This is the FunTech 903D TE Solid Mount Helicopter Muffler, and is intended for use with OS and YS .91 class engines. FunTech's unique design provides the right balance of back pressure so you get the best flow possible from your fuel system without limiting your motors performance capabilities, making this muffler a perfect choice for 3D fliers. This pipes flexibility and compatibility with a wide range of motors is unparalleled, allowing it to provide excellent performance with all OS and YS .91 Heli Motors and looks good in process. Plus, FunTech offers a full line of replacement parts including the muffler body at a price that won't break the bank.