Quite low
cooling drag (internal flow)...an attempt to prove the Meredeth Effect, i.e. thrust created by an exit velocity higher than freestream. And he did prove it, sort of. The published maximum exit velocity value (from Kitplanes) was 104% of freestream at 80 KIAS.
That's very good, but note that at 80 knots, the actual freestream velocity delivered to the diffuser inlet is much higher than indicated airspeed, due to the huge propeller outflow component. The prop outflow effect can also be seen in measurements of an aircooled installation:
http://www.vansairforce.com/community/showpost.php?p=1177277&postcount=198
Even if the exit velocity was less than 100% of freestream at cruise speeds (and I suspect it was), it is still very likely a
lot higher than the average Lycoming cowl. That part is a win, a reduction in drag due to internal flows.
Returning to the context of this thread, Ross wrote that he made no attempt to measure the
external aerodynamic drag of the installed belly pod. Further, the system as proposed retains flow through the cowl, another drag cost. I'm suggesting Charlie consider all his options before hanging a wheelbarrow-sized pod on the belly. If the exchanger(s) can't be fitted inside the cowl, then perhaps take a look at the beautiful underwing ducted exchangers we recently saw on an RV-8 in Oz. Simply moving the pod out of the propeller outflow would mean a significant reduction in external drag.