[aerohead]

An additional puzzle piece involved in a vehicles cooling system and drag is associated with the heat load imposed upon the radiator by the heat rejected from the air conditioning condenser, upstream of the radiator; which is a function of the total cooling load of the cabin.
The total cooling load for the cabin will include heat flux associated with conduction, internal sensible and latent heat sources, direct solar gain, infrared radiation, outside air ventilation sensible and latent heat, and HVAC system heat.
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* Conductive loads will have to do with the area of all cabin exterior surfaces, the inverse of all their individual R-factors, the design indoor/ outdoor temperature differential, including an engine compartment firewall ( when applicable ).
* Working temperatures for USA vehicles would be 75F-indoor, 122F- outdoor.
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* Internal loads would involve passenger occupancy density and both their sensible and latent heat contribution.
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* Direct solar gain would involve insolation value for Furnace Creek, Death Valley, California, USA, full sun, peak summer, solar noon, glazing area, selective coatings ( shading coefficients ), greenhouse effect.
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* Infrared radiation emitted from solar- UV absorbing/ long-wave re-radiating surfaces.
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* Indoor cabin volume and outdoor, fresh-air, ventilation, air-exchange volume rate cooling load based upon sensible/ latent enthalpy differential with indoor design air enthalpy.
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* HVAC heat load involves air handler electric motor losses and air handler fan losses when developing cfm and total pressure (Pt)
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1) cabin 'size' and 'volume' makes a difference
2) cabin 'construction' makes a difference
3) cabin 'glazings' makes a difference
4) whether or not there's an engine compartment firewall makes a difference
5) whether there's two-passengers, or eleven makes a difference
6) the AC system must perform to specification while parked, idling, no wind, all accessories 'ON'
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* Under the worse-case scenario, the radiator must handle not only engine/motor/battery/regeneration heat flux, but also navigate the additional worse-case scenario air conditioning heat load.
* As cooling system drag is related to radiator shutter position and ' blower-assisted' dynamics, the role the air conditioning system plays on overall cooling system performance needs to be appreciated.
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* With respect to the Porsche 911 Carrera cooling drag, consider its surface area, interior volume, and that it is a 2-passenger coupe, compared to the 1st-gen, Tesla Model S sedan surface area, interior volume, and that it is rated to carry up the seven (7) passengers, also lacking any facility to lose powertrain heat from an 'exhaust' system.