OE cats typically have 400-600 cells per square inch, plus as prev. mentioned, aftermarket cats do not have nearly as much washcoat (precious metals) applied to the bricks, thus the much lower price vs. OE. Also, much shorter lifespan & durability than OE. Also, the only aftermarket cats I'm aware of that are 50-state legal are Magnaflow - they ponied-up the $$ to gain CARB approval.

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