[overcoil]

from googling : "The covers reduce engine noise and protect the engine from dust, debris, etc, while adding a cleaner look to the engine bay. the "insulation" on the underside of a hood is not for noise, its a fire blanket. Its supposed to fall down on top of the engine fire as the plastic tabs holding it in place melt."

Now I always thought the engine covers were for pedestrian impact - as the head hit the hood the engine cover was a secondary barrier to the hard engine-head and attachments.

A far as heat goes, things like opening the hood when stopped or running the HVAC heat at high fan speed seem much more active in reducing heat than nixing the plastic engine cover.