[Fullback66]

Originally Posted by lust4speed

To go a little deeper into the operating characteristics of the clutch fan we can start out with a quote right off Hayden's website:

What is the difference between a standard, heavy and severe duty fan clutch?

Answer: Each fan clutch type is designed to simulate the performance of the original equipment clutch that it replaces. All fan clutches are for specific applications and should be applied only on the vehicle for which they are cataloged. Use of the incorrect fan clutch may result in poor cooling, excessive noise, reduced fuel economy or fan clutch failure.

• Standard Duty Thermal: Turns the fan 50-60% of shaft speed when engaged. Used with fans with lighter pitch (1-1/2" of pitch). Flat plate impeller design with 30 sq. in. of working surface.

• Heavy-Duty Thermal: Turns the fan 80-90% of the shaft speed when engaged for increased cooling. Used with deeper pitch fans (2 1/2" of pitch). Land and groove design with 47 sq. in. of working area allows higher operating RPM's.

• Severe Duty Thermal: Turns the fan 80-90% of the shaft speed when engaged. Used with deeper pitch fans. (2- 1/2" of pitch). Land and groove design with 65 sq. in. of working area. Larger working surface provides cooler running and longer life expectancy.

Their heavy duty thermal clutch characteristics were changed by Hayden a few years ago. Was 70-80% but now rated the same as the severe duty. I'd be okay with either one but the severe duty is much easier to locate and still seems to engage stronger.

Most important thing about a clutch fan is the viscous fluid and (as mentioned above) the ability to freewheel at speeds above about 4,600 RPM. The fluid normally hooks up the impellers but at the specified RPM centrifugal force pulls the fluid towards the perimeter of the clutch and keeps the clutch and fan from reaching any higher RPM and the fan simply stays at about 4,600 RPM while the engine winds higher. Also remember the over-driven water pump and fan reach speeds in excess of engine RPM.. The fan assemblies would become lethal weapons if allowed to RPM with the engine when you add in the 14% overdrive. I normally shift at 6,300 RPM and without the slippage the 19.5" fan would be turning 7,200 RPM.

Next thing is how strongly the different clutches engage, and the severe duty simply has a much higher lockup ratio. A worn out clutch fan might be down to 30% engagement or even less, and that's why it's best to replace it with a new unit.

Just like we add headers, aluminum heads, better intake manifolds and such, going above a stock fan clutch just makes sense.

So, are you saying you would go up on the clutch? Or, go stock? Your last sentence got me a little confused.

Thanks,
FB66