So I did some testing of a Mark VIII fan against a mechanical fan with a new clutch. This is only on a short trip but the datalogging was started at the second the tire hit the highway pavement and stopped once it hit the offramp pavement. Cruise was set to 60mph on the onramps.
I ran the electric fan once, came back, installed the mechanical fan, then ran the distance twice as both times heading north on I-5, people were going 50mph and in all 3 lanes! Sooooo irritating so my "North" numbers both had a brief drop (<30 seconds) to 50mph before coming back up to speed in overdrive - mechanical fan only. Mechanical fan took a few minutes to install due to the burning hot things around that area and lack of clearance with the 3 core radiator, fan shroud, and the engine accessories.
The numbers:
Electric fan - 10:26:40 AM
North - 30.4395 mpg
South - 35.6265 mpg
Mechanical Fan Run 1 - 11:25:26 AM
North - 30.5025 mpg
South - 33.9045 mpg
Mechanical Fan Run 2 - 11:49:19 AM
North - 30.5655 mpg
South - 33.6315 mpg
These numbers are around as close to a decent test I can do. If it wasn't for the South on the single Electric fan run, I would think that the economy was the same. Any theories on why this is? I assume due to the downhill nature of "south", little load is needed to keep the car moving and the mechanical fan adds some. On uphill, most of the power is spent moving the vehicle and not driving the fan.
Basically the electric fan may give up to a 5% fuel economy increase in ideal conditions. I'm not sure if flat terrain benefits or not as these trips are up a hill, back down, and up again. I do know that not using cruise control, some additional gains can be had. Idle, the electric fan used 6 ft lbs more torque at 737 rpm's in park than the mechanical fan - I blame air temperature for this. In drive, 628 rpm idle takes 9 ft lbs more with the electric than mechanical - again, likely temperature differences.
What I find odd though is going "downhill" with , both electric fan and mechanical have the same load, closed throttle voltages, they make the same torque at the same 1520 rpm's with the same MAF voltage. ACT is 2 degrees cooler with the mechanical and ECT is 4 degrees cooler with the electric fan.
A properly working clutch fan will "decouple" from the pulley during highway driving or when the engine is not hot. It doesn't stop spinning, but it slips a whole lot and draws almost no power from the engine. That's why it's called a clutch fan - you can actually hold it from turning with your hand when the engine's cold (NOT RECOMMENDED OF COURSE).
Meanwhile, the electric fan will use quite a bit of energy any time it's running. It takes a lot of power to turn an alternator to produce the 30+ amps an E-fan will draw. When you did your E-fan run, did you have it connected to run all the time, or was it connected to a thermal switch? If it was running constantly, I'd bet if you connected so that it only ran when necessary you'd see even greater gains.
I understand all this - the Mark VIII fan is powered by the DCC FK35. These NAPA 3G alternators with a 130A current draw almost kill a 5 horse electric motor. The fan draws 30-40 max (depending on whose figures you go by) at full output - draws almost nothing at half cfm. Anyways, I'm still not sure which part of the test is a fluke or if the electric fan actually did improve on the southbound interstate - ideally I would have ran that test twice also but I didn't feel comfortable with the lower mounts.
I had the electric fan wired at "194 degrees" with the temperature probe halfway down the radiator near the outlet. I have a stock thermostat. The fan spins barely enough to feel the airflow with a hot motor and in park - ECT sensor shows 198-206 degrees, about in the middle when in park. The mechanical fan flows a ton and gets roaring loud (unlike the old clutch I replaced) when done cranking the motor.
the DDC tirggerst the fan relay on/off? i assume it does.
interesting test you did, I would think the Efan would have been more cost effective on fuel consumption. That fan must be coming on and off a lot.
The DCC unit handles all the power itself via PWM - besides power and ground to and from the unit, it handles everything else. It hardly spins and likely not at all on the highway with the 3 core aluminum radiator - 3/4" each. I would need to run a wire from the fan to the cabin for a voltmeter in order to see when exactly it's on/off.
and yet, not know if the Efan is on or off still had no bearing on fuel economy. Thats amazing, id bet you would see a larger spread on numbers on a longer trip.
On the stock motor, I saw a 1.5mpg increase on the highway with a Mark VIII electric fan (that later died) - best of 34.5mpg on long trips. This is why I did these tests as electrical does have its own potential "issues" that mechanical is mostly free from - failure.