The biggest issue with thermofams is their operation being tied to a thermoswitch or a temp-sender/relay trigger arrangement.
Typically these have a trigger value in the range of 75-120ºC depending on the electronic package or sender type/value.
If not installed correctly, the sender will either trigger too late in the engines thermal load cycle or too frequently if installed in a position where the senders' range is narrower than the expected variation of the temperature and coolant flow rate.
So where they often fall short in an rrc application /use, is when they are incorrectly setup and used in a stationary or extreme low speed low rpm application, or with a poorly sorted electrical system with no increase in charging capacity or storage.
So many types of trigger/sensor setups available these days that in theory at least, a well-executed installation of thermofans should cater to just about any variable - but the reality is not quite the same truth - because the installation requires some actual research into airflow rates, air density and temperature and an acute knowledge of the engines coolant capacityu, thermal mass and heat dissipation factor.
This is all easily attainable, and in 99% of cases completely ignored by the end user wholesale, which leads to the endless complaints about systems not working properly or as a user expected.
The viscous coupling fan and shroud is a slightly more primitive and wholly mechanical method of a variable speed/load fan, which simply uses a clutched fluid coupling triggered by a bimetallic spring, which has a limited degree of adjustment in some cases and no adjustment in others.
Viscous coupled fans are efficient, but do draw a load mechanically, rather than electro-mechanically as is the case in an electric thermatic fan.
The disadvantages to a visco-coupled fan are in low-rpm high ambient temps, not at all dissimilar to the electric thermofan, however the visco-coupled fan suffers from being directly tied to engine rpm. When a vehicles cooling system is not operating at it's designed efficiency (i.e. not a well-maintained system) then the same issues of overheating occur - except that the visco-coupled fan is again directly tied to engine rpm, and it's operational efficiency when locked fully is only as good as the airflow at any given engine speed. Increase engine rpm, increase thermal load, may increase airflow, but airflow rate increase may not offset the thermal contribution by running engine at higher rpm.
The exact same can be said for thermofans when the airflow is suboptimal, except the primary load on the vehicle is electrical - and the alternator output is directly attributed to the Thermal load and the engine rpm. When the ambient temperature in the engine bay reaches a certain threshold, the alternator will de-rate and may not output it's full current for any given rpm, due to the thermal de-rating curve.
Again, increase engine rpm - increase heat, increases the de-rating level of the alternator, which reduces the output further, thus affecting the power delivery to the vehicles electrical circuits, one of which happens to be the thermofans used to cool the engine.
So you can all debate the relative merits and disadvantages of both until the cows come home. The facts however, are always there for any owner to observe, should they wish to learn.
Regardless of the type of cooling device used, when a fan is employed to create or increase airflow across a heat exchanger core, the fan needs to have a correctly designed venturi, the correct spacing from the heat exchanger core in order to prevent cavitation, reduce drag coefficients and provide the best possible airflow in both assisted and unassisted airflows across the exchanger core.
This is where the visco-coupled fan design provides an obvious lack of restriction in airflow for unassisted flow application. It's also why a visco-coupled fan was used instead of a direct driven fan or electric thermofan was chosen.
At the time of the vehicles production, electrically operated thermatic fans were far more primitive in their construction and operation, so a visco-coupled fan solution which was already a well-proven technology was used.
We're now more than 28 years past the expiration date of the vehicles production, so the 'rules' have changed somewhat.
Thermatic fans are now commonly PWM controlled brushless dc motors with highly efficient blade designs that do not impede airflow, and in many cases do not require a shroud of any kind. The intelligence is all in the controllers hysterisis loop and that is designed by engineers who know a thing or two.
Adaptation to an RRC nonetheless requires some consideration and planning. It may very well be the case that certain prefabricated solutions can be adapted to suit the operational requirements of the vehicle, but that does not mean they are optimal or the best or even a 'better' solution that what already existed in the original design. What they do depend on, as does any cooling solution, is a properly functional and perfectly sound cooling system, which also means every hose, every coolant passage and every mechanical device in that system be at it's optimal operational efficiency.
In other words - if anyone thinks that swapping out their visco-coupled fan for thermofans will wholesale improve the existing setup and fix a problem that is not directly related to a failed visco-coupling- they should be giving themselves a clip in the ear.
Unless that radiator is perfect inside, the thermostat and hoses are perfectly functional and the engines water pump, heater pipes, heater core and coolant passages in the block are absolutely spotless from corrosion, slime, scale and oil, then you're simply clutching at straws and wasting your time.
The visco-coupled fan is perfectly adequate when the rest of the system is in perfect working order.
If the coupling is slippy or the radiator has a bit of crud in it, or the thermostat doesn't open properly, or the heater core has a leak - or the header tank cap doesn't seal properly at pressure - then no thermofan will fix a runaway cooling system.
There's no debate - it is simply down to the facts.
Either know exactly what you are doing, or do the math and make it right if you don't. Ignoring the detail is what will get your goose cooked when the engine decides to become a piston-driven steam cleaner.
Roads?.. Where we're going, we don't need roads...
MY92 RRC 3.9 Ardennes Green
MY93 RRC LSE 300tdi/R380/LT230 British Racing Green
MY99 D2 V8 Kinversand
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Originally Posted by PhilipA
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