I know this is a homebuilt group, but for naturally aspirated engines-CFR
23.1093
says---" each engine shall have a preater to raise the inlet temp. 90 deg
F, from an
ambient of 30deg F w/ no visible moisture present.---AND-at only 75%
power. This is not remotely possible with under cowl air temps. The
pucker factor gets pretty high when your altitude is 600 ft over a forest,
or lake, and the engine starts choking down.
..Jerry

Ray, Toews wrote:

Has anyone ever tried using cowling air under the engine as heated air
for carb heat.
Because of tight cowling space I am thinking of building a door in the
lower cowl for fresh ram air and then closing it and drawing air from
under the engine, cooling air coming off the cylinders which seems to
run about 120 degrees as my heated air in case of carb ice.

I have measured the under engine air on two airplanes now and it seems
to run around 120 deg. so this must be fairly typical and would seem
to be more than adequite to get rid of carb ice. I have never had the
opportunity to measure heated air coming off exhaust manifold.

I would build it so fresh ram air is unfiltered and alternate, heated,
air would be filtered. in this way I would always taxi using warm
filtered air but in normal flight use cold ram air.

Makes sense to me, but then so does capitalism.

Ray