Corvair conversion engines

"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

"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

"Peter Dohm" wrote in message
. ..
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.

The 140 hp Corvair engines tend to lose valve seat inserts if overheated.

Rich S.

Rich S. wrote:

"Peter Dohm" wrote in message
. ..

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.


The 140 hp Corvair engines tend to lose valve seat inserts if overheated.

Rich S.


Same with VW.

Probably the same with any shrink in valve seats...

Richard

On Fri, 27 Jan 2006 19:13:28 -0800, "Rich S."
wrote:

"Peter Dohm" wrote in message
...
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.

The 140 hp Corvair engines tend to lose valve seat inserts if overheated.

Rich S.

ANY aluminum head engine can loose valve seats. The secret is not to
expect more of the engine than it can safely produce. They guy who
did mine pinned them, but I don't plan on putting the pins to the
test.

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.

Peter Dohm wrote:


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


For what it's worth, I'd have to agree, with the thought that you can
operate at higher power - until - the heads are heat soaked.
Then it's 45 HP or bust.

I think the thinking is that the engine needs to turn up fast to make
maximum engine power. Which may be true drag racing dune buggies.
Grab a gear and spin that puppy up!

Fixed pitch props don't do that.

Think one speed automatic transmission - with a lot of slip.
Pure torque converter.

IIRC, and it's been a while, the VW factory specs said max rpm ws 4200?
The torque curve peaked about 3000 (?)

Swinging a propeller at 2500 to 3000 RPM puts us on the rising part of
the curve - approaching peak.
The faster it turns - the more torque it makes - the faster it turns.
That's fun!

I think that's why VW powered parasols and biplanes tend to be faster
than their two-stroke powered brothers.

For a VW to turn a propeller ~~3000 rpm means...
1600cc 54-56" prop diameter
1835cc 56-56"
2180cc 58-60"

Our little low and slow airframes lean toward the long end.
Swing as big a stick as you can.
It pays off in prop efficiency, disk area, and tip circumference.
(compared to a 66"~68" prop on a Rotax 503, VW's climb like sea slugs)

Faster airframes tend to use shorter props and throw the excess torque
into pitch to go faster! But at high cost in prop efficiency (again).

Well, like I said, for what it's worth...


Richard

Richard Lamb wrote:
Peter Dohm wrote:


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


For what it's worth, I'd have to agree, with the thought that you can
operate at higher power - until - the heads are heat soaked.
Then it's 45 HP or bust.

I think the thinking is that the engine needs to turn up fast to make
maximum engine power. Which may be true drag racing dune buggies.
Grab a gear and spin that puppy up!

Considering the 356/912 Porsche engines have essentially the same
upper end as a Type 1 VW and they operate far higher sustained powers
than that (think a long 100+ mph Autobahn run or the military gensets
they were in designed to make 400 Hz power at continuous power
settings, depending on generator efficiency, between 55 and 70 hp), I
question this theory provided the cooling blower and baffling are
designed for the power in question. But it's easy to prove or
disprove-hang a VW on a oversized generator, hook a dummy load up, and
monitor RPM, power and CHT!

Gen seets make wonderful engine dynos.

"Peter Dohm" wrote

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.

You have no doubt read ditties from VeeDubber here, and one of them was on
VW engines, and how many HP they really can sustain.

Seem as though (from nearest I can remember) the amount of cooling fins in
the area of the valve guides and seats, and the cross area is insufficient
to carry away the amount of heat that is flowing through that area, if it is
run flat out with a big enough prop for very long.

Remember, heads in most auto engines nowadays, are cooled by water
circulating through the head. Flow some oil past the critical areas, and
heat can be dealt with. Air cooled engines must depend on cooling fins, and
enough metal to carry the amount of heat produced to the fins.

Perhaps VeeDubber will be kind enough to repost that treatise? g
--
Jim in NC

snip

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



I drove a harvesting machine that used hydrostatic drive and a VW engine
for power. We ran it on a governer at 3950rpm 24x7 all summer. Once I
was moving it from field to field and dropped a valve when I ratcheted
the motor up to 4100 because the machine travled at the speed of growing
grass. Of course this was after about 10 weeks of continous operation so
in terms of hours it was due.... The vale seats were pretty hammered as
I recall.

Lets see, 24*7*10=1680 hours.

Dave
PDX