Jerry Springer wrote in message link.net...

I would bet it has to do with cooling. It is a pretty simple matter to water
cool an engine in a boat.

Even that's not that easy. I installed a 283 in a 13' crackerbox
racer years ago, and had trouble regulating the cooling flow. You
can't use a standard thermostat, as small bits of weeds can be picked
up and they'll plug it real quick. And the stone-cold water can't be
dumped straight into the engine block; it need preheating via the oil
cooler and exhaust manifold jackets, if installed. I used a gate valve
and ended up with it wide open most of the time to keep head
temperatures in line, but the lower block temps were poor. It used too
much fuel as a result of not operating in an optimum range.
Outboards and outdrives have better means of picking up the
water, with screened crossflow inlets and so on. They use thermostats.
They were, after all, designed for marine use. Straight inboards get
their water off the bottom of the boat, and fine screens get smashed
up on the beach of plugged with sand.
Auto engines adapted to aero purposes face a whole range of
pitfalls, from cooling (radiators often too small for the max-hp
demands, and no serious thought to airflow to and from the rad), to
redrive resonance issues(prop inertia vs. crank/flywheel inertia), to
engine mounts (often complicated and prone to cracking by unforeseen
forces). And they're all pretty heavy for the power they produce. It
can take either years of fooling around and learning things the hard
(sometimes fatal) way, or years of education in engineering school, to
design an immediately successful conversion. Can anyone point out ANY
one auto conversion that has been in widely successful use for many
years? Even the VW and Subaru guys have scores of different
conversions. It makes me, after 30 years in and around airplanes and
the homebuilt movement, really skeptical of "new and improved"
conversions. I work on Lycomings every day, and I can change the oil
and plugs and let it go for another 50 hours without worrying whether
it's going to be back with some complaint.

Dan