Thermodynamics ... I have a degree in that, and spent most of my career cyphering on those principles, albeit with emphasis on what happens at Mach-alot. All this back-yard engineering and scientific experimentation at subsonic speeds is completely on point. Very good work. As to negative pressure in the engine compartment, that really does sound about right given the design of the fan mounted in the shroud, sucking air in to the tins at a high rate. That said, if one could increase the ambient pressure inside the engine bay, and decrease the resistance to air flowing from outside to inside, that fan will push more air. I will invest in an engine lid prop -- that can't be too hard. and will use it if/when required, and see if it helps.