Recently I pulled my motor to do a head job. When putting everything back together I installed a flexfan (had one laying around) in place of my stock clutch fan. The flex fan was lighter so I figured it would be easier for the engine to turn. After the flexfan self destructed (for some unkown reason & took half of my fan shorud with it) I reinstalled my stock clutch fan. It was unbelievable how much better my truck runs with the clutch fan. Here I thought I was having other problems because my truck wasn't running as good as it used to. I did before & after G-tech runs. With the flexfan 0-60 was in 9.5 seconds. With the clutchfan 0-60 was in 7.9. To me that a huge difference. Now I can't wait for my electric fan. Just thought I would share my experience.