Help picking radiator/fan for 440

303.
Answering your question in post #7.

You can see the impeller blades in post #7. Some pumps [ FlowKooler? ], you would not be able to see those curved blades because the pump has a 'back' on it. The bigger the gap between those open blades & the housing, the more coolant escapes through the gap.....never to be pumped through the engine, just churned up in a useless cycle.
Here is the problem: because of manufacturing tolerances, that gap is deliberately left large to ensure the blades do not contact the housing. But it makes the pump veeeeery inefficient. Think of shovelling dirt with a flat spade; dirt falls off the ends. If you use a shovel with curved sides, you pick up more dirt. By closing in the impeller, I have reduced the operating temp on dozens of engines; biggest temp drop was 20*.