I don't know if it applies anymore but I'm sure you've heard of the "hiss" that anything besides the OE cats produces. You probably won't get it as badly with HFC's versus (un)resonated test pipes.
Generally speaking, test pipes have shown gains roughly around 5-8 horsepower and 5-7 torques (or perhaps more torques...). HFC's should probably make just slightly less.
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"Since one of my brake light bulbs just went out, the forum told me that my piston rings are about to blow."
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