Some say good some say bad, i think bad. It may reduce heat that your manifold radiates but what it will do instead is disipate the heat elsewhere, ie to the head and valve area etc. Which IMO is not good, you will not be reducing exhaust gas temperature instead you will probably be increasing it. All the wrap is to me is an asthetic thing. It might reduce under bonnet temps at a stand still but as soon as the car starts to move there is no benefit. I should imagine the head would radiate the extra heat also