TEMPERATURE CREEP AT SPEED

Multiple sources rate the efficiency of an automobile alternator at about 60 to 70%  Taking the lowest number as a working number and then determining an efficiency statement based on HP to drive the thing vs power needed to run a fan is straight forward.  The application though is a variable.

If this is deemed needed at a drag strip for what ever reason the most efficient fan is non at all.  Mechanical if not there, and electrical  turned off gives the most efficient system.  Of course cooling the engine then becomes a function of need and time. 

But on the open road, where most people drive, the engine needs to be cooled going down the road, if it is not cooled properly for what ever reason the temperature will as stated Creep up and then the objective is to keep the engine from running in a too hot mode.  And there are a number of means to do that.

But reverting to the open road need of the most efficient way to cool the engine that being electric fans vs mechanical drive?  Depends. 

As referred to earlier, there is plenty of data that says an OEM mechanical drive fan with a thermostatic clutch on it that disengages when it gets hot enough is the most efficient and cost effective approach in the long run and a flex fan is not far behind it.

The only advantage an electric fan has is simply at a given running temperature, when heat gets to the thermostat control point, the fan is turned off.  Thus no power is applied to turning it. 

The advantage?  A powered mechanical fan at speed is in-efficient and actually serves as a wall when spinning that the moving air has to get around.

Even at that, a clutch fan is still the most efficient. 

However what power is used to drive an electric fan. One member has two 18 amp fans mounted in a radiator shroud. That means that at 14VDC (the amount of voltage an alternator generally puts out) is 252 watts (VxA) plus 2 fans =  504 Watts.  That is the power needed to supply these two fans.  However an automobile alternator is about 60% efficient. so the engine would have to supply 1.12 Hp in order to run these fans.  While that is not a lot, when turned off, it takes zero hp to cool the engine as in general the forward motion of the car supplies enough air moving through the radiator to cool the engine.

So applying this to an automobile going down the interstate at 70mph, there is a spare 1.12 hp available that is not being used.  That is the measure of efficiency.

As to the water pump cavitating at 3000 rpm, it probably does.  That is how it works.  But two things,  your car and I am assuming your street car has nominal gearing at the present, it might be geared for about  32 mph / 1000 rpm which in general most cars made today are geared around.  If that is the case, your rpm at 70 mph is about 2,185 rpm well under the hypothetical 3000 rpm and yes the water pump is cavitating,  But the theory behind FlowKooler water pumps is that cavitation is controlled better.  The act of cavitating puts air bubbles into the kooling system creating hot spots through out the engine. Eliminate the hot spots and you have a better controlled cooling system.

Todays modern cars use electric fans because engines are smaller and the hp loss is a greater % of the total available and turning off the fan is a smarter solution than clutch fan or a flex fan.  Radiators are more efficient, transmissions use their own separate cooling systems but A/C still has a condenser sitting in front of the main radiator.

Retrofitting any or all of this on a 30 to 50 year old antique car has to take into account the cost vs benefit ratio received by the builder.

Running the engine hotter with more radiator cap pressure to keep from boiling is one approach. I prefer the colder thermostat that establishes minimum value for engine heat and more wiggle room between nominal operating temperature and the actual boiling point. A larger radiator, relocating transmission cooling systems, a flow kooler or similar high volume water pump and a 50/50 anti/freeze + a water wetter. And a higher pressure radiator cap with an over flow tank to keep the radiator full up to the very top.

But it is over 100 degrees 1/3 of the year and today.  And I will drive to Flagstaff in a snow storm.

And this works for me, out on the highway with my AMC cars.