Considering a piston engined aircraft, what resistance pressure would be generated within its engine bay against its cooling intake? 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!
3 main points come to mind:


The space above the cylinders is a pressure plenum so that the air pressure and velocity flowing past the cylinders is the same everywhere.
The space below the cylinders is a low pressure zone and to achieve that the exhaust opening should be larger than the inlet opening(s) to get the highest differential at low speed, but can be reduced as ram pressure increases (hence a cowl flap).  The pressure balance has to take into account air bled off the high pressure plenum to pass through coolers or other services that need ram air.
The baffling that controls the flow between the high pressure plenum above to the low pressure exhaust zone below past the cylinders has be tight to the cylinders, actually touching the fins, so that the cooling air is forced between the fins.  Cooling air that passes beyond the edges of fins does no good.


As for details and numbers, you should probably obtain this book on piston cowling design.