Hey Rhett,
HFC or (High Flowing Catalytic-Converter) is basically the same device that's already built into your cars exhaust system that converts the exhaust gasses.
An HFC is just a smaller cell of them that flows free-er letting you regain some performance lost. It still suffices as a cat for those states that require them and do emissions testing...
A Test Pipe or Resonated Test Pipe has NO cat of any sort in it. Resonated ones carry a resonator, but just to drown sound and rasp, not any sort of chemical conversion. So what comes out of your engine, comes out of your exhaust. (Gas wise)
HFC's will give you better numbers than Stock cats, and (Resonated) Test Pipes will give you better numbers than HFC's due to the freedom of gasses to flow.
Hopefully this broke it down for you enough to make a decision based on your particular situation. If you already knew most of that, I was not trying to make it sound as if you were not knowledgeable, just pointing out all the details.
Chris in Avondale
