Radiator Cap--flow rate through radiator

[Johnny]

John Hess wrote:
> 
> You got it!  Did you get the thought about not trusting the temperature
> sender to tell you the actual coolant temperature, even at the point the
> sender is located?

I have done a few tests related to all this, and here is what I came up
with. Now, this is regarding the SBC, your milage may vary.

While experimenting to try and come up with the correct size radiator
for use in my airplane, I was up against several problems. As with all
aircraft stuff, weight is the big bad boogy man that you always want to
avoid, pretty much at all practicable costs. This dictated an aluminum
radiator, and it had to be compact enough to fit where I wanted to put
it, yet still cool the engine when run at 80% power continuous. The
thing I had going for me is that at altitude the ambient air temp is
usually below freezing, or at least much cooler than on the surface.
Also, at 200 mph there is plenty of air flow to get a decent pressure
drop across the radiator with. Problem is that you still have to
oversize things so that you can run the thing on the ground long enough
to taxi out and takeoff without the engine temp becoming a problem.

What I found that suprised me at first was this:

The cooling system wanted to go into thermal runaway during full power
ground runups within about 10 minutes when I had the 160 degree
thermostat in there. It took over 15 minutes with the 180 degree
thermostat. It took over 20 minutes with the 195 degree thermostat
installed. This is an exceptable amount of time, as in normal operation
you would never sit there like that for that long. Also, keep in mind
that there is no fan on the radiator and the only airflow you get when
sitting on the ground is from the prop blast. As soon as you are flying,
the amount of airflow through the radiator is greatly increased.

When measuring the air temp on both sides of the radiator I found that
the intake side was always the same (of course) and the outlet side had
the highest air temp when using the 195 degree thermostat. This wasn't
really a surprise, but to me it indicated that more heat was being shed
than when the exit air wasn't as hot.

This was confirmed when measuring the water temp at the inlet and outlet
of the radiator. The inlet side, when flowing, almost always indicated
the same as what the thermostat was (again, of course), but the outlet
side would vary widely depending on power setting. To level the playing
field, lets just assume we are comparing numbers at equal power
settings, say full power for example. What I found is that the inlet
temp would be about 160 with the 160 degree thermostat, and 195 with the
195 degree thermostat, giving a difference of 35 degrees. But on the
outlet side, the temp was much closer than that. I was getting only
about a 10 degree difference on the outlet between the 2 different
thermostats. I attributed this to the fact that the radiator did more
work when the temp differential between the water and the air was the
highest.

One of the most intersting things I noted was that when measuring water
temps in different parts of the engine, at the same time, I found that
as I removed the restriction at the thermostat housing, I would get to a
point where the rear of the engine started to get hotter. Or I should
say, the higher the temp differential got between the front (where the
in and out are) and the rear. This would indicate to me that the water
passages are designed so that the flow throughout the engine is most
equal with a certain amount of restriction in place at the thermostat
housing. When you remove the resistance to flow at that point
completely, the water must have a tendancy to short circuit the rest of
the block, and the stuff in the back must slow down, or go backwards, or
sideways or something. This would explain why some engines have a
tendency to overheat when you remove the restriction, more than others.
The one that comes to mind the most is the BB Chrysler. You can put a
huge radiator on there, and it just doesn't matter. If you remove the
thermostat, and then work the engine hard, it will overheat. At idle,
the same engine will run too cool. Another engine that has about the
weirdest cooling charactoristics I have ever seen is the 400 pontiac.
It's one of those where if you get a little air in the water pump, it
just won't ever purge it while it's running. If it runs low on water
while running, you have to shut it off, wait for it to cool, fill it all
the way up and then start it again. The fill while hot and running trick
just don't work on that one once it's in "foam mode".

Anyway, I ended up with a slick little 7 row job with no filler and AN
fittings. I am going to have to put a trap door on the belly scoop that
mounts it so that I can flow more air on climb out and then close it
down at altitude to reduce the cooling drag associated with taking air
in and then getting a pressure drop accross something inside the
aircraft... in this case the radiator... before you jetison the air back
into the stream, hopefully in a direction that is the opposite of the
one you are traveling in.

So, the moral of the story is this:
If you make more HP than before, you will also be making more waste
heat. You will not always be able to take advantage of an increased flow
of coolant if your engine design spazzes out when you start to increase
the velocity that the water passes through the passages at. Once you
reach the max flow rate that will work with your engine, you will have
to increase the amount of heat sinking capacity of the radiator. Going
aluminum, going bigger, using more fan, doing a better shroud job than
before, bringing more air into the grill... something. But no matter
what, the engine will work the best, make the most power, last the
longest, and get the best milage, if it operates at it's optimum temp at
all times. Under almost all conditions this means using a thermostat.

-j- (I usually use the highest thermostat and pressure cap that I think
the cooling system will tolorate)