I had recently had a look at the Ford Trimotor NC9645 (link), currently on an EAA-sponsored tour. The propellers were fixed pitch (possibly adjustable by mechanic on ground), and the engines were not supercharged. On the instrument panel directly in front of the pilot were manifold pressure gauges for all three engines. Oil temperature, oil pressure, and RPM gauges for the central engine were nearby on the panel. The same three gauges for the wing-mounted engines were out on the engine nacelles-- rather difficult to read precisely from the cockpit.
The pilot seemed to fly mainly by reference to the gauges on the panel, only occasionally (at least after warm-up and run-up was completed) checking the gauges on the nacelles.
So, why did this aircraft have manifold pressure gauges for all three engines on the instrument panel in front of the pilot, while other gauges for the wing-mounted engines were mounted on the nacelles? Is it simply easier for some reason to transmit the manifold pressure over the distance from the engines to the instrument panel, than is the case for the other parameters? Or is the manifold pressure significantly more useful to the pilot than are the other parameters?
In a non-supercharged aircraft with fixed-pitch props, does the manifold pressure tell the pilot anything useful that an RPM gauge would not?
Am I correct in believing that it's rather uncommon today to have a manifold pressure gauge for an engine with no supercharger and a fixed-pitch prop? Is this something that used to be more common on aircraft from the earlier days of flight?