Mark Sirota asked:
>On a related topic -- those of us towing with cars, not trucks,
>often find that the limiting factor isn't power or brakes, but
>cooling.
>Is there something we can do to modern street cars to increase
>their cooling capacity without spending an arm and a leg and
>screwing up everything? And given that everything is a tradeoff,
>what would we be trading off?
Back in the days when US car = Big RWD thing with a V8 and body-on-frame
construction, trailer tow packages were factory-available. Today, there's
only one car family in the US with a factory trailer tow package (to the
best of my knowledge, which admittedly is not complete) and that's the Ford
Crown Vic, Mercury Grand Marquis, and Lincoln Town Car family. And even
these have gone to electric cooling fans.
So, first off, if you're buying a car (or, for that matter, a truck) to use
as a tow vehicle, check to see if there's a factory tow package. If there
is, by all means order it. Tow packages can include a bunch of useful
stuff -- high-output alternators, bigger/higher-capacity radiators, engine
oil coolers, auxiliary trans oil coolers, trailer wiring leads, and
sometimes heavier-duty springs and shocks. Of course, if you don't have a
factory tow package available, there are a couple of options: a) buy
something else that DOES, b) pays your money and takes your chances, or c)
make some judicious additions to help your vehicle of choice do the job.
Towing does increase the heat output of the engine and transmission, so you
have to get that heat dissipated. There are 4 main avenues for heat to
leave an engine/trans combo: coolant, engine oil, trans fluid, and
exhaust.
The obvious stuff first: make sure the grille and the heat exchangers
aren't blocked with dirt, mud, dead bugs, dead birds (don't laugh... I've
seen my share of unfortunate avains impacted on radiators and a/c
condensers), and that the fins are intact and not squished flat (that
reminds me... the Probe's a/c condenser needs a good fin combing before
summer...). Make sure the cooling system is full of good coolant mixed
50/50 with clean water, and that the pressure cap is operational and seals
well against the filler neck. Make sure the oil and trans fluid are full.
Next, the exhaust -- obviously, a low-restriction exhaust system will help
with power production, but it also helps with heat dissipation. If you
upsize the pipes, though, be sure to keep good clearances to other
components (floorpan, suspension bits, and in particular fuel and brake
lines). A good rule of thumb is to have an absolute minimum of 1"
clearance from the exhaust to other metal bits, and minimum 2" to tubes and
pipe and plastic stuff.
Automatic transmissions, make a lot of heat, primarily through the slipping
of the torque converter. Most passenger cars have an oil/water exchanger
for the trans fluid that is located in the cold-side tank of the radiator.
If you're towing, adding an auxiliary oil-air cooler for the trans fluid
downstream of the oil-water cooler can help (there are some side-effects,
as you might expect -- more on that later). Make sure your has a thermal
bypass -- it will route around the cooler below a certain temperature (say,
180F), which is primarily to prevent extreme-cold-weather difficulties when
the trans fluid gels (at around -20F and below). You'll want to place it
where it will get as much fresh air it can, and minimize the length of the
hose run. Hoses should be specifically rated for trans fluid -- do NOT use
heater hose as trans fluid will chemically attack it and the hose will
disintegrate into little bits that will clog up the works. Adding an
auxiliary trans cooler may not help coolant temperatures very much, but it
will help transmission temperatures, and if you're towing with a car that's
really not intended for towing use, you *will* get the trans hot and it
*will* considerably shorten its useful life.
As a rule, engine oil doesn't have a cooler on most cars (there are
exceptions to this rule, don't send me "But *my* car has an oil cooler from
the factory!" notes. Two of my cars have factory oil coolers, too).
Adding an engine oil cooler will help a surprising amount toward helping
the engine stay cool. While there are oil-to-coolant coolers available,
I'd consider an oil-to-air cooler, again with a thermostatic bypass that
shunts around the cooler at oil temps below 180F or so (every
commercially-available bypass I've seen is set to 180F for some reason.
I'd personally prefer about 210F for oil, and I'll say why below). These
can be plumbed in with adapters that sandwich on the oil filter pad. The
most common factory engine oil cooler is a "hamburger" style oil-to-water
cooler that's on the filter pad and plumbed into the cooling system on its
water side (typically by using an orifice in the lower rad hose to induce a
differential pressure; the '96-up Mustang Cobra 4.6L DOHC has a "full-flow"
cooler built into the water inlet/oil filter adapter casting). If you add
an engine oil cooler and make no other changes, odds are pretty good that
you will see a noticeable drop in coolant temp as well, and may not need to
do anything else. I'd also recommend synthetic engine oil in cars seeing
tow duty. I'm very partial to Mobil 1, but Valvoline Synthetic is also a
fine product. Castrol Syntec will never, ever touch one of my cars.
For coolant, the first thing your average guy will say about increasing
radiator heat-rejection capacity is "throw a double (or triple, or quad)
row core in that thing and it'll cool." Well, maybe it will, maybe it
won't. It depends on whether you can get enough air through it, and if the
airflow is marginal already, increasing the resistance to airflow by
increasing the number of rows and the width of the fins it has to go
through won't help and may hurt.
So the first thing I'd do is look at what you can do to get more air
through the radiator. Bigger fans (if there's room), more fan(s) (again,
if there's room), and maybe turning them on earlier, which is the first
thing I'd try. It's a little tricky with modern computerized cars where
the fans are controlled by the PCM, but if you can find the relay(s), you
can wire in a switch into the trigger side of the relay (in parallel with
the existing trigger circuit). If you're really clever, you have a manual
switch on this circuit in series with an adjustable thermostatic switch --
when you're not towing, flip the manual switch to "off" and it runs the
fans at factory settings. Hook up the trailer, and flip the switch, and
now it'll run on your adjustable thermostatic switch, which you will have
set to turn the fan(s) on earlier than the factory settings.
The idea is to try to stay on top of the heat gain in the system. Most
factory electric fans I've had experience with kick in at around 220F
coolant temp on low speed (if it's a 2-speed fan), and hit high speed at
around 230F. There's a little hysteresis built in (on at 220, off at 215,
for example) so they're not just switching on and off rapidly, which is a
Good Thing. So you set your adjustable thermostatic switch to kick the fan
on (let's say you have a two speed fan and you've wired up both relays with
their own t-stat switches) at, say, 205F and it kicks up to high speed at,
say, 215F. It'll make a lot of noise, maybe, but it'll help with the
cooling.
Obviously, if you added trans oil and engine oil coolers in front of the
radiator and a/c condenser, you've added some resistance to airflow through
the "stack" of heat exchangers, so having the fans working a little harder
is an even better idea in that circumstance. This is the side-effect I
hinted at upstream. As with any other kind of development changes, it's a
good idea to change one thing, see what its effect is, and proceed to the
next change if necessary.
There's an "acceptable" range of temperatures for various fluids. For
engine oil, you'd like it up over 212F so that any moisture condensation
will boil out of it, but you'd like to keep it below 300F, preferably below
275F or so. For automatic trans fluid, again, you want to get over 212F,
but try to keep the sump temperatures below 250F. Manual transmissions
usually don't get terribly hot, but rear diffs can -- upwards of 350F in
loaded trucks! Synthetic gear lube is a Good Thing, so is some sort of
finned diff cover that can help dissipate the heat.
Coolant... lots of people panic if the coolant temp gets over 220F. Don't.
I can't think of a car available for sale in this country that starts
taking drastic measures below 238F (by drastic measures, I mean cutting the
a/c off, and/or going into some sort of spark-knock prevent strategy with
the fuel and spark curves), and the cooling system shouldn't have any
boiling problems in a operational system with 50/50 ethylene glycol/water
mix until you get to around 260F (a nominal 13 psi cap will give a boiling
point for 50/50 of about 259F, a 15 psi cap will give a boiling point of
265F). Mind you, if it hits 250F, I'd seriously consider backing off for a
bit and letting it cool down. My "magic number" for acceptable performance
is 245F in extreme conditions (here, we're talking 100F+ ambient air temp,
loaded to gross combined weight rating, and pulling a 4-6% grade). Most
folks don't do the kind of bizarre stuff we do when we're testing cars and
trucks (let me put it this way: we want it to be 100F at the TOP of the
mountain pass, over 5000 ft. altitude...), and thus most folks shouldn't
have any problems with their cooling systems provided they're maintained
properly.
Also keep in mind that factory temperature gauges, are, for the most part,
not terribly accurate. At least one automaker builds in a HUGE dead spot
in the middle of the response curve, for example, because they've found
their customers (in the US, at least) want to see that sucker get right to
the middle of the "normal" range and STAY THERE, and if the gauge is too
responsive to changes in coolant temp, their warranty claims for cooling
system problems goes way up even though there's absolutely NOTHING wrong
with the system. If you're doing a lot of towing, I'd consider adding
three aftermarket gauges -- an accurate coolant temp gauge, an engine oil
temperature gauge, and (this is the biggie) a transmission temperature
gauge for your automotive transmission.
Hope this is useful,
Jim Crider
autojim@delphi.com (home)
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