On 2/5/21 I heard Klaus Savier endorse the boat tail lower cowling. Changes to the plans burn up time, but the cowling is already a custom feature. Here's an explanation and picture of a "boat tail", then a discussion of baffling. I'll be cautious in planning, but it's where I'm headed.
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The plans version of the cowling involves a large curve that results in separated airflow. Somewhere along the way, someone came up with the boat tail. The idea is that by adding a tail to the back the slope of change can remain at or under the 3 to 1 ratio (for every three inches of length the curve only changes 1 inch or less). This allows a more gradual change in the contour which keeps the airflow connected - see the tuft testing photo below. Then someone went further and moved the exhaust to this location as there was space there and it smooths airflow over the cowling.

Nick Ugolini used an eductor in a standard cowling, and perhaps that can be used with both exhaust and with air exiting the baffles.
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Air through the cowling - tractor vs. Canard. A typical tractor-form airplane (think Cub) has pressurized air come through the nose and held tightly over the cylinders before exiting to the lower cowling and out a large and generally neglected exit at the bottom of the cowl. In a canard the air enters at the bottom of the cowl pressurized with intake air from the NACA. That air flows up through the cylinder fins, constrained by baffles, which guide around the cylinders and into a pressure box open at the rear. The air exits the cowling through largely unrestricted openings

Background - plenum & eductor. To increase pressure differentials, a lot of time and trouble goes into constructing tight baffles, with rubber / neoprene strips pressed tight against the cowl to hold air in and increase pressure differentials. When that approach is exhausted, people use plenums - see the BD-4 plenum below. A plenum is a pressurized chamber containing cooling air and forcing it over the cylinder fins. But I have an updraft cooling Cozy and it is darn hard to shape a plenum for the bottom of an engine - the induction pipes and oil drain, are hard to work around. Is there any benefit to a top side plenum with an updraft engine?

I have never seen one on the hot side, but perhaps it could be called a modified eductor. (Wow, straight from the Latin educare, "to lead out"!) Below is a picture of the Nick Ugolini "eductor". He used the eductor to exit exhaust gases, with the cylinder heated air exiting through ducts on the top of his cowling. (There is a slightly more detailed description of eductors at https://groups.google.com/g/cozy_builders/c/-YRfovdF1-E/m/j-1-d7pkOUUJ.) Could an eductor be used in a boat tail cowling - the cooling air exits through the eductors on the right and left side of the prop, while the exhaust pipes exit below.

In this approach the NACA is as well done as possible, pushing into the cowling as much air as possible and losing as little as possible to anything not cooling the engine or oil. The air escapes up past the cylinders into the plenum and out through an eductor exit. In the real world it is hard to pressurize anything, so I'm simply closing off big air exits other than through the baffles. Regarding exhaust, the pipes are kept fairly short, and 4 pipes exit below the prop hub. I'm going to use a 8" prop extension. That should give enough dissipation area for the exhaust not to focus heat on a given strip of the prop.

In summary,
- boat tail cowling
- plenum atop cylinders
- cylinder cooling air exits through eductors either side of the prop hub
- exhaust exits through eductor underneath prop hub

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And here are the half-done ideas on making cowling and cooling easy to live with.
• The top cowling “plate” should come off with nothing attached to the engine, so I get to see and service most things from the top. Dipstick hole in the top plate.
• The bottom cowling piece includes a ground adjustable lip to the NACA air intake, done via a metal lip on some screws. That lip includes the “fences” that keep NACA air from spilling out the corners.
• Exhaust pipes are pulled in close to and under the propeller – probably without a 4-into-1, just 4 pipes pointing aft. The exhaust exit is framed with an aluminum plate / mask, so airflow can leak around the pipes, but less than otherwise.
• Baffling will be high temp composite - I've ordered a half gallon of "Max HT", being the PCI product for high temps. It looks difficult to work with - at room temp's it can get to gel stage, then put it out in the sun for a "glass" stage (where it can shatter), then cook at 200 for 5 hours to get a hard cure that keeps strength up over 350 degrees.
• A hole in the forward baffle gives a dedicated air exit to the oil cooler, which is mounted on the spar with a 2” or 3” SCAT hose into the baffle.
• Cabin heat will be the EAA standard aluminum sleeve around a door spring around the exhaust pipe, with a bilge blower drawing air forward.
• With Dual EFI I won't need to blow air on a mag, so I don't need a cool air pipe.
• I have a prop-facing induction. I'd like air intake to come with the blessing of intake pressure, but the replacement systems all focus on making a vertical intake into a prop-facing intake (for instance, https://www.aircraftspruce.com/catalog/eppages/superior_sv78950.php). I'll keep looking for something that might work. Right now I expect to end up at an elliptical lip air filter box feeding a fiberglass connection to intake. I can use the Freeman modeling wax sheets to make the complex shape locking onto the induction box.