The stock exhaust on a 99-00 civic si is 2 inches and I'm going to be making a good 50hp more minimum. My type r is all "oem" inside but it does have a bit more compression than a "normal" b16b. It will still scavenge like it should but the dyno will need to decide that, hopefully the tuner can salvage some low end and mid range I'll lose with the big exhaust and bitching header. Test pipe is in, header goes in soon. Pics of the old cat with the new hardware.