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Gregte · 06-29-2008, 07:42 PM

I finally measured the load that my original belt driven fan puts on the motor of my 1994 2.2L GMC Sonoma. I mounted the fan (viscous fluid clutch type) onto my 1 horse electric table saw motor. I set the motor on a steel bench in such a manner that it was free to roll and used a spring scale to measure force to measure ft. lbs. of torque and thus calculate HP. The motor turned at 3450 RPM which equates to the GMC's crankshaft turning at 2500 RPM due to the pulley sizes involved. The speed of the unengaged fan does not matter as I am only concerned with the torque load on the motor itself.

It turns out that the fan load measured 0.19 horsepower. I then heated the bi-metal coil on the clutch to engage the fan. At this point my setup was too unstable to use my spring scale for making a measurement so I used my best judgement for feeling the pounds of twist on the motor and came up with 5 pounds. This calculated to be 0.996 HP.

It should be noted that the clutch virtually never engages on this fan in my normal driving, so therefore I am not including it's power load in my calculation below.

Assuming that the fan requires 0.19 HP to cruise down the highway at 55 (no tach so I don't know if that's 2500 RPM but seems it must be close) and assuming that it takes perhaps 10 HP to cruise 55 (just a WAG) and also assuming 34 MPG at this speed (quite easily achievable) I wonder if this means that the fan would cost about 2 percent of my fuel to turn it since 0.19 HP is about 2% of 10 HP.

If the above assumptions are true then that fan was costing me about 0.68 MPG.

I drive about 10,000 miles/year. At 34 MPG and $4 gas it must cost me $1176 per year.

Removing the belt driven fan would then be saving me $23.53 per year. I paid $85 or so for the fan w/thermostat so it will take 3.6 years to brake even with $4 gasoline.

Also, my new electric fan uses almost zero power since it only comes on for a very short time, only on very hot days, as I climb my 1/2 mile hill in 1st gear coming home. Therefore I consider its energy use too insignificant to matter. It only uses about 85 watts when it is turning and that is brief.

With the information I have now I would not have installed the electric fan. It was fun however so all is not in vain. It makes me a bit suspicious of some claims that I have read of people saving 3-5 MPG by replacing a mechanical clutch fan with an electric fan.

Or, maybe all of my measurements are screwed!!

06-29-2008, 07:42 PM	#12 (permalink)
Gregte EcoModding Apprentice Join Date: May 2008 Location: South Dakota Posts: 117 GMC Sonoma - '94 GMC Sonoma 90 day: 36.97 mpg (US) Thanks: 0 Thanked 4 Times in 4 Posts	I finally measured the load that my original belt driven fan puts on the motor of my 1994 2.2L GMC Sonoma. I mounted the fan (viscous fluid clutch type) onto my 1 horse electric table saw motor. I set the motor on a steel bench in such a manner that it was free to roll and used a spring scale to measure force to measure ft. lbs. of torque and thus calculate HP. The motor turned at 3450 RPM which equates to the GMC's crankshaft turning at 2500 RPM due to the pulley sizes involved. The speed of the unengaged fan does not matter as I am only concerned with the torque load on the motor itself. It turns out that the fan load measured 0.19 horsepower. I then heated the bi-metal coil on the clutch to engage the fan. At this point my setup was too unstable to use my spring scale for making a measurement so I used my best judgement for feeling the pounds of twist on the motor and came up with 5 pounds. This calculated to be 0.996 HP. It should be noted that the clutch virtually never engages on this fan in my normal driving, so therefore I am not including it's power load in my calculation below. Assuming that the fan requires 0.19 HP to cruise down the highway at 55 (no tach so I don't know if that's 2500 RPM but seems it must be close) and assuming that it takes perhaps 10 HP to cruise 55 (just a WAG) and also assuming 34 MPG at this speed (quite easily achievable) I wonder if this means that the fan would cost about 2 percent of my fuel to turn it since 0.19 HP is about 2% of 10 HP. If the above assumptions are true then that fan was costing me about 0.68 MPG. I drive about 10,000 miles/year. At 34 MPG and $4 gas it must cost me $1176 per year. Removing the belt driven fan would then be saving me $23.53 per year. I paid $85 or so for the fan w/thermostat so it will take 3.6 years to brake even with $4 gasoline. Also, my new electric fan uses almost zero power since it only comes on for a very short time, only on very hot days, as I climb my 1/2 mile hill in 1st gear coming home. Therefore I consider its energy use too insignificant to matter. It only uses about 85 watts when it is turning and that is brief. With the information I have now I would not have installed the electric fan. It was fun however so all is not in vain. It makes me a bit suspicious of some claims that I have read of people saving 3-5 MPG by replacing a mechanical clutch fan with an electric fan. Or, maybe all of my measurements are screwed!!