The story about coolant flowing through the radiator too fast keeps
reincarnating in various forms. Stuart is right; the hotter the radiator,
the more heat is dissipated. The erroneous notion about too high a flow
rate seems to have originated back with the flat-head fords. Someone
discovered a cure for their overheating problem which was to but about a
3/4" restrictor at the output of each head. Problem is, they mistakenly
atributed the improvement to restricting flow, whereas the improvement was
the result of increasing pressure inside the heads. Fact is, both flow and
presssure are good; you just can't have both at the same time. The optimum
is a trade-off with enough flow so that the temperature drop through the
radiator is small, while at the same time building as much pressure inside
the engine as possible. With a 15# cap and a good water pump you can
prbably get 30 or 40 psi inside the engine which prevents local boiling and
cavitation. The heat capacity of ethylene glycol is a lot less than water
(bad), but it has a higher boiling point (good). A compromise at about
50/50 seems to be optimum. It's what I use, although if you do racing it
may be forbidden because of the mess it makes when you spill it and also the
fire potential. Your motor will probably run a few degrees cooler with pure
water, but there is less margin for overheating. Regarding thermostats, why
would anyone use a thermostat less than the desired operating temperature?
I agree with Tom Hall and others (including all auto manufacturers in the
free world) that operating your engine around 195-205 degrees is ideal in
terms of performance (mileage, engine wear, and power). If you are doing
all-out racing, you would problably err on the cool side (if you're lucky
enough to have this option) to avoid local hot spots developing under heavy
load. I wish they made thermostats between 180 and 195. I'd probably pick
185 or 190, but certainly no reason to put in a 170 or 160, except maybe to
provehow good your cooling system really is. But operating an engine at
these temperatures for any extended period is acutally bad for it; it
increases engine wear like crazy and lowers gas milage. A lot of racers use
a restrictor hole instead of a thermostat. One advantage I can see is that
it is so simple it won't fail. Also, a thermostat may actually open up too
much and not provide enough restriction to maintain good pressure inside the
motor.
Enough of this for now. Maybe later I'll tell you what I think about single
versus multiple pass radiators. Mine is a Ron Davis three-pass aluminum I
got through Dale Akuszewski. It works excellent, but is only part of the
overall story of cooling the cat.
> First of all, my Tiger is mostly stock, and the gauge appears fairly
> accurate. Original radiator, rodded out once or twice. My worst case
> drive each year is coming home from British Car Day in Brookline. Mid
> afternoon, sunny, hottest day of the week, if not the month. This year
> was in the mid 90's. Less than a minute between stoplights for the
first
> half hour, or so it seems. Anyway, the temp gauge gets up to the 230
> area, but I don't blow any fluid. Once I get free of the
stoplights, it
> slowly comes back down to 215-220. I have a 170 thermostat, a 7 lb.
cap,
> and keep the fluid below the "half full" level of the tank.
>
> I will admit that I'm the wrong kind of engineer for this thermo stuff,
> but here's some things to think about.
>
> If you have "slow flow", the coolant comes out of the radiator much
> cooler than it went in. Fine, but remember that back in the engine the
> coolant is also flowing slow, so the temperature is rising....
>
> Heat rejection is proportional to temperature difference. If the whole
> radiator is at 100 C, as it would be with a really fast flow, it is
> rejecting more heat than the same radiator with slow flow, and the
> coolant coming in at 100 C and leaving at 50 C. Remember we are
talking
> about heat, not just temperature.
>
> Analogy: Your the hot water baseboard heat in your house. If the
furnace
> is running and the circulator pump is running real slow, the first room
> gets a little warm, but the rest of the house doesn't. And the
furnace
> runs only part time, since the pump is running so slow that the
water in
> the furnace can get up to the shutoff temperature. Ask me about Feb.,
> two years back. With the pump running at full speed, all the rooms get
> warm, though in the last room the baseboards are a bit cooler, and the
> furnace can run full time.
>
> I know there are folks who can site successes on both the slow and fast
> flow sides of the argument, but I just have a little more faith in the
> fast flow side.
>
> Stu
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