Rotax 914 coolant pressure

Please see the below explanation:

"To understand why the cooling system is pressurized, consider that for every 1 psi over atmospheric pressure, a liquid’s boiling point is raised 3°F. Whereas water boils at around 212°F.

At atmospheric pressure, if exposed to 1.2 Bar (17.4 PSI), the boiling point will now be 52.2°F. higher (3 psi times 17.4 psi cap). Under this pressure, water boils at 264.2°F. Due to this, altitude determines the boiling point. 

The higher in elevation, the lower the boiling point of any liquid since less atmospheric mass is present. Heat transfer from an engine to the liquid coolant is based on a temperature differential. A higher boiling point allows more heat to be removed from the engine due to the larger spread between ambient temperature and the coolant in the radiator.

The other purpose of a pressurized cooling system is to move the coolant through the cylinder head. Under high engine load, the coolant will boil at the contact point around the combustion chamber and become a vapor. When the liquid coolant becomes vapor, 97% of its thermal transferability is lost. The pressure created by the radiator cap forces the liquid and vapor to release from the cylinder head casting the water jacket wall. It then recondenses and allows lower-temperature liquid to come in contact with the surface to cool it"

The pressure cap is a relative pressure valve.

The pressure in the cooling sytem reaches 1.2 bar above ambient before the Coolant temp even reaches 210°F (99°C)
The expansion of the liquid as it heats in the closed coolant system that is 100% full will exceed the cap pressure long before the "Boiling" Point is reached.

Using round numbers....
Sea level pressure (Absolute) is 1bar (15psi).
The cap will vent at 1.2bar (18 psi) gauge pressure,  2.2bar (33psi) absolute pressure.
And Boils at ~254°F( 123°C).

This sets Rotax's MAX Cooling Temp at 248°F(120°C, a nice round metric number for convience).
It isn't because the engine is dangerously too hot,  The coolant is just too close to boiling.
When Waterless Coolant was approved, the Max  Head Temperature was limited to 300°F (150°C).
It wasn't because the engine was dangerously too hot,  The coolant was the limiting factor.

17,000ft is about the 0.5bar point.
At 17K the coolant pressure is 0.5 + 1.2 = 1.7bar (25psi) Absolute, and boils at 240°F (116°C)
At 17K ft. the air is thin and cold.  
The engine can barely make half power and generates much less heat and the cold air makes cooling easier.
There is little chance of  the cooling system ever approaching boiling at altitude.despite the lower boiling point.
Nothing needs to be documented about an event that doesn't happen.

"At 17K ft. the air is thin and cold.  

The engine can barely make half power"

The 914 is a turbo, so the normal computations for power loss with altitude gain do not apply.