I maintain that the conventional baffling structure, which has a
well-executed flexible seal at the upper cowl and a form-fitting seal
at the cooling air inlets, can be almost as effective as a plenum. The
gap to the upper cowl must be uniform, not less than 3/4 inch or much
greater than 1 inch. It should, ideally, be 1 inch all around. My
Lancair-supplied baffling had too much gap at the right rear cowl, and
too little at the left rear, because the engine is slightly rotated
with respect to the cowl. In addition, a major shortcoming of the
Lancair-supplied baffling is that it leaves a WIDE gap at
the cooling air inlets, with builders doing little more than adding some
loosely-fitting, ineffective seal there. It wasn't too difficult to
correct both of these conditions and add some other enhancements, which
resulting in lowering my cruising CHT's by 40 degrees, from 360-380 to 320-340.

As seen in the photos, I added a 1 inch wide aluminum strip
across the front, easily cut from a cardboard template. That not only
transformed the baffling into a rectangular box, making it easier to
seal, but allowed for the addition of custom fiberglass seals at the air inlets.
I formed those by trimming a block of foam to the required shape and overlaying
with duct tape for release. Their circular holes match the air inlets, with
only 1/8 inch gap to the cowl. In addition, I re-cut both of baffling side
pieces to their correct heights, utilizing .050 inch thick aluminum, and made
custom 1 inch-high inner pieces -also easily cut from cardboard templates-
so as to 'sandwich' the flex seals, further decreasing air leakage. I
carefully cut all seals to the correct height and shape and feel that
I've obtained a nearly 100% seal. I also reduced airflow
wasted through engine areas other than the cylinder heads.

 

Bill Rumburg

N403WR (Sonic bOOm)