The ECu lets me choose the temp that I turn on the fan. I have two 10" fans, which have been controlled up until now by a radiator mounted thermostatic switch. Now I plan on controlling one of them with the ECU and the other with the thermostat control, and have them come on in sequence. I can also set it up so one of them gets energized by the AC as well.

The thing is, the ECU gets its reading from down in the water jacket, which reads about 12-15 degrees cooler than the temp gauge in the head. So which reading should I go by for the ECU. They recommend having the fan go on at 205, and off at 185. That would equate to 215 on the head, which seems too hot. But is it really?. Maybe it is OK to have the head that hot. I don;t know. I can say that when the head temp guage reads 215, I start to hear things percolate in there, so things are trying to boil for sure. Whatcha think?
Sam

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You're on the right track doing the math to convert the temperature where you're actually reading it, to the temperature where it is assumed you're reading it.

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

Dan, while we're on the subject of coolant temps and electric fans, what would your recommendation be for the ideal temp for a slant to run at? When I start doing performance work, I'll be installing an electric fan that has a specially made controller that, instead of off and full on, will adjust the fan's speed to best maintain a certain set temperature.

Also, would you say the stock temp sender location is the best location for monitering engine temps?

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

Really depends on the particular engine and vehicle configuration and usage. I don't recommend using lower than a 180° thermostat, though some turbocharger guys prefer to use 160° units. The hotter you can run the engine without overheating it, the more complete will be the combustion...and the hotter will be the engine bay, which means you might run up against other practical limitations such as a carb and fuel delivery system that gag on higher ambient temperatures. Intake air is also less dense with hotter ambient temps, but this is a nonproblem 'cause you can duct cold air into the air cleaner (unless you're doing something counterproductive like using an open-element air cleaner) and the latent heat of vapourisation of fuel keeps the intake tract itself relatively cool regardless of thermostat temperature.
Yeah, I have such a controller from Flex-a-Lite. Haven't put it on a vehicle yet, though.
Best...? I donno...it works!

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

Since we re all used to reading the temperature from the stock location, and have years of accumulated knowledge based on that, then I think the stock loacation should be our guide. I would like to hear from people who have put an analogue, mechanical gauge in the stock location, and have experience with real numbers there. It is interesting that the water jacket down lower is cooler, but in thinking about it, I don;t think that should be the the temp used as a guide for turning on the fans, unless we had 40 years of data based on lower water jacket temperature.

According to the readouts on the lap top, even when the head is reading 215, the water jacket is just barely getting to the 200 mark. I have to say, it starts to scare me at 215 simply because I hear water boiiling in side the head when I cut it off then. It does not come out of the engine, but certainly has the potential at that point.

On a related topic, once the thermostat opens, does it really control the temperature anymore. I don;t see how it could. It seems as if it merely controls how fast the engine warms up. It seems like you would have to set the fans to come on lower as well. With a stock, fixed drive fan, that would be all the time, so maybe the engine would never reach the higher temp with the lower thermostat. But is still seems as if the ultimate determiner of temperature would be the radiator, coolant, and fan,(shroud, no shroud, number of blades, etc).
Sam

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I think these are all relative to each other. The thermostat has nothing to do with cooling capacity, aside from possibly it's flow rate when open all the way. It can only regulate the minimum temperature. And this is subject to how much of a temperature drop the rest of the system can provide. On engines that I have set up with heavy duty cooling, real big radiators, hd pumps and high flow thermostats, the temp cycles up and down on the highway when it is anything less than hot out. This is a range of about ten degrees, most of it being below the thermostats opening point.

The amount of air moved thru a radiator is probably the greatest factor, followed by the area of the radiator. The two can interchange somewhat. Once that thermostat is open all the way, it's all up to the radiator and airflow.

I would stick with using the stock location or a location right at the engines outlet for the main reading. It is the highest temp in the system that will determine overheating. That is kind of what we are stuck with. It is nice to know the variation between the lower jacket and head, but without a major revision we can't do much about it anyway. I suspect it will have less variation the hotter it gets. Ideally the block and head would have separate temps. Reverse cooling would likely be be better. A cooler head and hotter block should be the best of both worlds. Being able to control them separately would be even better. How much better, ???

My thoughts on setup. If this is a turbo EFI run a 160 thermo and set both fans at 170 - 175. For a normally aspirated engine with a carb run a 180 thermostat and set the fans at 190 and 200. If the one fan cannot control the temp set them both to the lower setting. If they are still on all the time raise the settings a bit so they shut off when cruising. If they never want to shut off or the temp rises much above the setting you likely need more radiator or bigger fans. In any event I would stick with your plan of having two control circuits so that you will have at least one in case of failure.