Flyboy Dave, have you ever polished aluminum? When you rub on aluminum it produces black residue, and lots of it. Give it a try, you'll be amazed. You can start with the pipe on your .61RF

Yes, you need to include the length of your extension. True pipe length is determined by measuring from the exhaust port to the halfway point of the converging cone of the pipe. Or the first baffle of a muffled pattern pipe such as the type available for the big Webra pattern engines.