wrote

Hoover write's about thrust being more important that HP. That jibes
with other stuff I've read.

The christmas engine blog sounds like he's doing all this direct
drive?

Since HP (and therefore heat) is the limiting factor with VW, you can easily
hit the maximum thermal HP and still keep the engine at low prop RPM's. No
need to rev fast with a PSRU and be limited to part throttle to keep it
cool. It would just mean more weight to haul around.

Can you really get max HP out of an automobile engine at lower RPMs
required for a prop not to spin too fast? Don't you pretty much have
to have a PSRU to match max auto engine HP to sub-sonic prop RPM?

No, if you go direct drive with most auto engines, you are being satisfied
with lower HP levels than would be possible with a reduction unit. It still
would be a sizeable amount of HP, but less HP per pound than airplane
engines, since auto engine blocks are usually so massive.

You look at an engine torque and HP graph produced from a dyno, and where
the torque curve crosses the HP curve is usually close to the ideal speed to
run the engine for maximum power and thrust. At those RPM's at wide open
throttle, the HP per weight is in line with conventional airplane engines,
and probably a little better.

Many times, you will see people run a little lower RPM than that
intersection, for noise, fuel economy, and for engine longetivity.

Well, okay this should be asked over on homebuilts. Anyway the OP was
all about having fun in low n slow. I think these guys are
outrageously low n slow.

Yep. If you want to learn more, there are many people over there that know
their stuff, and still quite a few that are anti auto engine maniacs. Like
always, you sort through it and use what you can.
--
Jim in NC