| Hello! As zfan21 wrote, high-flow cats are used if you need to pass emission testing. I got myself 3” to 2.5” with high-flow cats and passed emission testing pretty easily. At 2000 rpm emissions were very good but at idle I had little high hydrocarbon values (close to 100 ppm). You just need to heat high-flow cats a bit more than stock cats to pass emissions testing.
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