does anyone know what cfm the stock mechanical fan moves?
You might be able to find the numbers for a brand-new perfectly operating engine/truck; but, there are so many variables with a clutch-controlled engine-driven fan, that it would be near impossible to make a close guess.
The mechanical fan, providing the clutch is up to par, changes speed with engine RPM, so fan CFM changes constantly.
You may have an excellent design of blades that have the capability of pulling tons of air at a given engine-RPM, but the limitations of engine-speed and clutch quality not allow it to do it's job.
On several of my trucks, I took the clutch out of the equation by "pinning" it solid, as in the fan is now "direct-drive"; this helped a bunch with the A/C at low engine speeds, but still is not as much air-flow as I would like to have at idle RPMs. (By the way, I have not noticed any loss of power or mileage by doing so)
As much as I prefer mechanical components over electrical, I believe that a big electric fan is a better choice for our applications, so far as air movement is concerned; reason being, as I stated in another post, the electric fan turns "wide open" at all times, with engine RPM having no bearing on fan speed.
Thus, when one is sitting still, either stuck in traffic or waiting in a long line, the A/C condensor will have full fan volume.
