Efficiency was not their primary objective. Cooling and power to weight for performance were the priorities. 160 HP out of 8 liters at 1300 RPM and 325 pounds weight, in a era when compression ratios were 5 to 1 and fuel was probably 70 octane.
Vicious handling characteristics when you have 300 pounds spinning around in fron of an airframe that was basically a glorified kite. Compared to the same year Mercedes grand prix engine at twice the displacement and 2300 RPM producing 200 HP and you get an idea of the power in the context of the era when they were made and used.
As a fighter plane engine they were practically invincible, using engine torque to turn twice a quickly in one direction as the other.
With rods spinning around the fixed crank journal and combustion pressure actually pushing the cylinder head away from the fixed journal, there is very little reciprocation except amybe in the valve train. In the Gnome the exhaust valve was actually 2/3 of what you would normally call the cylinder head, and the valve spring was so weak you can actually open the exhaust valve with your finger. It depended on centrifugal force for "spring tension".
regards
Mech