First off datspeed, sorry for the thread hi-jack.

Dodgeman, I hope you didn't think I was having a go at you you, I just have a weird way of bringing my point across...

Anyway, I agree with your statement about it pumping the maximum amount of air at below 3000 rpm. However viscous fans have even more weight hence inertia to overcome than stock direct drive fans. It's almost like adding another flywheel to your car, with the difference that it now has "paddles" that tries to stop it from turning. One can literally hear/feel the difference, in my Mazda 2.5 TD for instance, between when it's coupled and when it's not. There is quite a difference in the speed at which the motor can spin up.

Other than that they still don't pump air as efficiently as thermos, because of the way they are "mounted" in relation to a thermo. Probably the biggest reason I dislike them, is because they are unreliable, I have very rarely seen any of them last for much longer than a 100K. They tend to either lock up or disengage altogether, with obvious consequences. That's also reason why I would then choose a stock fan over them...

Then onto the reason I would still choose a thermo over any of the above. With a thermo you can control your engine temp to within 5 degrees of variation, which is very, very good for engine life. You can't achieve this with a stock fan or a viscous, no matter how hard you try.

For instance we live in a country where the difference in temperature between winter and summer could be as much as 20 degrees, up to 30 and more in some places. If you now have a stock fan, or viscous for that matter, in the winter months it's constantly cooling the engine when it shouldn't. So you end up with an engine that never reaches it's working temperature.