It's not the cat that gives it that hiss. It's kinda just how the motor sounds. It's almost unavoidable with HFCs or test pipes. Resonated pipes help, but it'll still be there.

Most likely. That way you can the most out of the mod and turn off the CEL light

I would wait till you've added all the bolt-ons then have a tune. But rule of thumb - CAI + HFC + CBE + more = Need a tune. Otherwise you don't need to coz it's not going to change the A/F ratio that much.

Also I don't know if it sound stupid or not... When I sit in the car and stand outside the car(let other people rev it for me) the exhaust note are very different!:rofl2: When I am in the car, I don't really hear the hiss sound so I don't bother.:icon17:

I have had a CBE and CAI for over a year now so I just wanted more beautiful music coming from my exhaust. I am not the type to get all the bolt ons and then tune I don't race my car I only have all these for the looks department

Yeah, it's really only on the outside. The only time I've seen the hiss go away is when you stuff a pair of turbos in the exhaust. :stirthepot:

I have had the Stillen HFC's on for about 2 weeks now. I drive about 60 miles a day....NO CEL lights as of yet!!!!

What a freakin ROAR I have now with the combo of the GTHAUS and the Stillen HFC.

Ill keep you guys posted if anything funny comes about