Ducati engineers sized the cooling system components and chose the cooling fans temperature setpoints to optimize the operating temperatures such that the engine can develop its best power over the widest range of riding conditions and ambient temperatures.
Lowering the these setpoints is counterproductive — it will NOT lower the maximum operating temperature that you experience at a stop, and will adds a 60 watts per fan electrical load to the charging system for longer durations — and more often — as you ride.
The 916 cooling system was designed to keep the engine coolant temperature at a specific operating point, 180°F, when moving at speed on a hot 90°F day. This is the operating condition that is used to size the radiator, water pump and hoses. The overall design is based on the use of a 50/50 mixture of ethylene glycol and distilled water. If you use a different coolant or mixture percentage you change the design operating temperature.
The 998s have the same cooling system components but revised (less effective) fairing cooling vents and an additional radiator fan. The computer turns the fans on (LH) at 214°F, (RH) at 216°F.
The most common 998 gauge temperatures experienced are 210°F at speed and 250°F in traffic on a hot day. Substituting Water Wetter/water for the glycol/water coolant drops these numbers to 200°F and 220°F respectively. One owner checked the accuracy of the OEM gauge and found that it read 15°F high.
Typical 998 Operating Temperatures on a 90°F Day
200°F @ speed with WaterWetter/water
210°F @ speed with 50/50 glycol/water
220°F @traffic stop with WaterWetter/water
250°F @traffic stop with 50/50 glycol/water
When you get stuck in stop-and-go traffic, the temperature will begin to rise because now there is reduced airflow through the radiator. At around the three-quarter point on the gauge, the radiator fans kicks-in to provide the needed airflow. The fans stay on above this temperature. With the fan operating, when the temperature drops to just above the gauge midpoint, the fan stops running.
During these traffic conditions, temperatures in the upper quarter of the gauge display should be expected and not necessarily be a cause for alarm or an indication of cooling system problems. If your 998 is not discharging coolant it's not overheating.
Most stock-engined water-cooled sportbikes are designed to produce their best power when coolant temperatures are close to 200°F.
The problem with our bikes is they run at the correct temperature at speed, but get hot at low speed. This tells us that the radiator is big enough but the airflow (cubic feet per minute) through it isn't enough at the lower speeds. Hence the need for auxiliary fans.
Ducati fans aren't very big or efficient but adding a shroud will improve airflow. See davy.j's solution:
http://www.ducati.ms/forums/56-superbikes/84130-what-s-better-coolant-method-999-a-3.html#post816713
Finally, check the radiator for damage from road debris. Carefully straighten any bent fins that restrict airflow.
