What coolant temperature will produce minimum slip drive for a thermatic viscous fan hub rated at 155°F (68°C)?

The correct answer for the coolant temperature that will produce minimum slip drive for a thermatic viscous fan hub rated at 155°F (68°C) is 165°F (74°C). This temperature is slightly above the rated threshold, allowing the viscous coupling to engage more effectively without producing excessive slippage.

The design of thermatic viscous fan hubs typically involves a temperature threshold where they begin to engage based on the coolant temperature. As the temperature rises above the rated value, the fluid within the hub begins to thicken, leading to more effective transfer of power to the fan. At 165°F (74°C), the hub reaches a point where it starts to fully engage, minimizing slip and ensuring that the fan operates efficiently to cool the engine as needed.

This means that while the 155°F (68°C) rating is the point at which the fan begins to engage, it does not represent the temperature for minimum slip drive. Driving it to 165°F (74°C) allows for better performance and cooling efficiency. Higher temperatures listed in the other options, like 190°F (88°C) and 225°F (107°C), represent significantly increased engagement that may lead to higher wear and potential overheating issues, which is not ideal