On a recent flight to Chicago, my MAP during WOT climb was normal and ample all the way to cruise FL220, at which time I pulled throttle back to 31.5", dialed down RPM to 2350 (which increases MAP some, so readjust) and "Big Mixture Pull" to LOP 16.4gph.  All very normal.  Cabin Pressure 5 psid.

On my return, departing during 30*F temps and cleaning some overnight rain-ice off the wings (via hanger heat), I climbed out of Chicago WOT and had full MAP at departure, but then observed a decreasing MAP during climb (numbers are below).  Details below.

I leveled off at 17,500 and observed WOT ~31" MAP and RPM 2690, 32.3 gph, all engine parameters normal.  I did not reduce throttle, I dialed back RPM to 2350, and observed MAP reduce to 24" (and cabin altitude climbed with 1.5 psid).

T/O WOT 38.5" RPM 2670 FF 43 gph
8000' WOT 37" RPM 2670 FF 41 gph
12000' WOT 36" RPM 2670 FF 38.5 gph   Cabin 4.1 psid
15000' WOT 34" RPM 2680 FF 35.1 gph
16000' WOT 33"
17500' WOT 31.8" RPM 2690 FF 32.3 gph
adjusting into cruise settings:
17500' WOT 24" RPM 2350 FF 23.9 gph Cabin 1.5 psid

At home, I pulled the cowling, inspected everything, pressurized the induction system, inspected the already clean filter, check the Alt Air Door, checked turbos and waste gate controller (on bench), checked compressions: everything normal and in great working condition.

Today I test flew back to 17,500' WOT max RPM (full rich) and observed the same behavior.  This excludes intake ice or frozen alternate air door.

Also found that if I diverted incoming bleed air (feet vs defrost), the MAP remained same.  As expected, if I occluded incoming air, MAP increased to 28" but cabin (of course) climbed to >12,500'.  If I selected HOT air, MAP remained same; if I selected COOL air, MAP rose 0.5".  None of this is surprising, siphoning off the Upper Deck pressure.

Interestingly, while at 17500', there was no change in MAP when the throttle cable was moved from WOT (all in) to more than 2" throttle pulled out.

Descending back to 8000', the numbers were as above for that altitude, with access to 37" MAP WOT and higher cabin psid.  The plane has historically been able to maintain >36.5" MAP WOT up to FL210 (or maybe a bit higher) before MAP would start dropping off as turbos were max'ing out.

What suggestions can you make regarding this engine performance/behavior issue?

Jeff L
LIVP