Eric the Knuckledragger asked about coolant, and a bunch of people
responded.
Here's a response from someone who designs vehicle cooling systems for a
living (that would be me):
Strictly looking at the heat transfer coefficient, straight water is the
way to go. HOWEVER... straight water has its problems, notably a lack of
certain additives that prevent cavitation of the water pump at high speeds,
corrosion of the various metal bits present in all engine cooling systems,
surfactants to lower the surface tension of the coolant (allowing it to
"wet" the surfaces of the coolant passages better) and anti-foaming agents
to keep the surfactants from making big bubbles.
All these are present in antifreeze/coolant. The surfactants and
anti-foaming agents are present in Redline Water Wetter.
Water Wetter has limited to no benefit in a system using a commercial
coolant -- it's simply adding more of something already present in
sufficient quantity.
Another drawback to straight water is that the freeze point and boiling
points are closer together than a mix of coolant and water. A 50/50 mix of
ethylene glycol and water will boil at 220F at atmospheric pressure at sea
level, compare with 212F for straight water. A pressure cap, by the way,
adds about 3 degrees F to the boiling point per psi above atmospheric
pressure. And of course, water freezes at 32F, while a 50/50 EG/W mix will
freeze at about -35F. This of course is useful should you live somewhere
that gets cold (that would be most of the country this winter, it seems).
There are two types of base coolant stock available right now: Ethylene
glycol (EG) and propylene glycol (PG). Currently, no engine manufacturers
selling product in the US recommend PG (sold by Arco as Sierra brand
coolant), most caution against it (check your owner's guide). PG has a
higher boiling point than EG (straight), but has a lower heat-transfer
coefficient. EG coolants also come in several flavors, depending on the
additive package (more below).
BTW, PG isn't truly non-toxic. It's LESS toxic than EG, but PG coolant
contains various and sundry additives that aren't really good for you. The
LD50 (lethal dose in 50% of ingestion cases) for PG is about 4 times less
than EG's LD50, but that's means we're talking along the lines of 16 ounces
instead of 4 ounces for a small kid or large dog. And once it's been in a
cooling system, it's picked up contaminants (metals, etc.) that aren't
terribly good for you. Arco got in a fair bit of trouble and had to
re-label Sierra (including all the stuff already on store shelves) a few
years back when the FTC challenged their advertising claims and found them
lacking in accuracy. Basically, the less-toxic claim only applies if you
pour the stuff straight out of the bottle and onto the ground. Don't
bother with it. And treat *any* used coolant as low-level hazmat. Small
amounts can be disposed of in sanitary sewer systems, but you're better off
making nice with the operator of the neighborhood quickie lube place, who
will be able to take it off your hands and get it into the recycling
stream, sometimes for a nominal fee. You're already talking to him about
your used motor oil anyway, right?
The green-dyed EG "conventional" coolant we all know and love has an
additive package based around a silicate (and sometimes also phosphate)
based anti-corrosion additive. It's well-established and does a good job.
It can go 5 years/50K miles without worry.
A few years ago, someone thought a long-life coolant (original plan: life
of vehicle) would be a Good Thing. This lead to Organic Acid Technology
coolant (OAT), which is marketed as "DexCool" by GM and has been
factory-fill in their products (except C4 Corvette -- not sure about C5
Corvette) since 1995. It's the orange or orangy-red stuff. Someone along
the line decided the word "acid" was a Bad Thing to try to sell, so OAT was
recursively changed to Organic Additive Technology. It can go 5 years/100K
or 150K miles -- provided it's not mixed with other coolant. OAT has less
cavitation resistance than silicate-based coolant, and can attack certain
sealing materials, so it's not a good idea to convert a green-coolant car
over to OAT unless the manufacturer says it's okay. OAT also has a
tendency to stain translucent plastics in things like overflow bottles and
pressurized de-gas bottles with a funky brown crud. Oh, and OAT from one
manufacturer isn't necessarily compatible with OAT from a different
manufacturer. Texaco is GM's OEM supplier and is licensed to use GM's
"DexCool" trademark on their aftermarket packaging. I'm not aware of any
other company being licensed to do so.
Many European automakers use a hybrid of OAT -- HOAT (Hybrid Organic
Additive Technology -- clever, huh?), which is the OAT package with a small
amount of silicates added to increase the cavitation resistance and make it
less aggressive against those seals and gaskets. This is often pale yellow
in color. DaimlerChrysler is using it in several car lines now, too,
notably the LH sedans and the new minivans (It's possible the
Sebring/Stratus twins now use it -- I don't have that handy at the
present). This stuff seems to offer pretty much the best of both worlds --
it's not quite as long-lived as straight OAT, but it is much better behaved
in operation than OAT, much like conventional coolant.
Note that these three different additive packages are not really
cross-compatible. No, they won't eat the insides of your radiator if you
mix a little of one in with another in a pinch, but you'll be better to get
the system flushed out and a fresh mix of 50/50 whatever your car needs put
back into it.
In my own cars, I run a 50/50 EG/W coolant mix. I happen to own cars that
take conventional coolant, but if I owned a car that came with OAT or HOAT
from the factory, I'd likely stay with it. The anti-corrosion additives,
in particular, leave residues on the walls of the various coolant passages
(that's how they work -- the residues coat the base metal and prevent
corrosion), and it's tricky to convert an engine that's been run with one
style of package to use another package and get the full benefit.
Switching from conventional to OAT, for instance, requires a mild acid
flush of the cooling system after removal of the conventional coolant and
before pouring in the OAT if the long-life corrosion benefits of the OAT
coolant is to be realized. Just pouring the OAT in after draining the
conventional won't gain the full measure of added coolant life the OAT
marketers (notably Texaco) like to use as selling points.
Radiator caps are a whole topic unto themselves. Ask me about them another
time -- my lunch hour is over and I gotta get back to work. :)
Hope this is helpful.
Jim Crider
autojim@delphi.com (home)
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