I have some experience with the GMB pumps.  From what I can see the only difference between the standard and high flow is the impeller design.  One thing you could try is welding a disc to the back of the impeller.  Long story short, Milodon sells a pump that is literally a reboxed GMB pump with a disc welded to the back of the impeller.  The disc is to prevent cavitation in the cooling system (where the impeller spins, but isn't actually moving any coolant).  I added this disc to my GMB pump for my '89 GT build.  Its certainly not going to hurt anything.

I would not go to a 160 degree thermostat.  You should be able to keep the car cool with a 180.  If you can't there's a problem with something other than the thermostat.  Running a 160 risks running the engine too cool in colder weather, which will cause a dramatic increase in wear on the cylinder walls.  I know,. 20 degreesdoesn't seem like much, but if you've ever seen a graph of wear vs. temperature its not linear; its exponential. 

MS wrote:

Use the 20” 1969 radiator if lower hose is on passenger side. Use the 1970 20” radiator if lower hose on driver side.

These will bolt in with no cutting required. You will have to drill four new mounting holes and revise the condenser mounts slightly.

Use the 69 or 70 shroud and a 17” fan.

If the aluminum radiator in the 20” size with a good fan and shroud does not cure your overheating problem, your engine has other issues that will need to be addressed.

 
Are the 20" rads what would have been used on 69/70 6 cyl cars?

50vert wrote:

I'm not sure if you know that 212 is not boiling point of water in a pressurised system . Every pound of pressure, raises the boiling point 1 degree.
A 12lb radiator cap changes the boiling point to 224.

Thank you , i didnt know about that .  I was referring as water boiling point as a knowen point that all the world knows . Sometimes for me is not easy speaking in fareheight , pounds and inches There is only 6.6 degrees celsius
( 12  farhenit ) difference .not that much . 
Thanks !

The hotter temperature is not hurting the engine; the engine loves it.  Heat equals efficiency, efficiency equals power.  It is tricky to understand because you want the air going into the engine as cold as possible, but you actually want the fuel as hot as possible.  The hotter the fuel gets the leaner the air/fuel ratio can be because the fuel molecules don't bunch together as much.  The fuel increasingly becomes vapor, and the molecules spread out and there is a greater chance that each molecule will react with the air and contribute to combustion and power. 

Its not uncommon for race engines to run 20-30psi of cooling system pressure to allow them to run hotter without boiling over so they can make more power.  In sports like Indy Car there is a limit to how much boost you can make, but there is no limit to how much cooling system pressure you can run.  When one area of making power is restricted by the rules the smart teams start looking at the areas that are not restricted to get a competitive edge. 

The trick is not allowing the coolant to get so hot that is boils over.  An overheating engine doesn't make more power, because as the coolant turns to steam the ability to carry away heat is lost, and eventually damage can occur. 

Dkralc wrote:

I had a heating issue on my 66 Coupe that was not fixed by new water pump, new lower radiator hose with a spring, stock aluminum radiator, or larger 69 radiator (cutting the radiator support and adding dual fan Contour fans) also flushed the cooling system with Evaporust to make sure water passages were cleared.  Finally pulled the intake and found a shop had partially blocked a water passage with the wrong gasket restricking flow.  Installed the correct intake gasket & heating issue fixed.  Had a bunch of new stuff on the car that looked good but turned out to be over kill.   Good luck fixing your issue.  Dennis

 
After reading all of the posts and tech data on this subject I'm suspecting something other than the radiator is wrong. I'm running a 289 in my 66 with a mild cam, aluminum 3 row stock size rad, stock water pump, 17 inch 6 blade fan and NO shroud. I do not have a/c but I live in an area where 90-100 degree days with 80% humidity are not uncommon. We also have more than our share of draw bridges which means sitting in traffic. My car has never over heated. Soooo, I would say if you put the new rad in and it doesn't solve the problem then I would be looking at first, the intake gasket, then the water pump. Hopefully it's not the head gaskets but it possible.

Chaplin wrote:

MS wrote:

Use the 20” 1969 radiator if lower hose is on passenger side. Use the 1970 20” radiator if lower hose on driver side.

These will bolt in with no cutting required. You will have to drill four new mounting holes and revise the condenser mounts slightly.

Use the 69 or 70 shroud and a 17” fan.

If the aluminum radiator in the 20” size with a good fan and shroud does not cure your overheating problem, your engine has other issues that will need to be addressed.

 
Are the 20" rads what would have been used on 69/70 6 cyl cars?

 

The 20” radiators came on the six cylinder cars and on 289/302 and 351w engines without factory ac from 67-70.

Mounts were same 67-70 for the 20” radiators. They bolted to the side and there was a bolt-in 4” filler panel to partially fill the 24” wide hole in the core support. 

The 24” radiators, which were used on air conditoned small blocks, Boss 302 and all big blocks, was a clamp-in style. The 67-68 was same mount system. The 69-70 24” radiator clamped in similar to the 67-68, but the top clamp had a longer horizontal component.

When installing the 24” in a 65-66, use the shorter 67-68 top clamp. 67-70 lower saddles will work, but a simple fabricated u-shape saddle that accepts the factory rubber piece, welded or bolted to the lower diagonal braces, will work just as well.  For the 24”, it is required to cut the core support per the dimensions I show on the web page. Moreso to get the radiator in there than just for airflow.

DO NOT order the lower saddles from CJPony. They are total crap and will bend if you look at them too hard. Get them from Mac’s on ebay. His are quite sturdy and made correctly.

Hi Steve-
On the below link, they are calling it a 22” rad. Their diagram shows a 20” core but 22” overall width. I assume that’s the same rad that you’re referring to as a 20”?

https://www.speedcooling.com/1967-1970-Mustang-Aluminum-Radiator-22-DS-Outlet.html

Also, do you think it is worth installing the 20” rad in a 65/66 if you don’t cut the rad support?  I should have cut my rad support before it was installed and painted, but didn’t and I’d prefer not to cut it now.

Excellent info, thank you. I guess there’s no reason not to order the 20” than.

this is my fastbach 66 with  67 mustang or galaxie rad . to fit the fan is needed 1 inch cut in fan spacer . now i have one inch free space from fan to rad. 
the battery tray needs to be cut  1/2 inch only one side . this part is  for center the fixing screw on the metal that normally needs to be cut  to fit bigger radiator . in this way nothin has to be modified except a litte part of battery tray that anyway is useful



 

Chaplin wrote:

MS wrote:

The 20” radiator has a 20” wide core plus a 1” wide mounting flange on each side.
The 24” core radiator does not have mounting flanges on the sides, but does have a small flange where the fan shroud can bolt up.

When installing the 20” radiator in a 65-66, there is no reason whatsoever to trim the core support. There is plenty of air flow since the high pressure area in front of the core is almost fully open to the core. There is a front to back gap between the rad and support and that allows air flow to reach the entire face of the core.

The 24” clamp in requires cutting the core so it will physically fit and clear the mechanical 17” fan.

 
Hi Steve-
Now that I’ve got my engine in the car, I’ve been doing some measuring to see what I can fit in terms of a radiator and fan combination. I am planning on using an electric fan, but am surprised how little room I have between the rad support and the water pump pulley. Even with the Ron Morris adjustable mounts and the engine slid as far towards the rear as I can get it, I only have about 6.25” of space for the rad and fan, which seems to eliminate a lot of popular radiator/electric fan combos as most of them are at least 7” deep. Not sure how anyone fits an aluminum rad and electric fan in a 65-66 without cutting the rad support. Maybe they don’t?

Anyway, as I was measuring I remembered this thread about using the 20” rad instead of the std 66-66 rad. It looks like the 20” rad would interfere with the factory battery tray.

So here is the question, is there any reason I couldn’t install the 20” rad and offset it towards the driver’s side an inch or two? That would solve the interference issue with the water pump pulley and also eliminate the interference with the battery tray.