I agree with Bob. If the fan is on, the engine is too hot. That's the purpose of the fan. The fan not cutting in is not an indication of the engine running less efficiently, it's an indication of the temperature not being too high.
If the engine is running too cool then yes, the engine may not be working as efficiently as it could, but the water temperature gauge should show this. If the water in the head is at around 100ºC then all should be fine, if the revised cooling system is capable of pulling more heat out of the engine than the old system then all that will happen is that the thermostat will cycle more frequently to keep the water in the head at the correct temperature.
My radiator fan hardly ever cuts in except in slow-moving or stationary traffic in very hot weather. Normally the water in the head sits at 100ºC and the water entering the radiator is at 90ºC.
That makes sense Tim, my water temperature gauge remains the same as before at around 100, so the new alloy radiator helps maintain the correct running temperature without the use of the fan as much ?
