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Re: Thermostat/Oil Cooler (long... but maybe useful...)


On 14 Aug 2001, at 23:46, sml214@casbah.it.northwestern.edu 
wrote: 

> Forget the manuals, take apart an engine.  On that specific point
> regarding the T4, I am 100% absolutely positive that I'm correct. 

You may well be, but I remain skeptical. I've looked over the books 
that I have and I can easily see all the main features you describe. 
I just can't tell exactly how all these parts fit together when warm. I 
wish I had one to take apart right now, but I don't. I've replaced my 
share of type 4 thermostats and cables, so I am familiar with all 
that stuff, but the mystery is what happens just above the oil 
cooler. Yes, I know where it is.  ;-)

BTW, as I was falling asleep last night I realized that it really 
doesn't matter how much air is going thru the oil cooler when the 
engine is cold, because there is little oil flowing thru the cooler  
then.  

> First off, one correction to what I stated earlier: all T4 used the 80-85C (or 
> is it 85-90C?  I really don't remember offhand... I'll check next time I'm near 
> my parts and I'll check one).  Until 1974, the T1 used 65-70C, but afterward 
> they went to 80-85C.  I *think* that the T3 came with the 65-70C variety from 
> the factory, but on this point I could be wrong. 

I believe you're right, but that's only a small detail....

> > between iron alloys and alum alloys is something like a factor of 3. 
> > So I'm gonna claim tha a 10% difference in alum alloys is 
> > insignificant; calculate what difference this will make in cylinder 
> > wall clearance and we'll see that we can't even measure it. 

I'm at work now where I can see the handy table I have for thermal 
expansions. The ratio for alum to steel is closer to 2 to 1, so I was 
a little high there earlier.

> Sorry Jim, but that is entirely untrue.  The big difference is that 390 
> Aluminum is a hypereutectic alloy.  I'm not sure what you know about material 
> science, but please forgive me (and skip down a bit...) if I insult your 
> intelligence with the following explanation.  The eutectic composition of an 
> alloy of two materials is defined by the point when as much of the minor 
> constituent is totally dissolved in the major constituent as possible.  An 
> analog is salt in water.  When you put in a little bit of salt, it gets 
> dissolved.  That is like a hypoeutectic alloy.  When you put in lots of salt, 
> pretty soon it can't all dissolve and some falls to the bottom of the glass as 
> salt crystals.  That is like a hypereutectic alloy.  The point when the maximum 
> amount of salt as possible was in there and fully dissolved is like the 
> eutectic composition.  In the case of aluminum, many times the main alloying 
> agent is silicon.  It's the analog of carbon in iron for steel. 

I never heard the words hypereutectic or hypoeutectic, but I'm quite 
familiar with the term eutectic, which refers to the ratio of 2 metals 
which gives the lowest melting point of a binary alloy. Thus 63%Sn 
/ 27%Pb is the tin-lead eutectic and 72%Ag / 28%Cu is the silver-
copper eutectic. What you have defined above just sounds like the 
concept of a saturated and a super-saturated solution.  

I did a web search on "hypereutectic" however, and came up with 
stuff that supports your definition, however, so it appears that the 
metallurigists have saddled themselves with dual definitions of 
eutectic, or that the actual meaning of eutectic is along the lines of 
the American Heritage dictionary listing I also found on line which 
was along the lines of: "...in different proportions," which really is 
no help at all....

> Generally, the eutectic composition of silicon in aluminum (it depends slightly 
> on other minor constitents, but not much) is about 12% by weight.  So, if you 
> have 12% of silicon in your alloy, it's all dissolved in the aluminum matrix.  
> However, 390 aluminum has 16% silicon.  So, primary silicon crystals appear in 
> the material matrix.  This DRASTICALLY changes the properties of the metal.  It 
> becomes much stronger and resilient to wear, and pretty brittle (<1% elongation 
> at room temperature).  Additionally, its thermal expansion is greatly reduced. 
 
I found properties of similar alloys at: 
http://www.shef.ac.uk/uni/academic/D-
H/em/SSM/briteproperties.htm  (sorry about the line break)

They show an alloy with 17% Si that reduces the expansion 
coefficient by about 13%. While I have to agree that this is 
certainly significant, it really just reduces the steel/alum alloy ratio 
from just above 2 to just below 2. Still, it's a step in the right 
direction.

They also show other alloys with up to 40% Si which have even 
better (lower) expansion coefficients. One wonders how good their 
other properties are, however, as the ones they choose to display 
don't tell the whole story.

Some of the other material on these pages indicates that these 
aren't really alloys at all, but two-phase materials like glass fiber 
and epoxy resin. That's a pretty neat direction to be going! 

I hope John J. has been paying attention to all this. Maybe you can 
be designing composite pistons for Ford some day, John.  ;-)

> Modern technology is replacing the old technology forged aluminum 2618 pistons 
> with cast aluminum 390 pistons both at racing and OEM levels.  First of all, 
> the tooling used for casting 390 along with the different properties allows 
> less metal to be used than for forging 2618 without any degredation in 
> strength, so the pistons weigh less.  Second of all, the reduced thermal 
> expansion means that there can be a smaller piston to cylinder clearance, 
> resulting in less piston and ring wear when cold (less banging around).  
> Groovy, eh? 

I agree, pretty neat. All in all, the state of metallurgy is just 
advancing out of the dark ages.

> I thought that the post I responded to said the opposite... I thought it said 
> that too long a warmup period made cylinders crack more easily.  Perhaps it was 
> a misunderstanding my either myself (sorry!) or the poster, I dunno. 

I don't remember for sure either. I don't see why a long warm-up 
period would make a head crack. I'm just convinced that the 
warmup period is a time of high wear rates on things like pistons, 
cylinders, rings, and bearings, because of poor clearances when 
cold and poor lubrication from oil that is still too thick.  

> Per pound, that pricing is true.  Then again, the magnesium case weighs much 
> less. 

I've got a cute little book from decades ago that is titled something 
like "Designing for Alcoa Castings." In it, they point out that 
magnesium sometimes has the advantage when there are large 
thin walled sections in the casting. If cast in Al they could 
theoretically be made thinner, but the wall would be likely to fail 
from buckling, while the Mg could be thicker, thus resisting the 
buckling, without a weight penalty.

OTOH, I see that the thermal conductivity of Mg is only a little 
more than half that of Al. Maybe the alloys are closer than the 
figures I have for the pure metals....  

> But all that is speculation.  I am curious to know exactly what
> their reasoning is... 

Certainly a healthy dose of curiosity is good for each of us.  ;-)  

Micah is right, however, if the rain stops, we may take a break and 
walk down to Babcock Hall where the Dairy Science makes and 
sells the world's best ice cream.

-
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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