I noticed the Cessna 'New style' inlets are pie slice shaped, the wide part is away from the spinner; perhaps it matches the arc of the propeller blade passing to gain the most from the prop blast?
If that is the case, then Vans inlets are backwards, being taller near the spinner and shorter outboard from the spinner.
I also noticed the Sam James Rotary engine cowl has a single inlet below the spinner. I guess it is located right in the center of the stagnation zone. Perhaps those Ranger, Menasco and Walter - LOM engines have some of the best inlets?
If you are contemplating new inlets they should be low on the cowl where the predicted stagnation zone is, and form a radial arc that matches the trailing edge of the propeller. I.E. a single vetical slot below the spinner that is wider at the bottom. The divergent duct should curve around the left side of the prop shaft-extention (same as prop rotation) and up to the upper deck plenum for you down draft coolers. It would not have to enter the sealed plenum at the front.
While I'm being a total nutcase, I'd like to point out that some of the fastest piston powered planes ever built had hot air exits on the side of the fuselage, like Fw190, and Bearcat. The previously depicted cooling outlet ducting on the pusher plane looks like a candidate for side outlet of engine cooling air and exhaust. Then the bottom of the plane could be cleaned up and several inches of protruding ramp removed completely. the bottom is a high pressure area and drag there adds an extra penalty.
Now I know all this adds up to a lot of dam work, and it will look strange when it's done. It's probably harder to apply this inlet to a Flat Lycoming style engine. But if the side cooling / exhaust outlets made more room under the engine, the inlet diffuser could actually be 3 or 4 feet long and fill the plenum from the back, although it would not be straight.
Usual disclaimers, no harm intended, no axe to grind etc.
