Bryan,

I also have a cowl and plenum from Chris Zavatson, almost ready to fly but no data as yet.  The air inlets are smaller than the standard cowl to reduce drag, not larger.  These require a larger cowl outlet to allow the air to flow with lower pressure below the cylinders.  Chris' instructions to me stated that you have to have at least 50 sq ins of rear facing air outlet to get the cooling under control.  I do not believe that the standard cowl outlets meet this requirement.  Is it possible that this could be your problem, and the air cannot flow out of the cowl well enough to maintain cooling?  From your description, it sounds as though this might be the problem.

The following is a quote from an email that I received from Chris when I was first buying his cooling set up:  "Whatever you end up with, I would suggest planning on about 50 sq in of rearward facing exit area.  Unless you can build in a nice exit ramp, you don't want to just cut a hole int he bottom.  In a climb the underside of the fuselage sees high pressures that will prevent good flow through the cowling.  I have a photo of  plane that where this was done and the temperatures are astronomically high.  If you can figure out how to add good exit ramps to the firewall, the exit area can be reduced.  I've been tinkering with that idea for some time.  Lancair got it right on subsequent models, but for us they put the lower engine mount attach point right where the ramp should be."

It seems to fit your description of the problem.

Jerry Fisher