I'm buying a new radiator, stock w/o AC, and an electric fan for my stock supersix. I have seen several threads regarding the fan switch/thermostat topic, but never found anything about what is the ideal water inlet temperature for the slantsix. Since the radiator is of the downdraft type, it seems natural to me to have the thermostatic switch as close to the inlet to the water pump as possible, and not in the top tank where the fittings normally are placed. That will give the most stable temperature to the engine, but may not catch a situation right away if the engine is boiling - perhaps a setup with 2 probes will be ideal - one down in the hose and one tank-mounted? I will run a 195 or higher thermostat in the cold season, 195 or maybe a 185 in the summer, it's not exactly the tropics up here!
Has anyone got a tip regarding a hose-mounted thermostatic switch for the fan, or any other suggestion?
What should the start/stop temps be for the fan related to the thermostat temp when the thermswitch is in the lower radiator hose (or nearby), or in the top tank, respectively?
Olaf

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Aspenized

The hotter you can run the engine (up to the metallurgical limit) the more efficient it will be. Use a 195F thermostat year-round. If the radiator overcools during the winter, you can block off part of it during the cold months. The fan switch needs to be in the outlet tank of the radiator (bottom in your case). If you put it in the inlet tank, the fan will always run once the engine is up to operating temperature. The purpose of the fan is to provide radiator airflow when there's not enough airflow from vehicle motion. Thus you need to sense the radiator outlet temperature.

Ken

If you run a 195 degree stat use a 210 degree switch in the top of the tank. Then it runs very rarely, only in traffic jams waiting on lights because the airflow is enough to keep it cool. In the last 5 years driving to work, 65 miles the fan never comes on. That drive includes several stop lights. You can read about my setup in the FAQ engine sticky and click on the red link below my name to view pictures. Once they come up, double click for full screen slide show mode.

I have tried to measure outlet temps since you asked but they very allot. Using a hand held laser temp reader I see anywhere from 80 to 100 degrees at the bottom of the tank after a long run. Much colder on short local drives.

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Aggressive Ted

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True, but with a carbureted engine a practical limit on engine operating temp is in the fuel system, not in the engine metallurgy. Run too hot and the driveability problems aren't worth it. Likewise, without knock sensing and with a primitive combustion chamber, another practical limit on engine operating temp is tendency to ping.
180° or 195°, yep, depending on the specific parameters of the engine and car. Don't use lower than 180° or higher than 195°.
If the radiator "overcools", there's something the matter with the thermostat!
I agree. You can try and guess at what inlet temperature will correlate to the desired outlet temperature, but it's better and more efficient to sense outlet temperature directly, because that way you're only running the fan when it's really, actually needed.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for the answers guys. Ted, you illustrate what I am trying to find out when you describe the temp findings you could see by measuring the temp off the bottom tank, it will give the engine varying intake water temperature all the time. I was thinking about mounting a larger radiator, i.e. one for car with AC, but dicided against it. The reason is that it may be easier to control the temperature with an efficient fan for a small radiator, than relying on the capacity of a large radiator that will behave different according to surrounding temperatures and driving pattern. The question is what to make a priority, the best for the engine or the best for cold feet inside the car, and how to find the balance. I have come to the conclusion that I am going to mount the temperature switch in the lower hose, I found a very nice setup for doing that. I am also going to mount temperatur sensors in both upper and lower tank and try to monitor the temps over time to see how efficient the radiator/fan is and see if my decision is good. I have lots of old controller chips and memory lying about, so I'll try to rig some kind of logging function to read to a pc for making a spreadsheet with the results. I'll publish what I find, eventually. Any tips or comments for the setup?
Olaf

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Aspenized

And BTW, I am installing a Motorcraft carburetor and a mount for an Innovate lambda to see that the intake mix gets right.
Olaf

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Aspenized

Get some k thermocouples (thin gauge, perhaps #26 or so) and slip them between the hose and nipple. You can snug a hose clamp over the hose to hold them in place. Make the thermocouple leads long enough to reach the passenger cabin. Any good multichannel datalogger or decent DMM has an internal reference junction and can read the thermocouples.

Ken

Great idea, Ken, thanks.
Olaf

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Aspenized

Its always an engineering tradeoff. Remember that the radiator has to be sized to provide adequate cooling on the hottest summer day with the engine producing maximum power and the fan not running.

Collecting experimental data will give you an intuitive understanding of heat transfer. It can be mathematically modeled as an electric circuit. Go to the library and read any book on heat transfer if you want a thorough understanding.

Ken

Ken wrote: Ken, I don't totally agree with that. I am going to keep this engine relatively stock, and I believe the Mopar designers found that the small original radiator had sufficient cooling capacity, but that was with an engine mounted fan running all the time. A stock fan also have the advantage of increasing the cooling capacity by moving more air according to the load of the engine assosiated with higher rpm, and as I understand, the stock fan is a very efficient tool in that respect! My car also came without a fan shroud that would have given some additional help in cooling.
I can calculate up to a point what I can expect regarding heat transfer, but using an electric fan with a fixed speed that only runs for short periods of time, it will be some guesswork at the end, I am not sure I want to sit and do the maths, when I can have more fun by getting real world practical input from you lot! This forum is after all the best on the internet!
Olaf

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Aspenized

Radiator Outlet Tank temp must be less than 195 before the coolant enters the engine if the engine coolant temp is to be 195 at the thermostat?

Isn't the desired Radiator Outlet Tank temp variable depending on engine load since heat input varies with load?

Measuring the temp at the Radiator Inlet Tank measures the end effect?

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If the engine is heating the coolant then, yes.

If there is air flow through the radiator and the air is cooler than the coolant then the coolant outlet temperature will be lower than the coolant inlet temperature. How much cooler the outlet temperature needs to be to keep the outlet temperature at 195° will depend a lot on load and coolant flow rate.
Load, coolant flow, cooling air temperature and flow will all affect the radiator coolant outlet temperature. Many German cars have bypass type thermostats so that the coolant temperature flowing into the engine is regulated. Older American engines are slightly less sophisticated, but the coolant bypass does play a significant role in keeping the input coolant from being too cold. The engine would like it's temperature to be relatively constant throughout (no cold or hot spots) so an increase in coolant flow is preferable to colder inlet coolant. This has lead to inlet side thermostats on some newer vehicles.
Measuring the radiator outlet temperature is most valuable for activating an electric fan in the event that the coolant temperature drop across the radiator is inadequate due to low air flow.

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Joshua

Joshua, what you say about modern european engines is very relevant. The hose adapter that I will use lists only 2 temp switches, 92 and 95 degree C (197 F and 203 F), and they are used by, among others, Saab, VW, and Audi. But then again; these are fuel efficient light alloy engines that produces much less heat, and gets rid of it much faster than our cast iron slants.

Your collective input bring me back to my original thoughts about using 2 switches, one high temp in the upper tank to take care of potential overheating situations only (212 - 220?), and one lower temp (185-195?) in the lower hose for as even inlet temp as possible, and a 195 thermostat. Those figures may need some adjustment after testing.

Since my brain already spins in overdrive, he ideal construction would also include some kind of air restriction for low temp operation, perhaps some kind of temp controlled shutter ahead of the radiator.
Hmmm...
Trucks!
Hmmm...
Olaf

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Aspenized

Some additional observations; my dad's old VW Passat with stock electric fan worked OK, but in -33 F the heater turned to cooler, even with a cardboard plate covering the entire front of the radiator. The fan usually ran in city stop-and-go traffic, and had a very annoying habit of turning itself on the minute you turned your back on the car after parking, when the heat rose. After running hard in the summer, the fan could run for several minutes after shutting the engine. It never overheated.
Olaf

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Aspenized

A lot of old cars had thermostatic shutters on the radiators. I used to do restoration work on old cars so I've been exposed to Packards that had such a system.

I think one of the 3-hose Mercedes thermostats would be the easiest thing to adapt to a slant if one wanted to get a handle on coolant inlet temperatures, but this is probably solving a problem that hardly exists.

For the fan control you could easily have two switches to activate the relay. Either one could ground the relay coil and operate the fan. It would be interesting to isolate the switches with diodes and use indicator lamps to see which switch turned the fan on.

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Joshua