I have noticed, since getting the engine overhauled and the annual done, that the engine loses some manifold pressure in icing conditions.  First, be reassured this is not significant icing … a light frosting to trace mixed ice at flight levels in IMC.  

Instead of cruising at 34"/2500 (19gph) as before, I'm now cruising at 31.5"/2350 (16.4gph), which perhaps makes the turbos spin less fast and more vulnerable to restrictions on intake.  

As expected, the first sign of icing is a reduction in airspeed.  But I'm seeing a quicker reduction in MAP from 31.5" to lower numbers (maybe to 29"), requiring adding back throttle to restore MAP.  At the 31.5/2350 in the flight levels, there is still plenty of throttle available.

In the past, I have seen MAP reduce with light to moderate icing, but not so soon with trace, and not with minimal icing conditions without observed icing evidence.  Also, I have not heard the alternate air door open, as has occurred once or twice in the past.

Even with presumed icing (and reduced airspeed) but no visible icing, this will happen.

I wonder about obstructing intake icing tendency, alternate door function, and size or status of air filter as my possible issues.  Any other insights or suggestions by those with more experience?

Jeff L
LIVP
TSIO550E