My 172N with 180 hp O-360-A4M and fixed-pitch prop came to me with a manifold pressure gauge installed. I like it and use it.
The power charts in the Lycoming manual use manifold pressure in the calculation of horsepower. Yeah, it's far more complex and precise than necessary for routine operation, but you can't use the chart at all without manifold pressure data.
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I use the manifold pressure gauge for initially setting cruise power when I level off. The rpm will fluctuate with changes in airspeed and in up- and downdrafts, so setting power with the manifold pressure is quicker and more reliable.
Not sure where it came from, but my airplane has a little placard on the panel with a "rule of thumb" power setting chart (photo below). You take the MP in inches and add it to the RPM in hundreds. The sum corresponds to a percentage of power, example: 22" and 24(00) rpm = 46, which according to the table is about 68% power. The beauty of this is it works -- reasonably accurately -- at any altitude.
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Also, as shown above, on every low-altitude takeoff I can scare myself that I'm ruining the engine by running "oversquare" ...