actually, while the electric water pump is only saving 2-3 hp in normal driving at 2-3000 rpm, in racing where they are running at 6-7'000 rpm they are probably saving at least 10hp, nothing to sneeze at. Nearly all top drag cars with 700plus hp use electyric water pumps now.

In simple terms, the more electrical power that you take from the alternator, the more mechanical energy you need to put into it to make it rotate at the required speed to achieve that output. You get nothing for nothing, ... there is no free lunch.
This mechanical energy is provided by the engine & is transfered via the drive belt easy peasy.

As for the constant speed electric water pump, this bears a second look.
Because it is constant speed, the output of the pump is never really linked to the heat output of the engine except at the intended optimum.
At idle, the heat output of the engine is very low & coolant flow does not need to be great, & sure enough, the conventional pump rotates relatively slowly, draining little power from the engine.
The electric pump, on the other hand must be spinning faster than needed & drawing more power than necessary because it is locked into a constant speed that must be high enough to shift sufficient volumes to cool the engine at driving speeds. This means that at idle, the electric pump must contribute a needless power loss.

At full power, which is achieved at high engine speeds, the heat output of the engine is high & a much greater volume of coolant needs to be moved to keep temperatures under control. The conventional pump is spinning faster & moving greater volumes of coolant. The parasitic drag is greatest but so is the preformance of the pump.

The electric pump, tied to its constant speed, may well fall short in the volume of coolant required to be moved unless it is optimised for this operating condition. A power saving may be realised, but at what cost.

If it is optimised for an output that matches the maximum power output of the engine, then it is overdoing it at all lower speeds & therefore drawing more power than would be needed for cooling duties.
If optimised for idle, then the parasitic power drag is minimal, but obviously would fall short at higher heat outputs of the engine.

I think I will stick with the system that, even in a crude way. more closely matches water pump output, & parasitic load load, to the cooling requirements of the engine.

For racing use, it might be a good thing, ... perhap, but for street use, no thanks.

P.S. Serious race engines usually run revised pulley sizes to slow down the conventional pump to prevent cavitation that can occur at racing speeds, so a parasitic drag reduction is achieved right there.

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Sorry, this is not correct at all. As I mentioned in my earlier post, all electrical energy that the alternator delivers into the electrical system is provided by the engine. This means that at low electrical loads the engine load is small. As the electrical load increases the load on the engine increases. As the electrical load on the alternator increases the alternator gets harder to turn.

The alternator does not put out a large current that is bled off by the regulator. The alternator only puts out the current that the regulator tells it to put out. More load, more current.

To recap - The load on the engine is directly related to the load that the alternator is supplying to the electrical system.

There is no free lunch. The power has to come from somewhere. In this case the alternator loads the engine to provide the energy to generate the electricity.

hmm I doint know, u r suggesting that the alternator increases its resistance as more electrical load in accessories is drawn?
How does it do this, from what I can tell the internal brushes are at a light pressure to the magneto etc and that doesnt change.

dodgeman what u r saying about electric water pumps over supplying and over pressurizing the system before thermostat at idle is true, the question gets asked in Street Machine or Hot 4's write in sections often and they always recommend
its not a good idea for cars built mainly for street use.

Elecricity is created by passing a conductor through a magnetic field [or vice versa] That's oversimplifying it but it's also true.
In an alternator, the magnetic field is provided by the rotating element inside the unit & the strength of this magnetic fiels is varied by the amount of current passing through its windings. This clearly means that it is an electro magnet & the regulator 'regulates' just how much power is fed into these windings.

In it's simplest form, the lower the electrical load, the lower the strength of the electromagnetic field, but as the electrical demand rises, so too does the strength of the electromagnetic field [controlled by the regulator] & as a consequence the electrical output of the alternator also rises.

The mechanical load that the engine sees is provided by the magnetic field inside the alternator. The engine must pull the [electro]magnets away from the stationary iron poles of the stator within the alternator during the normal rotational operation & if you have ever tried to separate two strong magnets, you will get some idea of what's happening in there in a continious cycle.
The more electrical power output that's needed, the stronger the magnetic field, the greater the mechanical load.
No free lunch.

Electric water pumps not recomended for street use? Not hard to see why.
My comments refered to volume of coolant that is pumped but made no reference to 'over pressurising' whatsoever, so I don't know where that came from.

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this `is where people are getting confused.
the alternator never ever gets harder to turn, the ENGINE has to work harder to turn it so it can supply the needs of the car.

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the resistance to turn the alternator doesent change. your confused with what people are saying.

The alternator is a form of electromagnet: As the alternator spins, the magnetic fields cut through each other to produce a voltage/current output.
Higher electrical load requirements by the vehicle mean stronger internal magnetic fields, which equates to greater rotational resistance (the alternator is harder to turn, so robs more hp from the engine).

A friend has developed a system where he can open circuit his alternator via a dashboard mounted switch. It gives him extra hp for hillclimbs etc (dyno proven), but can only run for a short time (otherwise it kills the battery). This is dependent on your ignition system though - his is aftermarket (distributorless with coilpack) so doesn't seem to suffer when voltage drops from 14v to 12v.

Peper is right in this regard, the 'mechanical' resistance to rotation doesn't change [brush drag & bearing resistance] but the electromagnetic drag, well that's different & this is seen at the pulley in the same way that it would see mechanical resitance.
It's just 'load'

yes dodgeman, i meant to add "mechanical"

ok cool understand.