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Carl,

Hope this doesn't sound like one of those "how come you don't know that" sort of a question, but how do you use the manifold pressure gauge to set mixture. I've used an EGT and RPM to find mixture setting, but never a M.P.G. - is it the same sort of thing, wait for a drop and push back a little??

Please explain....

Cheers

Beats me! The 140 pilots were talking about it. That's one of the reasons I want to install a temporary gauge and do some experimenting. I definately believe it can help detect the onset of carb ice, even before a power loss is evident. I think it might help set mixture, since the amount of fuel in the mixture can affect cooling of the induction air, and therefore density and breathing, which might show up ever so slightly on the gauge. One thing that puzzles me, is why (from what I'm told) Cessna hooked the gauge to a boss on #2 cylinder, above the intake valve. Seems like that would cause a flux everytime #2 intake opens. I'm not about to drill and tap a hole in my cylinders just for an experiment (the boss is there, just not used. A holdover from the old fuel injection system, I think). I'll tap into the induction spider, instead.

I don't expect any industry standard revelations here. Just covering old ground our predecessors thought worth the effort.

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.