ark (01/18/83)
Every piece of plausible evidence I have seen indicates that there is no magic in the general relationship between manifold pressure and engine RPM. In other words, one should not assume that if an engine is operating at 25" MP, it should necessarily be run at 2500 RPM or faster. Consider: (1) it is only a coincidence that manifold pressure and engine RPM are measured in units that come out to similar numbers. Suppose MP were measured in atmospheres, or Pascals, or that engine speed were measured in radians per second? (2) It is stretching the bounds of plausibility to ask me to believe that all engines have the same operating limitations regardless of design. Engines with fixed-pitch props attached are almost always run "over-square" during takeoff and climb-out. Turbosupercharged engines are run "over-square" in level flight too. If I recall right, the takeoff setting for a Mooney 231 is in the vicinity of 2700 RPM and 41 inches MP (!! -- someone correct me if I'm wrong) The most reasoned advice I have seen is that if the airplane's operating handbook gives a range of allowable MP and RPM and does not restrict the combinations in which they may be used, it is all right to use them in any combination. Now to specifics. I am a member of a flying club that operates a Cessna Cardinal RG. The handbook for this airplane says that normal operating conditions are 15" to 25" MP and 2100 to 2500 RPM. It is usual practice in our club to cruise at 25" and 2100 RPM at all altitudes at which we can get 25" (up to about 5000 feet), and full throttle if we can't get 25". The reason is that the club charges by tach time and this power setting gets the most distance per tach hour, so it minimizes trip cost (unless there's a big headwind). One of our members is a certificated mechanic, and completely approves of this procedure. The engine ran more than 200 hours past recommended TBO before we finally decided to have it overhauled. The overhaul shop (Mattituck) said that it was in much better than average shape when they tore it down. Whether this advice is correct for your particular airplane and engine, I can't rightly say. Perhaps you might ask the manufacturer, or a good mechanic or overhaul shop.