Having a problem with my 261 over heating since I installed a/c. It does fine while running at lower speeds, but when I'm running up over 75 MPH with the a/c on it will over heat. I'm going to install a 3 core aluminum radiator. I have that ordered. But, I also want to put a larger water pump pulley on to slow down the speed of the coolent. My theory being to give the coolent more time in the radiator to cool down. I have tryed different temp thermostats. Thinking to increase the size to 7 3/8" or a little larger if I can find one to fit to bolt pattern. Any suggestions?

Have you tried an auxiliary pusher fan?

At that speed air motion isn't the problem. Another fan is an obstruction, in fact above 50 the fan can be safely removed altogether.

Think of your radiator function is to heat the air passing over it. The hotter the radiator, the more it will heat the air. This heat comes from your engine. Therefore if the water in your radiator is hotter the effect will be a cooler engine.

Above argument debunks the myth of "give more time for the water to cool off in the radiator" to solve cooling issues.

I think this myth may have started when someone observed an engine running cooler with a thermostat than without and erroneously assumed that less water flow was the reason. More likely reason is the thermostat restriction created back pressure in the engine block which inhibited boiling, thereby increasing heat transfer and cooling system performance.

I have a 1940 Chevy with a 302 GMC engine with AC. It has the same diameter pulley as the original. The 40 Chevy radiator is much narrower than your 53 Chevy's and has an electric fan on the front of the radiator. With that setup my engine did over heat on really hot days pulling up long hills at high speed.

I had a radiator guy replace my 2 coil copper core with a 3 coil. And, I built a sheet metal shroud to duct the air from the upper grill into the upper radiator core, instead of just escaping blowing into engine compartment. Some cars come with this shroud but not the 40 Chevy. Your 53 may already have a shroud. Anyway, my overheating problem went away with these 2 mods.

(inline with strokersix) This blog post over at Champion Cooling suggests otherwise: "The idea that too much water flow causes overheating is a myth."

It also points out that a water pump is designed to create a specific amount of pressure in the block (against the closed thermostat) so under-driving the pump may have other consequences.

I think that the increased radiator capacity is a step in the right direction. But maybe all you need is a new stock core.

I had an '86 P/U that ran fine for years and then suddenly started to overheat on the freeway. The temp gauge would just rise-and rise the longer the cruise. Ambient temps didn't matter.

The problem was radiator deposits. At cruise the thermostat was cycling so quickly that the (deposit-insulated) radiator couldn't keep up. Eventually the thermostat would be held open with overly-hot coolant in both the block and the radiator. The issue was not flow rate but dissipation rate . . .

I'll just about guarantee it's the radiator. The passageways get clogged over the years.

Thanks everybody for your responses. I have desided not to put a larger size pulley on the water. As I said, I do have the 3 core aluminum radiator coming. I don't think there is a cracked head, block or blown head gasket. Timing might be an additional problem. The distributor is an HEI. Going to pull it and put it on a machine to make sure that it is getting enough advance at higher speeds. It's not a problem right now with the sub-zero temp we have had for the last week. But, summer is only 4 months away.

The radiator that in it is aluminum 2 core and is only about 5 years old. When I installed the a/c late last summer this problem started.

Where did you put the AC radiator when you installed it?

Also you might have clearance issues with the 3 core. I did. That's when I went with an electric fan. I could have machined the fan spacer instead but opted for less perisitic drag on the engine.

Take a look at the Nov/Dec 2017 issue of The 12 Port News, Page 20, Tech Questions. Roger Whitley is asking for help with a heating problem on a 57 Chevy Truck with a 261 engine. Pat Smith gives a good rundown on heating problem causes and what might help.