This has been a successful way to overcome the heat rising from the engine and it is standard to have one or two of these fitted under the intercooler on most quality top mounted units. We don’t consider these a great long term option for those looking for large power increases from more fuel and increase boost during the hotter months of the year.
We have customers who have needed to find a replacement to their top mounted intercooler simply because the fans stopped working just when they really needed to do their job. One customer quoted 130ºC as the max temperature for his fans, before they turned off. Positioned with a hot engine below and the intercooler above (experiencing inlet air temps climbing over 200ºC on higher boost engines) this temperature doesn’t seem too hard to reach. On an electronically controlled engine, where the system backs off the fuel with rising inlet air temp, it just means less power, but for the mechanical pump setups it means your EGT is going to rise rapidly.
The max temperature rating on 12V thermo fans is limited by the motor winding insulation. The most expensive units seem to be rated to 115ºC and cheaper models are around 100ºC.
BUT, aren’t they designed for the job? They are designed to be mounted at the front of the engine bay, pulling air through a radiator filled with coolant between 90 and 120ºC. The air coming through will be cooler as its coming from the front of the car and your radiator isn’t 100% efficient at removing the heat.