Its VERY important to realize that an air-cooled engine needs a LOT of air to keep the heads cool as thats the key to maintaining power. This has always been the struggle and the latest 930 fan setup (245mm & pulley) is the closest thing you'll get to the flat fan.
Flat fans were intended for engines that ran high cylinder head temps and for those app, its a godsend (but not anywhere close to water cooling). You just cannot get enough airflow to keep them cool enough (under 225 deg F) for best VE. This is precisely why Grady did his "Rubbermaid Solution" on his race engines.
A local gent used a 935 flat fan to cool his 410+ HP, 3.5 litre race engine and IMHO, it was worth the power loss to maintain full output on hot days at the track.
