Rocket Man VW Heads... What alloy?

On Jan 13, 3:00*am, Stealth Pilot
wrote:
On Mon, 12 Jan 2009 22:38:54 -0600, "Maxwell" #$$9#@%%%.^^^ wrote:

There is also the possibility of molding this in plaster, but that's another
story.

unless you've tried it you would have no idea how dangerous that
suggestion actually is.

firstly the strength of plaster vanishes to nothing beyond a certain
temperature. that temperature is just 5 degrees celcius above the
melting point of aloominum.

That is because it is water molecules that bind the other molecules
together when the plaster cures. When you heat it, you break those
bonds and drive off the water.

You can test this out by weighing the water and plaster you mix,
then weigh the cured solid when it appears dry. It will still weight
a lot more than the dry plaster did. After baking it, weigh it
again.

As long as the weight is greater than the original dry plaster, you
know there is still water in there.


I have tried the technique to cast a mills 0.75cc diesel crankcase.
this is a tiny thing about an inch and half by an inch and a half in
overall dimensions. relatively tiny and easy to work with.
it was done lost wax.
a day after the plaster was set the mold went into the oven to melt
out the wax. it was then cooked up to about 180 degrees farenheit for
a while (2 hours afaik) to evaporate off the moisture. it stayed in
the oven overnight (I've never been allowed to use the oven again) it
was then given 15 minutes in the microwave to really dry it out.
it was wrapped in a cloth and raced out to the workshop hot.

it looked alabaster white and as dry as a piece of chalk.

when the aluminium was poured into the mold a guyser of molten
aluminium ejected to the ceiling of the workshop. *this seemed to
continue for some time. an examination of the mould later showed it to
be totally empty. I dont know how dry you need to get it but I gave it
a fair go at getting a bone dry mould.


I've no experience, but there is a nice newsgroup called
rec.crafts.metalworking or something like that where you
can get good advice from people who routinely have a
less exciting experience.

Plaster of Paris retains water of crystalization when it is
cured. It takes a lot of heat for a long time to get all of that
water out, like maybe 5 or 6 hours at 500 F and you should
keep it hot when you our. If I understand the chemistry, by
the time it is dry enough to cast aluminum you will have returned
it to much the same material that it was before you mixed
it. Since you didn't do that, it was the molten aluminum
that decomposed the plaster releasing the water with the
observed result.

Jewelers work with much higher melting point metals, platinum
even. They use investment, sometimes called investment plaster.
The difference between that and ordinary Plaster of Paris is that
it has materials added like silica that do not decompose when the
mold is burnt out, and other materials (maybe wood flour) that are
there to create microporosity when it is burnt out. A high quality
investment may be overkill for casting aluminum, but it will be
better than a molten aluminum geyser.

But please do check with people who, unlike myself, have
actually done this.

On to the subject of cooling fins:

I HAVE designed cooling fins back in a former life when I
was working in the nuclear industry. I believe you will
get the best heat transfer with pure aluminum. Alloys
will have a lower thermal conductivity, although the
more important parameter is heat capacity, the product
of thermal conductivity with density. But for an airplane
you probably don't want denser (and therefore heavier
materials).

You can improve the heat transfer and reducing the
weight by tapering the cooling fins from root to tip. A
good undergraduate engineering book on heat transfer
should address the optimization of cooling fins.

Remember also that convective heat transfer is
governed by Newton's law of cooling:

q = mdot * h * A * deltaT

DeltaT is the temperature difference, A is the surface
area (which you increase with fins), h is the convective
heat transfer coefficient (usually a function of Reynold's
number) and mdot is the mass transfer rate of the cooling
fluid.

Bumping up the flow increases both mdot (directly) and
h, by increasing the Reynold's number. It helps a lot.

IOW, concentrating on improving the airflow may be more
helpful than improving the fins.

Hope this helps

--

FF

On Jan 12, 11:04*pm, "Maxwell" #$$9#@%%%.^^^ wrote:

...

What's the point? If you are going to buy bentonite, why not buy a quality
molding sand like silica or olivine as well?

Kitty litter is cheap.

--

FF

IOW, concentrating on improving the airflow may be more
helpful than improving the fins.
---------------------------------------------------------------------------------------

Roger that. In spades. Most homebuilt cooling systems aren't. Go
over to AirVW and give a gander at what one of the Monnett's had to
say about VW heads.... and later what he had to say about VW cooling
systems.

x-number of inches of water-pressure differential between the inlet
plenum and the outlet plenum... takes something on the order of 90mph
to get that amount of pressure from ram air alone. And you're driving
a plane that cruises @ 75mph

So now you REALLY get to see the Wizards at work... guys like John
Thrope, with his equations for the inlet AREA of the plenum vs the
outlet area, with pieces of duck tape here & there and flat THROWING
the stock oil cooler across the hangar because it's making such a huge
HOLE in the back wall of his inlet plenum that there ain't no way to
get the required about of pressure... and the Great Designer insisting
he's NEVER had any 'trouble'... and it reminds you of the guy in
Oregon with his three engines but never any trouble, or the guy in
Canada with 1300 hrs on the SAME engine over a 20 year period and
NEVER ANY TROUBLE... but forgetting to mention the ANNUAL 'freshening
of the valves' and the occasional replacing of the bearings because
THOSE THINGS DON'T COUNT as 'troublle' since he's never had any in all
those long, long twenty whole YEARS of flying.... which worked out to
something like 65 HOURS OF FLYING PER YEAR, between June and
September, because that is the flying "..season..." according to this
particular expert with his high-time VP1 now a'dangling in some
museum.

The only thing more lovable than a VW-powered air plane is the folks
that owns them.

-R.S.Hoover

On Tue, 13 Jan 2009 08:42:55 -0800 (PST), "
wrote:

On Jan 13, 1:00*am, Stealth Pilot
wrote:
On Mon, 12 Jan 2009 22:38:54 -0600, "Maxwell" #$$9#@%%%.^^^ wrote:

when the aluminium was poured into the mold a guyser of molten
aluminium ejected to the ceiling of the workshop. *this seemed to
continue for some time. an examination of the mould later showed it to
be totally empty. I dont know how dry you need to get it but I gave it
a fair go at getting a bone dry mould.

Stealth Pilot

Your experience pretty much matches mine. I was able to get dry
enough plaster to make castings without porosity, but it took WAY to
long to dry to that point. My first pour too resulted in a steam
geyser and I would have sworn I baked that thing long enough. I was
also having fill problems. One plaster cast per go made the
experimentation too time consuming. I gave up and went back to sand.
I was trying to make fined valve covers for my1/2VW and was having
trouble getting molds in sand to stay together while puling the
pattern. At that time I didn't have a muller and could not get oil
based sand to work at all - thus the try at plaster. I finally
managed to modify my sand formula and got satisfactory results. A
touch of sugar did the trick.

The fins on the valve covers were enough of a challenge that I won't
be trying to do any head casting in sand, oil or bentonite based.
IMHO buying/building a muller is worth the time and trouble as the oil
based sand is good stuff. The humidity in my part of the world is
such that an uncovered covered heap of sand will dry out too much in a
matter of hours. Oil doesn't and for that reason alone I made the
switch.

Just in case you don't have the formula, even though I'm sure you
do...........................

http://www.foundry.ray-vin.com/k-bond/k-bond.htm
http://users.hal-pc.org/~lwhill/sandsystems.html

=======================
Leon McAtee

priceless references.
I've never tried oilsand just a seemingly continuous tweaking of my
bentonite based mix getting it better and better.

maxwells investment reference gives little actual detail but they must
do some cunning stuff with the additives that they allude to in their
plaster. they've certainly developed the process way beyond what I've
achieved.
Stealth Pilot

On Wed, 14 Jan 2009 12:52:18 -0800 (PST), "
wrote:


IOW, concentrating on improving the airflow may be more
helpful than improving the fins.
---------------------------------------------------------------------------------------

Roger that. In spades. Most homebuilt cooling systems aren't. Go
over to AirVW and give a gander at what one of the Monnett's had to
say about VW heads.... and later what he had to say about VW cooling
systems.


I changed some baffles on the O-200 to get around a corrosion problem.
airborne moisture was impacting the fins, blasting off the paint and
then leaving the bare hot steel moisture coated. rust city.

later I noticed some harsh running sounds and while investigating some
oil leaks had the need to replace rocker gaskets.
on one cylinder there was a noticeable build up of carbonised charred
oil on the top of the exhaust valve.
I've since placed a hole in the baffle face to direct air over the
exhaust fins of that cylinder, rather than using the plenum pressure
directed airflow.
The harsh noises have not reoccurred. during the annual in a few
months I'll check for carbon buildup again.

do you find in your vw engines that carbon buildup on the exhaust
valves is any indication of a cooling deficiency?
what indicator do you use for health of a cooling setup?
in other words how can anyone know that they have it right?

in my case oil temps were all totally normal. it seemed to me that the
charred oil was an indicator of a local hot spot that I needed to
address.

Stealth Pilot

On Tue, 13 Jan 2009 07:16:12 -0600, "Maxwell" #$$9#@%%%.^^^ wrote:




At any rate, good luck on your green strength test. Even the finest
seasoned
and mix green sand will in no way meet the expectation of your 2" x 12"
sample as stated.


rubbish.
I *actually* do castings.
the shake test is one I can demonstrate in 5 minutes with two
handfulls of greensand from the black bin in my workshop.
I'm writing from first hand experience.

can I suggest you get off your arse and *do* something in life.
your pleasure would increase immeasurably.

Stealth Pilot

Then whip one up in 5 minutes, and send us the pic.


after the last time? c'mon even george bush thinks that twice is
stupid.

On Jan 14, 5:23*pm, Stealth Pilot
wrote:

do you find in your vw engines that carbon buildup on the exhaust
valves is any indication of a cooling deficiency?
what indicator do you use for health of a cooling setup?
in other words how can anyone know that they have it right?
----------------------------------------------------------------------------------------------

Detonation gets your attention pretty fast.

SPLIT in the exhaust valve.

Guy sez temps are good and you're standing there with you IR
temperature gauge, with it say 530F and he's still yaking on about his
'good temperature,' with the thermocouple under his sparking plug.
Ask AGAIN and he starts talkng about fishing or some dum-assed thing.

-Bob

On Jan 14, 12:46*pm, Fred the Red Shirt
wrote:

Kitty litter is cheap.


PLEASE don't mention anything about "kitty" around a casting sand
heap ........ Bad Karma :-)
=================
Leon McAtee

"Stealth Pilot" wrote in message
...
On Tue, 13 Jan 2009 07:16:12 -0600, "Maxwell" #$$9#@%%%.^^^ wrote:




At any rate, good luck on your green strength test. Even the finest
seasoned
and mix green sand will in no way meet the expectation of your 2" x 12"
sample as stated.


rubbish.
I *actually* do castings.
the shake test is one I can demonstrate in 5 minutes with two
handfulls of greensand from the black bin in my workshop.
I'm writing from first hand experience.

can I suggest you get off your arse and *do* something in life.
your pleasure would increase immeasurably.

Stealth Pilot

Then whip one up in 5 minutes, and send us the pic.


after the last time? c'mon even george bush thinks that twice is
stupid.

Once again, I didn't think so.

"Fred the Red Shirt" wrote in message
...
On Jan 12, 11:04 pm, "Maxwell" #$$9#@%%%.^^^ wrote:

...

What's the point? If you are going to buy bentonite, why not buy a quality
molding sand like silica or olivine as well?

Kitty litter is cheap.

=========================

Yeah, but the castings it produces is pure ****.