There should be an inner liner of sheet metal like aluminum inside the carbon fiber tube. The carbon fiber epoxy binding matrix cannot handle the heat of the exhaust directly. The carbon fiber tube will turn gray and look like an ash color if the epoxy binding matrix is overheated. There are some special carbon fiber products used with a high-temp epoxy binder, but most of the tubes you see for consumer use are not. Even at best, they can only handle 500 degrees F for the high-temp ones, and the regular ones can barely take 300 F.