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I wish I knew how to post scanned images here. If so, I could show you illustrations out of Dr. S. Hoerner's "Fluid-Dynamic Drag" page 9-16, with cites and sources.
The optimum outlet is not as shown in the BMW photo, or anything close. It is more like the bottom drawing D in the above illustration, but with the outlet channel as close to parallel to outside flow as possible, and with the downstream intersection radiused.
Fast flow of the outside air means low pressure, which in turn sucks the outlet discharge into the free stream. If the outlet is as close to parallel with the free stream as possible, and the downstream intersection radiused, the outlet air can merge with the free stream with the least possible turbulence and greatest efficiency. Think of it as an on-ramp to a freeway, where the cars represent air molecules.
BTW, some folks use NACA submerged inlets and turn them around backward as outlets. Ungood. These NACA arrowhead-shaped inlets look sexy and work pretty good as inlets, but are not efficient as outlets.
Bottom line: If outlets and internal ducting are good, the stagnation point inlet at the nose can be quite small.
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