...regarding the Wright R3350 radial engine efficiency (from Wiki):
"...By this point, reliability had improved with the mean time between overhauls at 3,500 hours and specific fuel consumption in the order of 0.4 lb/hp/hour (243 g/kWh, giving it a 34% fuel efficiency)." [Wiki]
...
color emphasis is mine.
...however, you have to remember that these engines were "turbo-compound" (
both supercharged AND turbocharged), not just normal aspiration.