I had a chance to compare some the internals of some different fan clutches whilst tracking down the overheating in the cruiser.
Whilst I can’t actually dyno the behaviours, the pics below tell a story.
The operation of the OEM clutch is explained here - http://neuralfibre.com/paul/4wd/tuning-and-understanding-your-toyota-viscous-fan-clutch
As the pics below show, the OEM clutch has far more coupling rings, more surface area, a better adjustable valve system, and more fluid. The aftermarket is a Daikin unit.
My bet is the OEM unit will couple much more tightly.
![P3230011 [1600x1200]](http://www.landcruiserwiki.info/wp-content/uploads/2010/04/P32300111600x1200.jpg "P3230011 [1600x1200]")
OEM – Lots of rings
![P3280003 [1600x1200]](http://www.landcruiserwiki.info/wp-content/uploads/2010/04/P32800031600x1200.jpg "P3280003 [1600x1200]")
Aftermarket – not many rings
![P3230001 [1600x1200]](http://www.landcruiserwiki.info/wp-content/uploads/2010/04/P32300011600x1200.jpg "P3230001 [1600x1200]")
OEM – deep reservoir, adjustable
![P3280004 [1600x1200]](http://www.landcruiserwiki.info/wp-content/uploads/2010/04/P32800041600x1200.jpg "P3280004 [1600x1200]")
Aftermarket – shallow reservoir, fixed valves
![P3280005 [1600x1200]](http://www.landcruiserwiki.info/wp-content/uploads/2010/04/P32800051600x1200.jpg "P3280005 [1600x1200]")
OEM on left – more volume
![P3280006 [1600x1200]](http://www.landcruiserwiki.info/wp-content/uploads/2010/04/P32800061600x1200.jpg "P3280006 [1600x1200]")
OEM on left – more volume