John Kleber -

I only know about the 360 plenum, a beautiful piece of fabrication by Chris Zavatson, but here are my observations which are relevant to your situation.

1)  Chris Zavatson claims better cooling with his plenum, and has data to prove it.

2)  I see no reason why using a separate plenum cover rather the the inner surface of the cowling would improve cooling.

3)  On the other hand, the diffusers that Chris includes with his plenum explain (to me) the superior pressure recovery that he experiences.  In other words, the pressure on top of the cylinders is higher when using a diffuser than when simply having a hole in the front of the cowling which dumps air into the space above the cylinders.  This is because the air slows down with less energy-robbing turbulence than with a standard installation.

4)  Higher pressure above the cylinders means better cooling.

5)  I'll bet that you could improve the pressure recovery (and therefore cooling) with diffusers alone, without the separate plenum cover.  The diffusers would have to blend into the cowling top and baffling reasonably well.  This may be easier to accomplish with a separate plenum cover but you could probably improve the situation considerably with just a diffuser.

6)  There are two other advantages to removing the pressure load from the inside of the cowling, and you'd need the separate plenum cover to do that.  The pressure load on the cowling caues it to flex a little and it may ultimately crack near the front openings.  Also, the cowling would lift upwards slightly, loading up the fasteners at the back.  Scott Kreuger recently posted something along those lines.  Anyway, both of these adverse effects would be reduced significantly with the separate plenum cover.

I have attached a picture of Chris Zavatson's diffuser on my engine.

- Rob Wolf