On Fri, 27 Jan 2006 21:49:21 -0500, "Peter Dohm"
wrote:


"Morgans" wrote in message
...

"Peter Dohm" wrote

I always tend to think of these little engines in terms of installing
them
in go-fast machines, for their power. To me, that means turning a 52
inch
propeller about 3500 to 3600 rpm.

Willim Wynne does not reccomend turning the Corvair engine at more than
3000
RPM, and pretty much says right out, that doing so has been shown to break
the crank, in all of the cases he has studied.

So 95 to 100 hp may not be all that
crazy--especially with a rear drive installation such as offered by
Great
Plains for their VW based engines.

Some pretty experienced people (in VW engines) have said that the valves
are
the weak link, and that much more that 45 HP will melt the valves down, if
run at that level for more than a few minutes.
--
Jim in NC

That is an interesting assertion. I really don't know. The number would
conform quite nicely to a 1600cc/96cid engine producing around 60hp; as well
as most older normally aspirated aircraft engines, which seem to have
maximum continuous power figures in the 0.5 hp/cid range. There is even a
rather famous dissertation, supposedly published in Contact!, which suggests
failure due to heat dissipation problems in the valves and/or heads of
automotive engines operated above continuously that power level.

As it happens, I am really on both sides of this issue--and may not be
technically qualified on either. (Required disclaimer as I am neither a
mechanical engineer nor a mechanic)

On the "this is unadulterated manure" side: there is a dissertation
attributed to an engineer at one of the big three auto makers and posted
here a couple of times, possibly by Corky, asserting that manufacturer
testing includes a 100 hour run at full rated power--and that the failure
about which they are concerned is the harmonic dampener. That is in keeping
with articles I read more than 30 years ago in my school days, which stated
that the exhaust manifolds glow incandescent during this proceedure.
However, the colant and oil are maintained within their normal temperature
range during that portion of the testing proceedures. On modern automotive
engines, this equates to more than 1.0 hp/cid; and 100 hours is clearly much
more than a few minutes.

However, my real problem with the valve assertion is that I really don't
know anyone who managed to run one of these little air cooled engines long
enough and hard enough to burn a valve. I do know of two broker cranks on
Corvair conversions (same person) and at least one, and possibly two, broken
cranks on VW conversions (same other person). Both are mentioned on the
FlyCorvair site, so I am really not adding much that is new. I am convinced
that all of the failures were torsional damping issues. The only burned
valve that I know of on an automotive conversion was on a liquid cooled
Geo/Suzuki engine and was traced to a carburetion problem--which was run at
a much higher power level. I was told that the carburetion problem was
corrected and has not recurred.

On the other hand, I strongly suspect that very high power levels equate to
accelerated wear; and I really dislike very short TBOs. So all of my own
scratch pad doodles are based on continuous power levels of less than 0.5
hp/cid, and usually significantly less.

Peter

On VW engines the problem is a lack of fin area on the heads, combined
with limitted thermal mass. Anything over 40HP produces more heat than
the heads can dissipate, and the thermal mass is low enough to limit
any operation above that level to a matter of a very few minutes.

Corvairs do not suffer this lack of fin area.