aFe MACH Force-Xp 2-1/2in Stainless Steel Axle Back Exhaust w/CF 15-19 BMW M3/M4 (F80/82/83)Aggressive Sound: A free flowing, transverse muffler is used to absorb high pitch tones producing a powerful sound and unleashing all available horsepower and torque. The muffler is finished with a high temp powder coated metallic black finish. Exhaust Valve: This Axle back exhaust system incorporates a factory style exhaust valve which uses the factory exhaust valve motor (not included). The valve helps regulate, direct and control the flow of