● Material: Phastek offers exhaust system materials in aluminized, T304 Stainless Steel and T409 Stainless Steel. The material your exhaust is made from does not effect the sound, but the durability of your aftermarket truck exhaust. aluminized steel. T304 is the highest-quality to resist rust. T304 Stainless Steel is also good, but the metal will discolor from the heat. If you're wondering if you have a T409 or T304, then try to stick a magnet to it. A magnet will stick to T409 stainless steel, but not T304 stainless steel.