Dylan Smith wrote
The accepted wisdom is that aviation engines are tough, because they can
be run at full rated power for hours on end, and auto engines are
fragile, and must not be thrashed or they won't last very long. It was
even mentioned in a thread in the last couple of weeks - I don't
remember who said it, but they said "operate your car engine like that
and it wouldn't last half an hour".

In reality, that's nonsense and everyone knows it. The standards of
testing used by Detroit (never mind the Japanese) are way tougher than
anything the FAA ever thought about doing. This has been discussed
extensively on rec.aviation groups. Check out this link, or just
google it yourself:

http://groups.google.com/groups?hl=e...30% 26hl%3Den

I've never really thought about it, but this weekend I learned how to
drive the winch at the glider club. After a few launches it got me
thinking - this thing about aero engines vs car engines is probably an
old wives tale, possibly promulgated so people don't feel so bad about
spending so much money on aircraft engine parts when yet another
cylinder is cracked.

That's basically it. The aviation engines we use are obsolete. Sure,
they're more reliable and have better power-to-weight ratios than auto
engines - as long as you compare them to contemporary auto engines.
That's contemporary to the design age - meaning somewhere in the
1950's-1960's. At that point, real aviation went to gas turbines, the
designs of aviation piston engines were basically frozen, the
engineering talent went away, and no further progress was made.

When was the last time you heard of a modern (made in the last 10
years) auto engine that just died without giving weeks (or months, or
years) of warning? Not got stalled by a ham-fisted shifter or run out
of gas, but actually died? I'm sure it's happened, but it's a huge
rarity (whereas in 1955 it wasn't). On the other hand, I don't know
ANYONE with more than 2000 hours in piston GA who hasn't had an engine
failure. These things eat valves, they crack jugs, they throw rods,
their carb floats sink - you name it.

The truth is that aviation and auomotive use are very, very different.
They have very different duty cycles, cooling requirements, and
performance requirements. It really should not be a viable
proposition to adapt automotive engines to airplanes - you will wind
up with engines that are overdesigned in some areas and underdesigned
in others. However, automotive engines have advanced tremendously in
the past half century; piston aviation engines have not. Now we even
have manufacturers actually using autmotive cores (which are optimized
for a completely difference application), dressing them up for
aviation, and selling them - that's how Thielert works. It's very
much suboptimal - but the existing engines from Lycoming and
Continental are such disasters that even this is a viable business
plan.

Homebuilders have been adapting automotive engines for years. The
safety record has been abysmal. Oddly enough, it's never the engine
core that fails. It's always fuel systems, ignition systems,
reduction drives - all the stuff it takes to make the conversion. In
other words, all the stuff that is amateur-designed rather than
professionally engineered. That's why I'm not a great fan of auto
conversions. It's not that there's anything wrong with the engines -
despite being suboptimal for aviation, they're way out in front of the
crap Lycoming and Continental are selling, even for aviation
applications. The problem is that adapting the engine for aviation
use is a major task, and not something to be done by a garage mechanic
on the cheap.

Michael