My 451 Dart runs hotter than it should at highway speeds. But I can't decide if this is due to the old 26" copper/brass radiator having insufficient capacity, maybe in need of a rodding-out, or replacement with modern aluminum, or tif he Contour fans are acting like an obstruction when off.

Idling at a standstill, the temp will slowly climb to 198 at which point the fans kick on and fairly quickly drop it back to 192, then they turn off. Repeat indefinitely.
When driving at 30 (or 60) mph, it wants to run 196 or so.

If I manually turn on the fans while at speed, the temp will drop some, but never all the way down to the 180 thermostat.
Think I just answered my own question - most likely the radiator.

Any recommendations for an aluminum one that won't break the bank? It has to be fairly thin to clear the Contour fan assembly to water pump pulley, and they are a perfect fit across a 26".

I'm not sure exactly what I put in there for pulley ratio or water pump (a/c or non-a/c) - could it make that much difference at 2000-3000 rpm on the highway?

First verify the Gauge is giving you the correct reading. 198 in my opinion is not to hot. What is the ambient temperature ?

It's an Autometer mechanical (capillary) gauge. Can't remember if I tested it in boiling water or not (think I did a long time ago). From the underhood heat and touching various cooling system parts, I think it's reading about right. I agree, 198 maximum is not too hot as long as it doesn't keep climbing, and I've got lots of oil pressure. But I would rather have it cooler as protection from detonation on pump gas. And the thermostat ideally would be the sole determinant of operating temperature...

Ambient temp varies a lot here, of course, but it's been doing this since I got it running four or five months ago (50F or 95F, same behavior).

under 200 is very good .if its at higher rpms, it will not keep the water in the rad.as long . so its not in the rad. long enough to cool it. if it runs hotter at rpm its either air flow or water flow. you ran the fans and it helped some so, the air is good when its on. sounds like more pump pulley to slow the water pump down . or try improve air flow . my race car would respond big time to air flow through the back of the car if it was trapping the flow under the hood! kind of like a hole in the bottom of a beer can ..

I would check it with a pyrometer in several areas. Did you test the thermostat? Maybe it is opening at a higher temperature than the rated 180 degrees. One item I like is the ambient temperature does not seem to have any effect on running temperature. In reality if you were running a 195 stat the temperature may not be any different. So make sure that thermostat is right. One thing a pyrometer could tell you is the face temperature of the radiator core. If you have irregular readings it could indicate restrictions. Before I wasted any money on a radiator I would verify that thermostat and core temperatures.

Sorry, this urban legend has been around for a very long time, but that is just not how thermodynamics works. There is a given amount of heat that must be rejected by the radiator regardless of the speed at which it's moving. Slowing down the coolant would increase the temperature drop from radiator inlet to outlet, but less volume would go through the radiator being cooled more, so the same heat output would occur.

The only reason to slow down a water pump is if it's turning so fast that it's cavitating and not pumping efficiently.
In fact, slowing down the flow through the engine excessively can cause hot spots in the heads due to localized boiling...

Anyhow, thanks for the reminder to check the t-stat opening, NitrousN. I think it's right though, because when warming up from dead cold, I can see the gauge climb to 185 and then suddenly drop several degrees as the cold coolant in the radiator starts flowing.

the coolant must have enough time to release the heat as it passes through. if you double the speed you cut the time in half. that means it is less efficient and does not remove as much heat .if you could double the air flow across the rad. you would increase its efficiency . either way, it is a fact. localized hot spots are mainly due to poor circulation in the heads passages and pre ignition when the water turns to steam. this car is running under 200 degrees.

No, either way that is still wrong. Temperature difference between the source and the sink is what drives heat transfer. Not the time.

See, for example:

https://flowkoolerwaterpumps.com/bl...t-need-more-time-in-the-radiator-to-cool

But if that old saw about "time in the radiator" works for you, feel free to keep believing it
Anyhow I agree that the engine is not overheating - but I want it to run at 180-185 on the highway.

What type of T-Stat you got in there? You running the Robert Shaw designed unit or the typical parts store replacement?

Parts store I think

I need to run a test this morning and re-verify that it won't go down to the t-stat temp at idle with both fans running continuously.
(Much less heat to reject at idle vs. the 20-30 hp required to maintain 60 mph).

If that is the case, I almost certainly need more radiator.

Highway overheating could be insufficient t-stat flow (as I think you're implying?)

Get a Robert Shaw design (Flow Kooler, Milodon, or CDP unit off Ebay) and try that before chasing ghosts with the Rad. Stay away from the Mr.Gasket unit as they seem to fail fast per Big Brother Google searches.

Personal experience: Have a 4.10 geared car that stayed cool-ish around town with a US Radiator 3 core High Efficiency 26", but when going over 2500RPM on a long drive it started to creep up to 200+ and stay there. Using a high flow pump, Clutch fan, 7 blade OEM, shroud and all and it would not stay cool at speed. I was too cheap to spend $30 on a T-Stat and thought a T-Stat was a T-Stat....wrong!!!! I figured $30 was a last ditch effort before spending $1000. I put in a Milodon 180 and it has corrected every issue I was having for the last several years.

That's what you need to do. Don't waste time do it now.

please correct me if i'm wrong, but the number on the thermostat is the temperature that it starts to open, and to be fully open can be 10-15 degrees warmer.
example, my caravan has a 205 thermostat, but runs all day, every day, at 220 or so. never has overheating problems.
in my mind, something with a 180 thermostat will run at 195 or so, and be perfectly within design parameters of the 185 thermostat.

This ^^^
That why I run a 160 in my Charger & let my fans do the rest

I had a parts store 180* thermostat in my 69 Dart 383. It would run cool until I got to higher speeds, then it would get hot and there was no cooling it down. I had a brand new Cold Case radiator, so I bought a high flow Robert Shaw thermostat, and did a quick home flush of the radiator. Now the car will run 180-195* all day long (except sitting in traffic, but that is because I don't have a shroud yet).

Thanks for all the t-stat experiences! I'll definitely try replacing it with one of the recommended brands before I change out the radiator. A modern aluminum one would be nice but I don't plan to spend $1000 regardless...

More data - today was low 80's. With the fans on, idling and stationary, it will stay on the 'stat (182-185). Driving at 25 or 55 mph, temp creeps up until the fans kick on (198), then it comes back down to 192 and the fans turn off. If I keep the fans locked on, it will stay at 192 regardless of speed, and eventually drops to the stat temp if I stop and idle for a while. On the hot days (90's) I don't think the fans ever shut off, since the temp would not go back down to 192 while driving.

So either not enough radiator, or (hopefully!) too restrictive a thermostat. Will see what I can find locally (probably have to mail-order it as usual).

so if you are heating a small pot and it boils over , use a bigger pot and it will take longer to boil over . either way it boils over.. you either need less heat being put in or do a better job of taking heat out . ie> improve the efficiency of the air passing through or give the rad .more time to remove it... thousands of fools are using the correct pulley size to get the water flow where it needs to be and the parts stores are glad to be selling them what they dont need. just trying to help. not offended ............

the people that think slowing the water flow thru the radiator cools it better are forgetting that slowing the water thru the radiator also slows it thru the block thereby picking up more heat, if your theory was true then it's a wash.

Thermodynamics does not work like that.

The biggest factor in heat transfer efficiency is temperature differential, not time.

Your analogy of a bigger pot vs a smaller pot fails because unless you are heating the same amount of mass it's not a valid comparison.

Which is what I already explained (with references)...

Thanks for Sniping As Thomas Edison allegedly said when firing an assistant: "It's not what you don't know that bothers me. It's what you don't even suspect".

de nada

One other point you could make is that if the flow rate was important to cooling and slowing it down helped then it would also hold true on airflow thru the radiator and I never hear anyone argue that one.

I try not to get into the math too much as it rarely helps.