Twenty or so sounds about right, 100BMEP, but what I really remember is the torching involved, when a PRT let loose.
Quite a sight at night.
BEagle,
The 'ole R3350 turbocompound engine had a SFC of 'round about 0.37 pounds fuel/HP/hour, so I suspect this was about all you could get out of an air-cooled radial piston engine, now or then.