The most aggressive-sounding street and strip muffler in the Flowmaster product line. The 40 Series generates a powerful, aggressive sound outside the vehicle. If you are looking for the traditional ''Flowmaster sound,'' the 40 Series offers more of it than any other muffler. The 40 Series generates interior resonance within the vehicle as well, so if you want to really hear your exhaust system inside the vehicle as well as outside it, the 40 Series is for you.