Engine Instrument Systems
What each gauge really measures
Instrument questions center on what a gauge measures and how its markings are read.
Tachometers show engine or propeller RPM and may be mechanical (cable driven), electrical (tach generator), or electronic. Manifold pressure (MP) is the absolute pressure in the induction system, measured in inches of mercury, so with the engine stopped at sea level it reads about 29.9 in Hg - not zero. That single fact appears repeatedly. RPM plus MP defines power output on a normally aspirated engine.
Other instruments: oil pressure and temperature, fuel pressure and flow, cylinder head temperature (CHT), exhaust gas temperature (EGT), turbine inlet temperature (TIT), torque (turboprop), N1/N2, and engine pressure ratio (EPR). Know the standard markings: green arc for the normal range, yellow arc for caution (no continuous operation), red radial line for maximum, and the red line on the MP gauge. Markings and limitations must agree with the FAA-approved AFM/POH.
Traps
Mixing up EGT and CHT limits; treating a manifold pressure gauge as a fuel pressure gauge; assuming an ammeter displays system voltage. Also remember the EGT probe is mounted close to the cylinder for fast response.
Rule of thumb
At a fixed throttle setting, manifold pressure drops about 1 in Hg per 1,000 feet of altitude gained - a quick sanity check on climb power. For mixture, best power is typically about 100 degrees F rich of peak EGT; best economy is at or slightly lean of peak within the manufacturer's limits.