Here's my outlook on electric water pumps.


A mechanical pump varies its flow amount with engine revs, therefore you are not always obtaining the flow that gives the optimal heat transfer time for the radiator vs supplying cooled fluid to the engine components.

i.e. if the flow is too fast it will not give enough time for the radiator to sufficiently transfer heat from the coolant to the air passing through the core fins and therefore higher temp coolant will then flow back to the engine.
If the flow is too slow then it will allow the coolant in the engine to become excessively heated and possibly start localised boiling, probably worse than having the flow too fast. Also, when the engine is below normal operating temp the thermostat closes slightly and therefore causes the water pump to operate against a higher resistance.

So lets say for instance that the optimum flow of coolant from the mech water pump is acheived at 3000rpm, revving to anything above this, in regards to coolant flow, is meaningless and an unnecessary use of power.

Compare this to using an electric water pump with a constant flow rate.
You can keep the water flow at the optimal rate whether you're engine is revving at 800rpm or 8000rpm.

Torque required to operate the alternator to, in turn, provide enough energy to operate the electric pump at the optimal flow will be the same as the torque required to operate the mech pump at the optimal rate, no more, no less. To push 100 litres / minute through a given size hose is going to take 'X' amount of energy, no matter how big the impeller is. General physics states that energy cannot be created nor destroyed, it can however be converted. The mech pump uses mechanical energy to turn the pump impeller directly from the crank, the elec pump uses mechanical energy to turn the alternator which converts the energy into electrical energy to operate the elect pump's motor, which converts the electrical energy back to mechanical to turn the impeller.

Then theres always the extra weight argument, I'd rather bolt an extra 2 or 3 kilos statically to the car than have another 1/2 kilo having to be rotated by the crank. Most people know the theory about rotational masses.

The only other point is that of reliability. Both pumps have bearings that can sieze and seals that can leak. The mechanical pump uses a belt that can break whilst the elec pump uses windings in the motor that can fuse. Either design has its own reliability issues and should be taken into account individually but kept in context to the other. When it all comes down to it, if the temp gauge goes up, pull over.

You can probably guess that I prefer the electric pumps..

This is just from my head, my view, my opinion.

Sorry about the long post.