High Volume Water Pump or Not
Not sure on ratio vs impeller vs cavitation and I haven't found any info on high capacity and impeller size, but if the coolant doesn't spend enough time in the radiator it can't be cooled.
When I had this model A with the 400 11.1 motor (chevy) sorry, I tried everything to cool it The walker 4 core did it and even used a Tuff Stuff Water pump with the super cool option they claim 30% more flow and 20* F cooler temps. The passage in the pump were smaller than stock and I called and talked to there tech and he told me it slows down the speed and keeps the water in the radiator longer? high flow? to me it really doesn't make sense, smaller hole = less volume higher speed?
Given the speed at which air is pulled through the radiator, it would be fairly difficult to flow water through the radiator too fast. The smaller impeller was likely done for pump efficiency at the driven speed, not because it would circulate the water too fast. Plus, the faster the water goes, the more trips through the radiator it makes, so you have to balance the heat lost with each trip vs the number of trips through the system. Kinda like looking at the speed the impeller is turning, just because the impeller is smaller doesn't necessarily mean it's not pumping more water.
If anything, the water can be circulated through the
engine too fast. The heat uptake rate slows as the water is heated, the hotter the water gets the longer it has to hang out to absorb more energy. Plus, in the engine the only thing moving is the water over the heated surface. In the radiator, you have both the air and the water moving, so, the water is exposed to a large volume of air moving past it. And the hotter the water is, the faster it wants to shed that energy and the larger the temperature differential is with the air, again improving the rate of energy dissipation. That's actually the argument for a standard vs a high flow thermostat too, to keep the water in the engine longer to better absorb the heat.
I've heard the whole "water flow through the radiator too fast" argument a bunch of times. If you look at the thermodynamics of how it actually works, that argument doesn't look nearly as good. Obviously at the extreme end it's possible, but given the application here there's usually another explanation for why something isn't cooling efficiently. That's not saying the whole system doesn't have to be matched up, it does, and the speed of water circulation is part of that. But it's not the biggest driver of cooling efficiency. Surface area and the volume of air flow are by far the biggest.