Quote:
Originally Posted by Xist
The argument I remember is that cardboard and coroplast have negligible insulative properties.

Thanks for all the helpful links, Xist! I thought I couldn't have been the first person to think of using more insulation in the engine bay. It seems like such a straightforward idea.
As for the insulation, it's often misunderstood about how it works. The RValue of dry, noncirculating air is ~ 3.5 per inch. All it takes is a
sealed envelope to achieve that, and that's how the vast majority of insulation works. It's how fur keeps animals warm, sleeping bags are disproportionately effective, aerogels offer insane protection, etc.
When calculating the effective temperature drop, we need to account for the static air gap as well as the actual material itself. So while a 3/16" of cardboard is only R 0.05ish, if it traps an inch of air then its really 3.5 or so.
This is significant when calculating the flux of heat through the surface. Metals have very high heat transmission properties; it's why stuff like copper and aluminum are used at heat sinks. The heat flux is a function of the
temperature delta between source and sink. So the smaller one makes that (say, through insulation) then the smaller the thermal flux becomes. Optimally, one wants exactly enough insulation so that we have the engine operating temperature on one side and ambient on the other, with no flux through the surface.
By the way, there are more subtleties to insulation. We've been talking about resistive insulation, which blocks conduction. But there's also reflective insulation, which blocks radiation. And trapping air is effectively blocking convection.
Hope this helps!