One thing I've noticed on big-bore engines is there isn't a big difference in MP indication (maybe 3/4" rise, at most) when running with the mixture full rich---say 155 lbs/hr flow---to a leaned setting with 380 degrees F CHT---about 100 lbs/hr.

When I was instructing in a 182RG (O-540J3C5D, to be exact) I had one instance of carb ice. We noticed a drop in MP before we could hear a difference or see anything else make much change. Carb ice is not generally common in this plane, but it happened because the conditions were exceptionally ripe for it. Of course, this being a constant speed plane, RPM remained constant. And, since a 150 is fixed pitch, RPM is the first indicator. That's why it'd be interesting to see what Carl's results would be. I would think the gauge would not necessarily give warning in advance of RPM drop, but I guess it would depend upon the deflection and change of MP. The change in the 182RG was actually almost two inches in a few minutes after having done maneuvers without changing power settings.


Labor omnia vincit.
KDAL/KGKY and beyond.