Fan clutch problem?
Thread Starter
Drifting
Joined: Mar 2011
Posts: 1,886
Likes: 9
Fan clutch problem?
'74 smallblock w/AC.
Put in a lightweight steel fan (Flex-a-Lite) to address a clearance issue with a spreader bar.
Now it seems as if the clutch never really disengages. I can hear the fan turning all the time. When I shut off the engine, the fan stops turning almost immediately; it never seems to "freewheel".
I always thought that the failure mode of a fan clutch is to freewheel all the time, not stay locked up.
Can the fan be too light?
Put in a lightweight steel fan (Flex-a-Lite) to address a clearance issue with a spreader bar.
Now it seems as if the clutch never really disengages. I can hear the fan turning all the time. When I shut off the engine, the fan stops turning almost immediately; it never seems to "freewheel".
I always thought that the failure mode of a fan clutch is to freewheel all the time, not stay locked up.
Can the fan be too light?
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
'74 smallblock w/AC.
Put in a lightweight steel fan (Flex-a-Lite) to address a clearance issue with a spreader bar.
Now it seems as if the clutch never really disengages. I can hear the fan turning all the time. When I shut off the engine, the fan stops turning almost immediately; it never seems to "freewheel".
I always thought that the failure mode of a fan clutch is to freewheel all the time, not stay locked up.
Can the fan be too light?
Put in a lightweight steel fan (Flex-a-Lite) to address a clearance issue with a spreader bar.
Now it seems as if the clutch never really disengages. I can hear the fan turning all the time. When I shut off the engine, the fan stops turning almost immediately; it never seems to "freewheel".
I always thought that the failure mode of a fan clutch is to freewheel all the time, not stay locked up.
Can the fan be too light?
The clutch for a clutch fan is designed to allow the fan to not turn the same speed as the engine when it's not needed reducing the drag on the engine due to it' aerodynamic resistance.
So the clutch is designed to turn a fan that presents more resistance to turning than the flex fan is going to give it. I would expect it to turn that flex fan at a higher speed all the time than your stock fan.
If it doesn't overheat and the noise doesn't bug you too much I can't see It causing a problem.
Thread Starter
Drifting
Joined: Mar 2011
Posts: 1,886
Likes: 9
Flex a lite also makes a replacement rigid steel fan that bolts on to the fan clutch, and theoretically operates the same way as a stock fan. It is somewhat lighter than the factory fan. That's what I installed.
Racer
Joined: Jan 2014
Posts: 407
Likes: 14
From: Mulberry Florida
You got two different systems combined. The flex a lite fan is designed to allow the blades to flatten out at higher rpms to reduce the drag of a straight driven fan.
The clutch for a clutch fan is designed to allow the fan to not turn the same speed as the engine when it's not needed reducing the drag on the engine due to it' aerodynamic resistance.
So the clutch is designed to turn a fan that presents more resistance to turning than the flex fan is going to give it. I would expect it to turn that flex fan at a higher speed all the time than your stock fan.
If it doesn't overheat and the noise doesn't bug you too much I can't see It causing a problem.
The clutch for a clutch fan is designed to allow the fan to not turn the same speed as the engine when it's not needed reducing the drag on the engine due to it' aerodynamic resistance.
So the clutch is designed to turn a fan that presents more resistance to turning than the flex fan is going to give it. I would expect it to turn that flex fan at a higher speed all the time than your stock fan.
If it doesn't overheat and the noise doesn't bug you too much I can't see It causing a problem.
Thanks
Larry
Race Director
Joined: May 2006
Posts: 18,648
Likes: 2,557
If you installed a light weight fan it has less mass and does not have sufficient weight to continue to freewheel once the engine is turned off. It will also spin faster than a heavier fan when the engine is running.
A 76 with A/C should have a clutch fan.
A 76 with A/C should have a clutch fan.
Racer
Joined: Jan 2014
Posts: 407
Likes: 14
From: Mulberry Florida
I guess I will need to add that to the list.
Thanks
Larry
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
At higher rpms the fan will stay with the engine rpm until it either cools down a bit and the clutch allows it to slip or the rpm of the engine increases enough that the drag on the fan is increased enough to cause the fan to turn slower than the engine due to the clutch.
Thread Starter
Drifting
Joined: Mar 2011
Posts: 1,886
Likes: 9
The fan clutch allows the fan to freewheel until the underhood temp reaches some predetermined high value, at which point it locks up and drives the fan at almost engine speed. With the stock fan and clutch, the fan on my car never locked up unless it was a very hot day with the AC on.
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