ice / plenum temperature

Ice is going to form whenever the temperature is below 0°C AND also below the Dew Point.
If the ambient temp is -10°C with a dewpoint of -20°, and the engine is at idle and Boost is low; the induction (Plenum) temps might be -15° to -5°.
This is still ABOVE the Dewpoint of -20° so the water in the air will remain as a vapor and NOT precipitate into Ice.
The Fuel Injectors, Intake Valves, and the Walls of the Intake Manifold are all attached to the HOT (~100°C) Head and will be well above 0° under all running conditions.

Some light frost may develop in the Throttle Body or Airbox at idle but will melt off very quickly as soon as any power/boost is applied.
All of these conditions make Ice formation nearly impossible.

Hi All

The injected engines are very unlikely to develop any ice.  First as you compress any air it will heat.  (915 has a turbo) if you are recording lower than 0 C temps in the manifold it is only for the short time while you start the engine and ambient temps are below that point.  The real differences are the fact that the intake, throttle body and plenum chamber, are dry.  There is no fuel vapor.  Fuel vapor as in a carb engine will have a dramatic cooling effect and without it any risk of ice is really not an issue.  (fuel is sprayed just above the inlet valve) 

Even in the type 912 iS, without turbo, there have been no reported icing issues noted in testing or in the field.  This engine has been out for more than 10 years now so I suspect it would have been seen by now if there was any problem.  I note that on the injected engines to prevent excessive heat bleed to the manifolds these engines have a paper gasket rather than the typical O-ring used in the carbureted series.  This was expressly done to reduce heat transfer from the heads. 

Cheers