located close to the engine's exhaust manifold. This unit heats up quickly due to its proximity to the engine, and reduces cold-engine emissions by burning off hydrocarbons from the extra-rich mixture used in a cold engine."

Catalytic converter - Wikipedia, the free encyclopedia
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