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I'm fairly certain the rectangular section is a resonator. When I swapped in aftermarket catless headers, you could really smell the unburned fuel. The car smelled like it was catless. Since then I've added tiny Magnaflow cats (too small to fool the ECU), and the fumes are reduced by about 80-90%.
If you ever want to pass OBD2 readiness, the rectangular section is about the only place you can add cats large enough to pass emissions without sacrificing ground clearance.
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