Regarding mjgermer's comments, the greatest factor in heat exchange is the size & airflow through the radiator, and how long the coolant stays in the engine. If the water pump moves the coolant too fast, it will not be in the engine long enough to absorb the heat.This explains why removing the thermostat from an engine will not cure over - heating. A fast water pump also whips up the mixture, adding unneeded air. A 50/50 mix of water/coolant has been proven to raise the boiling point over plain water by 20 - 30 degrees, assuming the cooling system is properly pressurized.
BTW, you should never use tap water in your vehicle cooling system; use distilled water to eliminate the mineral impurities that clog a radiator. A helpful tip from an old desert rat...