Consider a piston engine with all its coolers and components within an aircraft engine bay. To cool this engine, cooling intakes are positioned facing the direction of flight. Now, the condition at the inlet surface need not reflect the flight conditions exactly, one reason for this would be since the resistance pressure from within the engine bay would influence the conditions at the inlet itself.
The question is specific to this resistance pressure as a first approximate at least. I know there are a variaty of other reasons as to why the conditions at the inlet surface would change (e.g pressure distribution at location of intake position, inlet type, etc...) but those aren't a concern here yet.
A good answer to this based on actual data, or statistical data or even experience would be greatly appreciated. Sources as to where one may find the answers to this would also be much welcomed. Thanks in advance!