What the factory configuration did was apply manifold vacuum to the distributor advance during overly-hot idle, this raises the idle speed and thereby increases the fan speed and coolant throughput.
This won't help you with an electric fan although I guess you could try it... get a 3-port vacuum switch that opens at a high temperature... this type switch would have connections for two vacuum sources and one connection for the device receiving the flip-flopped vacuum supply. It'd raise your idle speed and coolant flow but I think you'd be applying a Band-AidŽ on a symptom instead of treating the cause of the problem which is most likely someplace else.
Well you´re right a E-fan won´t help me when I use a vacuum switch, I did not know it works by raising the idle speed. The more I think about it I feel it´s the wrong way.
Guess the E-fan doesn´t pull enough air. On a really hot day in a traffic jam the temp slowly raises from 82° C up, at 88° C the fan kicks in but it fails to reduce the temp, just holds around 90° C and raises more and more under some conditions.
The cooling system is OK, as I said before hot tanked block, new radiator block, new water pump, a correct 180° F robertshaw thermostat, new VDO gauges ..... opening the rad cap shows a good flow of fluid.
I suspect the engine build-up with a big improvement of power from 150 to 360 HP has changed the whole arrangement. Maybe I´m going back to the old belted-driven fan and wait for hot weather.
Thanks a lot for this input !