The other thing the racers have discovered is that the shape of the duct
immediately behind the intake is critical. The NC lathe machined airfoil
section
aluminum intake tube shown here was a very noticeably improvement in the
amount of air actually ingested into the intake. This manifested
itself in a large drop in cylinder head temps allowing the diameter
of the holes to be reduced for the same cooling.
Paul Lamar

Mike Wills wrote:

  OK guys, a couple of days late but I want to go back to this discussion
of internal versus external diffusers and how it fits with the example
given by Paul of Steve Barnards round inlet cowl. Its taken me a couple of
days to digest this stuff. Pardon my ignorance and please explain to me
what is happening here with Barnards airplane. Ive never seen the airplane
uncowled so dont know what his cooling plenum looks like. But I do know
that these machined inserts in the inlets are airfoil shaped - i.e. the
inlet area reduces as you go back into the inlet. Seems to me that this
would accelerate the flow/reduce the pressure as the air passes through.
Whether discussing an internal or external diffusion scheme it seems to me
that this is the wrong thing to do. So can someone explain (in laymens
terms) what is going on here?

  Thanks to all (particularly Bill and Jeff) for the excellent posts on
this subject over the past week or so.

  And thought I'd mention that Steve Barnard uses a pretty heavily modified
IO360. I think he said its up around 235 - 240 HP. So the aero tweaks arent
the only thing that makes this plane go fast.

Mike Wills

No Mike they open up slightly. The duct then smoothly transitions to a molded
fiberglass plenum over the cylinders. Tire inner tube sections are used
to connect both sections of the ducts with hose clamps. Tracy Saylor (245 MPH RV6) 
has done somewhat of a better job in this regard  and Steve is planning on 
improving his. I may have some pictures around and if I do I will scan them in.
 
Paul Lamar
 
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