Good to see others out there are thinking about design considerations! However, please consider the following:
Assuming the turbo is not touching the valve cover (
very critical), the main method of heat transfer from the turbo will be radiation. Since radiation behaves similar to light, it only affects things that are in line-of-sight (unless you happen to have a double slit experiment going on
). In our case, a mostly solid sheet of aluminum will be covering the valve cover, blocking line of sight between the VC and turbo.
As with light, radiation is reflected very well with a shiny surface (like a polished aluminum sheet). A good example of this can be seen on the heat shields protecting our windshield washer fluid fill tubes and trunk area (above the mufflers). For this reason I would argue that we would ideally want to keep the top surface of the engine cover polished aluminum (clear coat maybe).
The underside is a trickier subject. For now, I would say that putting insulation under there will primarily just dampen injector noise. Moreover, if the insulation is actually in contact with the engine cover and the valve cover, it will be hindering cooling two-fold. First, it will provide a path for conductive heat transfer that isn't present with an air gap. Second it will hinder convective flow underneath the engine cover, making the louvers less effective.
The thing I'm not sure about though is this: should we paint the underside black. Black bodies are very good at absorbing radiation (coming from the VC area) and metals conduct and distribute heat well. So there might be some heat that can be extracted away from underneath besides just just the convective cooling form the heat rising out of the louvers. No idea if this is legit or not, but might warrant some testing.
TL;DR: Keep the top surface reflective, block line of sight from turbo, and ensure air gaps. Maybe paint the underside black?
