so how would keeping more heat inside the pipe affect the flow characteristics of a hot gas? el diablo had his HPR pipe coated, no change in performance only less heat on his leg. changing the inside diameter of a pipe by coating it makes sense with a fluid flowing inside of it but how is a coat any different from the build up in a pipe while its used. the inside of the tune pipe after use gets coated with oil film which is as thick as a coat of paint. so why wouldnt the build up of oil film inside the pipe affect its flow characteristics?
not trying to disprove you pipester just asking questions for the sake of gaining knowledge.