HFC's. Stillen or Berk???

Phimosis

the stock cat doesn't use an extended O2 port. Having a long tube makes me think the O2 sensor would give an inaccurate reading because hardly any fresh exhaust gas is moving by the sensor. What is the logic behind using a long O2 sensor tube?

semtex

The same logic as using anti-foulers with test pipes (which close off the O2 sensor from the exhaust entirely, btw). The extended tubes are basically built-in anti-foulers. Also, keep in mind that there are O2 sensors in the headers, and that's where the functional data is being gathered from. The sensors in the cats are just there to make sure the gasses that have passed through the cats aren't 'too dirty', so when you go with test pipes especially (HFCs to a lesser extent), giving those secondary sensors an inaccurate reading is actually a good thing. In other words, you want to 'trick' those sensors into not sensing the dirtier exhaust gasses. Again, not so much a concern with HFCs as with test pipes, because with HFCs, they're still cats. But with test pipes, you're not filtering the exhaust at all, so you definitely want to shield the sensors. I ran test pipes in my 350 and within a couple of days it started throwing CELs. I'd reset it, and it'd just come back. So I finally broke down and installed anti-foulers to segregate the secondary O2 sensors from the exhaust completely.

One other thing I forgot to mention is that the Berks have built in ports for wideband O2 sensors, and the Stillens don't.

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