How many cfm doest the factory cluch fan produce when working properly?
On a E36

That would depend on RPM since it's connected to the water pump.

on average ? at idle?

I don't know what the engine driven fan is rated for. But I can tell you that a 14" 1400cfm fan isn't adequate in +90 ambients. A 16" 2400cfm fan has proved to be adequate so far. The acid test will come this weekend when temps are forecast to top 100. The car doesn't have A/C so that isn't a consideration.