The recommended pipe for the OS46 VXDF is the MACS #1250 8.5cc muffled tuned pipe. At least thats what it is for use in at Turbax ducted fan and spins on the order of 23,000 RPM. Check with Larry Wolfe at Jet Hangar Hobbies <
http://www.jethangar.com>. He can tell you the correct pipe length, which coupler to use, etc.