I’m not sure why you went to less blades on the pump. I use Milodon high volume pumps and they have 8 impellers.
You should be using a mechanical fan.
The water pump should turn no less than crank speed. So 1:1 should be the slowest you run the pump. Over driven is best.
Don’t forget the fan turns the same speed as the pump. Slow down the pump, you slow down the fan. That’s the last thing you want.
At high ambient temps and low rpm situations you need as much coolant and air flow that you can get. Slowing down the pump is bad policy.
