Thermostat data, not opinions

[krswen]

I’ve been pondering the advisability of using a thermostat. Here’s why.

The Model A cooling system is very primitive compared to more modern cars. Today, when we design an engine, the water pump is situated so that it pushes coolant into the engine, usually the block first, then through the cylinder head(s) and finally to the radiator. The impeller is designed with very specific blade shapes so that the “incidence angle” of the incoming flow is minimized. The engine represents a resistance to flow which plots as an upward turning parabolic when plotted on a pressure vs flow “performance map” of the pump. We match the resistance curve with the pump size to center the curve in the “sweet spot” of pump efficiency.

In the case of the Model A, the pump is at the engine output side of the flow circuit and all the pump does is to raise water to the height of the upper radiator header tank. Density changes, which result from the heat exchange process in the tubes and fins, cause to water to sink down to the bottom of the radiator. The pump DOES NOT push water through the radiator. It cannot since there is “head-space” in the upper header tank and it is not pressurized.

On modern engines with a thermostat, it is plumbed so that when the thermostat is closed there is an “escape route” back to the engine inlet so the flow is never stagnated and the pump is never “deadheaded”.

In an A with a hot water heater (rarely done as far as I can tell) the heater circuit would provide similar relief.

However in a typical A with a thermostat in the upper hose, when it is closed the pump will be deadheaded and the impeller will be churning in non-flowing water. For modern water pumps this would be a VERY bad thing, leading to possible cavitation and impeller erosion. The A impeller has only three blades and seems to run with rather generous (by today’s standards) clearances. Maybe it doesn’t care.

There seems to be a lot of discussion in the threads I’ve found about the desirability of “slowing the flow down” in the cooling circuit. Let’s think about that for a minute. In a compact heat exchanger (the term we use in engineering to describe automotive radiators) one of the performance parameters is the flow velocity through the water tubes. The heat exchange generally improves with higher, not lower, velocities. Balanced against this however is what we call “residence time”. The longer the coolant is in the radiator the more heat it can reject to the air (in a cross-flow design which is what radiators are). However, that same increased residence time means that coolant in the engine has more time to gain heat … maybe enough to reach boiling. Also slower flow through the engine means that the temperature difference from inlet to outlet is larger, possibly leading to increased thermal stress and distortion.

So here is my question: has anyone done controlled experiments to determine if adding a thermostat has any beneficial, or deleterious effects on steady-state cooling? No argument that during warm-up (what we call “transient conditions” in engineering) you can get the engine up to temperature faster. But is it worth the possible downside effects of impeller wear and the greater thermal gradients that have to occur in the engine with longer residence time and slower flow?

Thoughts or experiences from those who have done experiments with PROPERLY running cooling systems before the thermostat was installed?

[marc hildebrant]

krswen,

Interesting note, but the way that you have structured the question causes failures in the responses. By this I mean that you have set the engineering "bar" at a too high level that only a Model A original designer could answer !

There have been some posts regarding thermostat in/out showing that the steady state temp is higher with the use of the thermostat.

May I suggest that you try some measurements on your own ?

I am interested also in the use of a thermostat and would look forward to data you or others could produce.

Marc

[MCHinson]

krswen,

Thanks. I enjoyed reading your post.

I do not have any data. My thoughts have always been that it was designed to run without a thermostat and none of my Model A's over the years had an overheating problem without a thermostat. Properly restored, as designed, the original system works, so I have never felt the need to tinker with it.

[krswen]

Hi Marc ...

Alas, I don't have my A yet. Awaiting retirement (soon I hope). Then I certainly will do some experiments!

Ken

[Bob Johnson]

I am currently collecting more data and will post it when I have it. The thermostat that was used in the test I believe was a 160 degree thermostat. However I could not read the markings on it and it was quite old. I have since replaced it with a new 160 thermostat. The new thermostat does not have a bypass hole right now. I am collecting data without the bypass hole and will then drill a bypass hole and do more tests.

Bob

[Bruce_MO]

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Based on my experience as an engineer who installed a lot of pumps in a chemical plant, I would suggest that cavitation should not be a concern. Where I have encountered cavitation, it was in situations where there was "high lift", i.e. a good chance of creating extremely low pressure or a vacuum on the inlet side of the pump...kinda like vapor lock. I don't think that the inlet to the "A" water pump would get a low pressure situation like that. And simply deadheading a pump doesn't necessarily lead to low pressure at the inlet.

Regarding any data that someone might generate, it should clearly state the ambient test conditions, the condition/age of the radiator, etc. My bottom line is that I think that there's so much variability among owners' cars & conditions that I would think it's pretty hard to generalize if a car should/should not run with a t/stat.

[Frank Miller]

The pump DOES NOT push water through the radiator. It cannot since there is “head-space” in the upper header tank and it is not pressurized.

It sucks it through. At least it will if there are no restrictions in the radiator.

[Special Coupe Frank]

My '28 has a new Brassworks "heavy-duty" radiator (10 fins/inch), new "leakless" pump, new headgasket, and no thermostat.

Put a temp gauge on it about a month ago ( new water outlet with a 1/2" NPT tapping).

When the engine is thoroughly "warmed-up", it runs around 155-165 deg F.

When the ambient air temp is below 70 degrees F, it can take 15 minutes / 10 miles driving to get "thoroughly warmed-up".

Generally, any trip with the Ford is at least 20 miles long...

I do have a 160*F stat for the upper hose (modern stainless "poppet" type, mounted to a sleeve), with a 1/8" "by-pass" hole. I plan to install this soon, and see what effect that has on things.

I am confident that most of my cooling system is fairly "clean"; certainly the radiator is ( less than 1,000 miles on it).

Will report my particular findings.

[Kirby1374]

Originally Posted by Frank Miller

The pump DOES NOT push water through the radiator. It cannot since there is “head-space” in the upper header tank and it is not pressurized.

It sucks it through. At least it will if there are no restrictions in the radiator.

While reading this all I could think of is what Frank Miller said

[Jim Huseby]

On the subject of varying densities, varying temps and the resulting possibility of thermal stress (and areas of the engine, especially near the back of the head, where it has been proposed that steam develops and the coolant cannot carry away the heat effectively from those areas) : does pressurizing the cooling system allow the stock water pump to push coolant through the system instead of just raising the coolant to the upper tank? 'Seems to me that closing and pressurizing the system allows the pump to both push and pull. In my layman's mind, I imagine that those "steam voids" would then have a better chance of having liquid coolant forced through them. 'Seems to me also, if I remember from physics classes more than 40 years ago, that water can be pulled upward, against gravity, a certain number of feet (32?) , so as the stock pump in the unpressurized system raises water to the upper tank, it must also pull water through the system except for those "steam voids", where the pulling action may exacerbate that situation. What are the facts here relating to pressurizing?

[krswen]

Hi Frank ....

Yes, the pump does suck somewhat on the circuit, but the radiator is on the far side of the engine. The pump has to suck water from the lower radiator tank through the restrictions in the engine. Pumps are much better at pushing than sucking. If the suction gets too deep, cavitation can occur. It would be interesting to measure the pressure at a number of locations in the circuit, especially since the system is unpressurized, so the highest achievable "head" is just what's necessary to lift the water up to the upper tank.

Apologies to all if I'm making too much of a science project out of this. The curse of being an engineer.

Good discussion thoughtful discussion though.

[Ron in Quincy]

My great uncle purchased his new 29 AA from the Ford Dealer in Stockton, Ca. just prior to harvest time. He told me that when idleing the truck in the field it would get hot and boil over. He said he started experimiting with the water pump impellers as he felt it was pumping the water too fast into the upper tank. He said he found with a 3/16" hole drilled in each impeller it stopped boiling over when idling in the field.

With each and ever A I have owned or restored since 1962, I have, when rebuilding the water pump, drilled a 3/16" hole in each impeller. I fill my radiator, either 28-29 or 30-31, just to the bottom of the baffel, allowing room for expansion. I, myself have never used a thermostat and respect those who choose to use one. I've had a number of radiators recored and always insisted on a heavy duty core ( 10 fins to the inch). Never wanted to ruin a new rebuilt engine trying to use an old original radiator; some old radiators will flow fine but if the fins are loose on the tubes its not going to cool properly.
Another advantage to slowing down the volume water into the upper tank is to reduce the chance of cavation in the engine block.

This is how I feel and what I've done over the years; I'm sure some of you will disagree with me and you certainly have a right to your opinion.



Ron

[Special Coupe Frank]

I overfilled the radiator on my A the other day, and while it was running at a fast idle ( 600-800 RPM ?), I figured I'd just dump some water from the pet-cock on the lower radiator pipe.

Well... much to my surprise, when I opened the petcock, instead of water draining out the pet-cock, it began to flow vigorously from the overflow tube.
I closed the pet-cock, and the water stopped flowing from the overflow tube.

Apparently the "suction" on the lower-end of the cooling system was great enough to prevent water from draining out the pet-cock, and instead it drew-in air, allowing enough ... something... for the pump to push water from the block up into the top tank and out the overflow tube.

It's a new, clean Brassworks HD radiator, so clogged tubes are not the issue.

When I was done with this unplanned experiment, I idled the engine down to its normal idle (300-400 RPM), and checked the water level: it was back down to the baffle, so I put the cap on and drove away...

I guess I learned that the Ford water pump will move quite a volume of water, given the opportunity.

[Larry Brumfield]

The Model A cooling system works on the principle of thermo-syphon just like the Model T except with the aid of a centrifugal pump. The water gets hot, rises and moves forward to follow the slant of the water hump toward the front where it is assisted by the action of the pump. The movement will naturally suck water or syphon at the block entrance.


Larry B.

[1931 flamingo]

I think many install the t/stat to slow the volume of water being sucked and try to prevent it getting into the o/flow tube. S/bulletin says to "form" the o/flow tube to rar. I was losing quite a bit of water before doing these two mods and have almost totally stopped loss of coolant. It worked for me.
Paul in CT

[krswen]

here's a link with a good description of how cooling systems work:http://www.arrowheadradiator.com/14_...utomobiles.htm

[marc hildebrant]

Group,

Recently I cleaned out my radiator with "simple green". I also replaced the 50/50 antifreeze coolant with water. Good reason for a road trip.

Air temps in the mid 80's today. Started out at 5000 ft. The engine temp ran up to 160 as I approached the Foothills of the rocky mountains.

Drove up to Jamestown CO at 7000 ft. Great ride up through a nice canyon. Car temp stayed around 160 the whole way up.

Drove back down...through the canyon the temp went to 100 as I dropped 2000 ft.

Back home in the city, around 150.

I do not have a thermostat in the car....but....seems that I could use one to keep the car from cooling down !!

Marc

[Gord. B by the bay]

Krswen as you asked "just the facts" Rad 10 tubes to the inch---- fluid used water---average speed 45-50 mph---various elevations---- using a 160F thermostat---- drove across the USA ---- constant reading---- 160F ---
50,000 miles on engine--- running well ---30 Model A roadster
Have a great Day
Gord, B by the Bay

[40 Deluxe]

Just throwing out something to mull over while lying awake at night: several posts refer to the "suction of the water pump", but technically there is no such thing as "suction"! Picture drinking a glass of water through a straw. You expand your lungs creating a low pressure area in your mouth and throat. Atmospheric air pressure (14.7 lbs./sq. in. is pushing down on the surface of the water in the galss) always moves from an area of higher pressure to an area of lower pressure. Thus atmospheric pressure pushes the water up the straw into your mouth as it seeks the low pressure area of your lungs. You are not really sucking water up the straw! In the Model A, as the pump pushes water up into the radiator, there is less water now at the pump inlet so there is a low pressure area there, so atmospheric pressure in the upper radiator tank pushes more water into the pump impeller as it tries to reach that low pressure area. The pump doesn't really "suck" anything. A vacuum cleaner doesn't suck up any dirt, either! It creates a low pressure area inside the tank or bag and hose so atmospheric pressure again tries to equalize the pressure inside the vacuum cleaner so it pushes its way into the hose, carrying dirt with it (introduce a low pressure area above the dirt, and the atmospheric pressure under the dirt pushes it into the vacuum). One more point: Can you pull (or suck) water through a hose? How? Do you put a hook in the water to pull it, or tie a rope to it? No! A liquid (like air) can't be pulled, it has to be pushed.

[marc hildebrant]

Gord. B by the bay,

When you traveled across the USA, did your car have a thermostat in it ?

Marc