Direct drive V8's have been done on EZ types, where prop diameter is limited by airframe characteristics. 200-230 HP should be easy with an LS motor at ~3000 RPM. But you'd still need an external bearing (and support) to take the bending loads off the crank. And done right, it can be a significant % of the weight of a redrive. If you can accept the prop turning 'backward', an airboat reduction drive is worth a look. Relatively inexpensive (~$3k), not terribly heavy (usually around 70 lbs) for their HP handling (way more than we'd need; around 1000 HP).Having been out of the PSRU quest for some time, (flying certified), I wondered if anyone has learned anything useful about the (usable) design of a modern PSRU unit. I used one or two on planes I built in the late 80`s to around 2000, but as was previously stated, a lot of manufacturers promised a lot of things, and somehow the produced unit always seemed to not quite work as well as promised.
If we are no closer to an answer re. a modern PSRU unit, it looks like I will have to go and drag the old IO-470 out of the shed and give it a bit of a once-over.
I realise this is a redrive forum, but another avenue that could be pursued is to just get the humangous aluminium GM or Ford engine that tickles your fancy, and invert it, and accept the fact that you will "only" get 1 hp out of every 2 ci of your engine capacity, for a normal size prop.
I am investigating this option quite seriously for an install on a BD-4 I am resurrecting, and I have a book written by a person that did just that with a ZZ4. Minimal mods, with most needed to the oiling system, but way less hassle than a redrive, imho. Your individual requirements may differ from mine.
Erik in Oz.