This is just for conversation and opinions,would a crank pulley and water pump pulley the same diameter cool better than a crank pulley that is larger than a water pump pulley? If the crank and water pump pulley were say 6.2 inches diameter or if the crank is 8 inches and the water pump pulley is say 6 and a 1/4 inches,which would cool better overall.

one stumbling block for the cooling system on pont. v8 engines is water pump efficiency. Upgrades by Pontiac include better waterpumps for 69 and overdriven waterpumps for 1970 when they went to the larger crank pulley and smaller waterpump pulley. There are other mods that can also be done including clearancing the water pump impeller to the divider plate, the 421 mod for better cooling between center cylinders, altering steam holes between the block and heads or adding return lines from the rear of the heads to the water outlet to promote more even cooling.
I am changing to the later overdriven pulley style when my 455 goes in in a few weeks and I believe it will improve my cooling efficiency. ( just my opinion of course)
Neill

X2
I always overdrive my water pumps on my engines. I have used junkyard pulleys off other GM engines that line up perfectly.

I would say a larger crank pulley than pump pulley.

Ok Sounds good to get a bigger Crank Pulley...... Any suggestions on what make model, Part # etc?

I installed a smaller pump pulley. Seemed to help.

As people have alluded to, if you have the crank pulley and the water pump pulley the same size, then they run at the same speed. A smaller pump pulley (or larger crank pulley) will run the pump at a higher speed.

But I agree with Neill above. The biggest issue with the cooling on these cars is the water pump efficiency. I bought a new pump and divider plate for my engine (I'm running the '69 and later 11 bolt on my '68) and the clearance between the divider plate and impeller was almost 1/4"!!! After coming across the Wallace Racing article on desired clearance (there is a post to it in the Hall of Fame section) I massaged the divider plate so the clearance was 0.025". You can imagine how much of a difference that will make in the pump efficiency.

So the reason you have to overspeed your pump is most likely because it is internally bypassing way too much coolant due to larger clearances. Running the pump faster helps that, but it's not correcting the root cause of the issue.

The Pontiac engineers seemed to think a smaller water pump pulley made a difference. They have a smaller water pump pulley and larger crank pulley on AC cars than non AC cars. I'm assuming the AC cars work harder than the non AC cars and generate more heat, that added with the heat from the compressor and other AC equipment would require extra cooling. I could be wrong about that. Not only does the smaller WP pulley turn the water pump faster resulting in higher coolant flow it also rotates the fan faster resulting in more air flow as well. I would think that would aid cooling considerably, especially at a red light in hot weather with the AC running.
A 1968 eight cylinder PS car has a 7-1/4" dia crank pulley and an 8" dia WP pulley. Every rotation of the crank results in only 0.91 rotation of the WP. The same car equiped with AC and PS has an 8" dia crank pulley and a 5-11/16" dia WP pulley, resulting in 1.41 revolutions of the water pump for every rotation of the crank. If my math is correct [a big IF] that's a 55% increase in water pump rotation over the PS only car.
Replacing the 8" dia 9786819 WP pulley on a PS car with a 5-11/16" dia 9788886 WP pulley and leaving the crank pulley stock will result in the WP turning 1.27 times for every crank rotation, an increase of 39%. Again, IF my math is correct.

I wonder if AC equiped car water pumps wore out faster?

That being said, the Pontiac engineers did a pretty good job designing our engines including the cooling system. I've never had overheating problems from any of the stock Pontiacs I've owned. Including a Tropedo Back, Chieftain, Catalina Wagon, GTO and now the Firebird. If the cars are in good stock condition they should run fine the way they were designed to run. There are gobs of 60s Pontiacs out there running every day summer and winter with no cooling and/or heating problems. When we start to modify them it's a different story. They were designed to run the way they were manufactured not the way they get modified.

That's my 3 cents.