Carbon is used successfully in making intake manifolds for top fuel dragsters.As for making tubes I belive that making your mold in two parts will give you the best results.\n  First make a "buck" by making a model of the intended shape and get it as smooth as possable then lay fiberglass or pour a durable plastic material around your model,making sure you can seperate it into two halves when cured.Pop out your model and you have a mold in which to lay up your Carbon piece.