Clutch fan vs derale single 2400cfm fan

When disengaged, it turns the fan at 25-35 percent. It’s used with deeper-pitch fans (2 1/2″ of pitch), and works well with higher operating rpm. Severe-Duty Thermal Fan Clutch: Severe duty thermal fans turn the fan at 80-90 percent of the shaft speed when engaged and 20-30 percent when disengaged.

Understanding Fan Clutches — and When it's Time to Replace Yours - OnAllCylinders