Originally Posted by SauceSquatch
You can also set the temp you want your fan to kick on in TuneECU.
This is unfortunately a incorrect statement. The 990 does not leverage a ECU's capability to turn the cooling fan on/off, this is only performed by the thermo switch that threads into the right radiator tank.
EDIT: Also, to my knowledge, there is not a lower temperature thermostat, the device that regulates the engine coolant temperature. The Thermo switch is the device that tells the fans to turn on/off dependent upon a set temp range. OEM temp is ~220F on. My experience with the lower temp thermo switch (~200F on) is as follows, and I almost completely agree with BillyD's statements, with deviation on that with the fans coming on sooner there is a greater potential for a period of "rest", so MAY allow for the system to catch up sooner. Granted, we are not increasing the capacity of the heat exchanger, we are increasing the exchange of heat by forcing the flow of air through the exchanger (radiator), still there are limits and in some cases the limits will still be met when the engine is producing more heat than can be rejected to the ambient air.
EDIT2: What I typically experience with the addition of the 4" fan on the right side of the radiator (my experience is with the ADVmachines fan so I cannot speak to the BDCW fan) is a reduction in fan cycle time by about 40-50%. This is unloaded and with the engine idling on the lift. The cycle time between on and off is reduced again by about another 30% when adding the lower temp thermo switch, this switch turns both fans on at the same time. During normal riding the fans will cycle more frequently and will tend to come on sooner (with 200F switch), but again will turn off sooner as well. The engine temperature is still regulated by the thermostat and most reports that I have heard back from other people who use this system say that typically the gauge rarely exceeds 4 bars (950), and almost never goes past 5 bars unless in extreme conditions and loads. Even at 6 bars of engine temperatures the cooling system should still be "stable" and not puking coolant uncontrollably.
Personally I run the ADVmachines rally fan.