How thick is the radiator? How many rows? The factory A/C and/or HD radiators had additional rows.
Story: purchased a used Econoline 300 for towing from a Ford dealer (circa 1975). It had a 302 and the heads were redone and had a new radiator. Always overheated, even without a trailer. Turns out they put a 6 cyl radiator in the truck. I purchased a brand new 302, a/c, towing, hd radiator from another dealership. It was twice as thick. Never had an overheating issue again.

 

It’s got three rows of tubes in radiator, and is about 2 inches thick. Factory brass n copper for 302/w AC

 

From my spec book there were three radiators for the 390 in '68. All were 24". Thickness and fins/inch were: Std - 1.5"/9, Std - 1.5"/11 and AC/HD - 2.25"/12. It did not list number of cores and did not explain difference between the two standard radiators (I'll bet the one with more fins was for the automatic). I have the 2.25 in mine; the a/c was added by a PO (Vintage Air).
Hope this helps.

 

When I put a 390 into my 67 Cougar (replacing the 289), I had to buy a bigger radiator to keep the engine temp down.

 

So you can run a 7-blade fan with a 24" radiator? Thanks. I didn't think that would work. Do you know which fan/shroud combo you're running?

 

Hayden 2765 clutch, SD 7 blade fan C9ZE-8600-E, and the SD 24 inch shroud for 428 in 70 mustang. I don’t have the part number, but it is heavier duty than the factory one.

 

The BB radiator outlets were located differently than the SB, but were otherwise quite similar. The 7-blade OEM BB clutch fan is reproduced, although pricey. If your clutch is not the thermal type, or is not functioning properly, the engine will overheat quickly when stopped. And even if the fan is OK, if you don't have a shroud you have no chance of keeping cool.

 

So other than outlet differences...the radiators are the same? I am running a thermal clutch, shroud and 7 blade fan. I don’t want to go the dual electric fan route unless I HAVE to. I could not fit my 100 amp Taurus alternator on the 390, and had to go back with factory 60 amp. Its not really up to par for contour fans or others? Still chasing my tail.
It does not boil over, per say, but will puke a small amount when shut down and in heat soak. It also drags the starter pretty heavily when its shut down after a run to get gas or a drive down highway. I’ve backed timing down to 6*, factory spec. Underhood temps are down, but the air cleaner and carb are almost too hot to touch. My gas percolates and makes for hard starts. It’s a Summit 750, and I’ve added a 1/2 phenolic spacer as well. No headers, 428 exhaust manifold, Edelbrock performer 390 intake. The exhaust passage under carb is NOT blocked off. I think this is the problem of too hot carb. I have bought more take gaskets, and may go back into it and block those exhaust passages that go from head to head through the intake/ carb base. Thoughts?

 

Where is your alternator at? I have a 3G on my 428 Cobra Jet, Alternator is on the Pass side top.

 

Yeah I have passenger side alternator, up high, and I do not have smog pump, nor the dual tensioners for AC. The Taurus 100 amp is a physically larger case and hit the block, so would not bolt up. It did fit on the 302, though.

 

As others have said, the big block radiators have the the lower hose exiting on the driver's side of the car while the small block radiators have the lower hose exiting on the passenger's side of the car. I ended selecting a US Radiator brand copper/brass (24" wide) 3 core for a 351W with A/C. Made in USA and very nice quality. I will be installing it along with A/C later this summer. Note that the present no name brand 20" wide non-A/C radiator With 18 1/4" diameter clutch fan with shroud did a splendid job of cooling the engine on the way to Hot August Nights in Reno so the added margin of a 24" radiator should be enough to pick up the added thermal load of the A/C. I ended up purchasing the US Radiator part # 051581ANN described above from Summit while in Reno that was re-branded by Scott Drake (#SDK- 338-3) and drop shipped by US Radiator.

 

Not related to your radiator but may help your cooling. I did a citric acid flush and it brought my temperatures down 10-15 degrees. Even though my fluids were normal there were some passages blocked internally

 

Ok, after dumping all coolant, flushing the running motor for 30 minutes until it would run clear as possible, I refilled with water and burped it. . Went for a ride to the corner store and came back. Let it cool down (runs at 170-180) and shut it down. Once cool enough to open radiator again, I looked into radiator and I’ve got brown foam. I think it’s a head gasket issue- Or possibly an intake issue. No water in oil, but possibly oil in water? So what do you all think?

 

Was you previous coolant brown? It could just be rust as well. I would try the thermocure or citric acid flush. I am pretty sure you know how many times I cleaned my radiator out, lol. I am running a 24 inch aluminum that I bought off of eBay. It is shaped like a stock radiator, with stamped top and bottom and is set up for passenger side inlet and outlet. Although my ultimate issue with over heating was a warped head. I did just buy new pulleys from cvf, apparently crank pulley is smaller now so my fan turns a little slower and it idles a little warmer than before.

 

Yeah Jacob, I think I’ll keep flushing and hope I don’t have to go down your path with this cooling issue. Next will be intake swap...we shall see. Whatcitric flush did you use?

 

A couple observations and a question or two...

1. RETARDING the ignition timing actually INCREASES engine heat load.
2. It is possible, when combining non-factory pieces, like a radiator and a/c condenser not originally designed to work together, that airflow through the radiator core could be affected due to too dense a fin count in either or the condenser placed too close to the radiator.
3. A/C installations included a carburetor (Autolite models) with a hot idle compensator in the air horn and, on Holley-equipped models, an external hot idle compensator to modulate a small vacuum leak when ambient air temperatures exceeded a set limit to increase idle speed to pull more air through the radiator.
4. Vacuum advance "plumbing" included a DVCV (distributor vacuum control valve) switched by coolant temperature to modulate the signal to the vacuum advance unit between timed spark (ported vacuum) and full manifold vacuum to reduce heat loading.

One, or a combination of these factors, could be contributing to your issue.

 

ok, thanks for the replies guys...I’m still thinking this through.
to answer some of Bart’s questions:

A couple observations and a question or two...

1. RETARDING the ignition timing actually INCREASES engine heat load.- yes, I’ve played with timing and have run the gamut from 20* down to 6*. 6-12 seems to be in the “happy zone” where it sounds good, underhood and running temps are good, good throttle response. Getting higher does not ping, but it sounds awful, and starts to load up at idle and stop lights, running/operating temps are higher.
2. combining non-factory pieces,-airflow through the radiator core could be affected - its factory 302 HD radiator. I’ve got aftermarket AC condenser and a trans cooler in front. AC not hooked up yet...can’t get temps in line yet....that is why I began to question the ability of this radiator with this post....
3. A/C installations included a carburetor (Autolite models) with a hot idle compensator in the air horn and, on Holley-equipped models, an external hot idle compensator to modulate a small vacuum leak when ambient air temperatures exceeded a set limit to increase idle speed to pull more air through the radiator.—yes my 302 had this stuff. But not using on 390 with summit carb. That will be addressed when I get to AC install...aftermarket air cleaner now.
4. Vacuum advance "plumbing" included a DVCV (distributor vacuum control valve) switched by coolant temperature to modulate the signal to the vacuum advance unit between timed spark (ported vacuum) and full manifold vacuum to reduce heat loading.—same as above...on my 302, but not using on 390. I did not have a dual diaphragm dizzy for 390, it was a single point galaxie dizzy. I swapped for new spectrum duraspark dizzy.

One, or a combination of these factors, could be contributing to your issue.

to answer 22’s questions:

With the thermal spring on the front, right?—- Yes!
If the radiator is too full, this is actually normal.—- I’ve filled it to just barely over fins in radiator, and it will puke a little. Next run, same thing. After a few days/trips, it’s low on coolant, below the fins. So I top it off to just over the fins. It only takes a small amount...a cup or less To get back over the fins.

Normal in a BB.-I plan to get pgmr starter eventually.
My gas percolates and makes for hard starts.
Also normal now that it's polluted with ethanol.- I’m running 90 octane non-ethanol specifically for this reason
It’s a Summit 750, and I’ve added a 1/2 phenolic spacer as well.
Good carb, good choice of spacer.

The 69 Mustang 390GT engine had a water-cooled carb spacer. Although I think it was mainly intended to prevent carb icing, I can't help wonder if it would also prevent the carb getting too hot.-I want to block intake exhaust passage to address this issue, I’m not sure if it is the answer...but I believe it is a great contributor to heat issue.

 

So other than outlet differences...the radiators are the same?
I am running a thermal clutch
With the thermal spring on the front, right?
, shroud and 7 blade fan. I don’t want to go the dual electric fan route unless I HAVE to.
Probably would not be an upgrade from a properly-working 7 blade BB fan.
I could not fit my 100 amp Taurus alternator on the 390, and had to go back with factory 60 amp. Its not really up to par for contour fans or others? Still chasing my tail.
It does not boil over, per say, but will puke a small amount when shut down and in heat soak.
If the radiator is too full, this is actually normal.
It also drags the starter pretty heavily when its shut down after a run to get gas or a drive down highway. I’ve backed timing down to 6*, factory spec. Underhood temps are down, but the air cleaner and carb are almost too hot to touch.
Normal in a BB.
My gas percolates and makes for hard starts.
Also normal now that it's polluted with ethanol.
It’s a Summit 750, and I’ve added a 1/2 phenolic spacer as well.
Good carb, good choice of spacer.
No headers, 428 exhaust manifold, Edelbrock performer 390 intake. The exhaust passage under carb is NOT blocked off. I think this is the problem of too hot carb. I have bought more take gaskets, and may go back into it and block those exhaust passages that go from head to head through the intake/ carb base. Thoughts?
The 69 Mustang 390GT engine had a water-cooled carb spacer. Although I think it was mainly intended to prevent carb icing, I can't help wonder if it would also prevent the carb getting too hot.

 

Besides number of rows, there can be a difference in tube size as well as fins per inch. All radiators are of not the same quality. You may want to look around the specs on this site: U.S. Radiator: Made in the USA. I believe NPD sells some of their radiators.

 

So after sleeping on it, I’m going to continue with flushing, maybe some citric flush, and a vinegar flush to see if I can get it to run clear. And stay clear. I’m not holding my breath though. I think I may have an issue with intake leak...I do have oil residue at corner of intake and head, so maybe I might be getting a leak into cooling there as well, not sure? It also has the cork end gaskets, and they are squishy. But pulling intake will allow me to block exhaust passover. Also I think this performer 390 that is on there is not the best, nor in the best condition so I may need to source another intake prior to doing the swap. Anyone got a good FE intake they want to part with? I’ll trade?

 

UPDATE: I started with another flush. I used some household items that were handy. I started with a flush of water and "cascade" dishwashing powder. Its low suds...and just what I needed to stop all of the frothy milkshake. After filling and going for a short 20 min. drive, I came back and checked after a cooling cycle. It was no longer frothy...just dirty. So I must have had a fair amount of Oil in the system that just would not clear out with the water flushes and the thermocure. I will continue to flush to see if I can get it to run clear and stay clear....so I would call this a little progress.