John Deakins says:
for aspirated engines (8,5:1 compression ratio) - 14,9HP/GPH
for fire breathing turbos (7,5:1 compression ratio) - 13,7HP/GPH
Observation: those numbers are for lean of peak operation only
THE mandatory link for everyone who wants to lear A LOT about recips:
http://www.avweb.com/news/columns/182146-1.html
Thank´s John. You made much much less ignorant.