What I had heard was that the high flow cat thing came up back in the early days of catalytic converters. The early ones were made of a bunch of ceramic beads. Then the newer design was the honeycomb version that flowed better since it had straight through passages. So today most cats are the honeycomb design, so are all high flow versions. I suppose there are some differences in the really high flow cats, like larger passages, larger cross sectional area of the matrix, larger bore inlets/outlets, etc. Probably makes a little difference.

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