Which hfc's ?
 

Are there better hfc's than others? Do they really increase power? Will my check engine light go on? How are they different from test pipes?

Test pipes just that a pipe with no cat, HFC's which one are better there is a lot of info on the board but there are a lot of people who are happy with Berk HFC and I will be getting mine this weekend. There are a lot of other companies such as Fast Intentions (FI), Stillen, and maybe more that i have missed.

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does it make a difference if you install cbe before hfc or other way around? i'm hearing all kinds of different crap like you won't even gain any power.

You will gain power from whichever exhaust mod you do first. You will get a diminished return on the second mod although they do work nicely together.

Just go with a reputable company as listed above. I haven't heard of to many issues if any with HFC's throwing a light. I personally love test pipes. For some it's a sound thing and others would love to have but emissions stop them. I don't have emissions here where I live. Which I love.