If memory serves me right
the original equipment viscous clutch locks up when the air is about 220 degrees
but the heavy duty version from NAPA locks up at a much lower temperature
and you will hear it more often
and your MPG might take a little hit.

 

that figures, i got mine from napa and I do hear it more often but no growling. havent noticed mpg loss yet but havent really paid that much attention. I live wiht that. thanx richie