Considering a piston engined aircraft, what resistance pressure would be generated within its engine bay against its cooling intake?
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.