The PA-28 Archer uses a fixed pitch propeller. Typically an aircraft with such a propeller, they are not fitted with a manifold pressure gauge as a means for gauging power output. Power output on a fixed pitch propeller is gauged by the engine tachometer. A Beech Bonanza is equipped with a constant speed propeller and will also be equipped with a manifold pressure gauge. This gauge will be used as a measure of power output for the engine.
You really won’t get into the use of manifold pressure until you start flying complex airplanes which have a controllable pitch propeller.
It’s not that a PA-28 engine does not have manifold pressure associated with it. It does. But there is no gauge mounted in the aircraft to measure it.
Neither manifold pressure, nor engine speed are exacting indications of power output; they are sloppy indications and can just be used as a general yardstick to approximate how much power an engine is producing, similar to the torque gauge in a turbopropeller airplane or an N1 or EPR gauges for measuring thrust output from a jet. In fact if you look at the cruise performance charts for a complex airplane, you’ll know that a particular power output requires both a manifold pressure and propeller speed setting in order to ensure an exact power output from the engine. Similarly in an airplane with a fixed pitch propeller they will do a general gauging of power based on propeller speed in the cruise performance charts. Note this power varies with altitude i.e. you will not get the same power out of the engine at 2400 RPM while flying at 6000 feet, versus flying at 3000 feet.
Remember that in an engine manifold pressure describes in fact the pressure of the fuel air mixture entering the engine manifold. When the engine is not operating, the manifold pressure will return to whatever the ambient atmospheric pressure is. Should an engine failure occur, the manifold pressure will not change, and will remain constant with a throttle setting provided the engine continues to turn. Typically manifold pressure is measured in inches of mercury, so for that bonanza if it’s sitting on the ground, engine off, at sea level @ STP, the manifold pressure gauge would read 29.92 inches of mercury.
Neither manifold pressure nor engine RPM are a reliable indication of an engine failure and should never be used as such for the reasons stated above. That was the problem the pilot was perceiving when he lost engine power. As soon as the engine power went the manifold pressure gauge returned to ambient atmospheric pressure, which coincidentally is exactly the same as if the engine was operating with a throttle wide open. Propellers can often windmill at maximum speed in a dive, which also prevents them from being a reliable indication of engine power. Your two most reliable gauges for determining engine failure in an AvGas powered piston airplane are 1) the fuel flow gauge and 2) the exhaust gas temperature (EGT) gauge or turbine inlet temperature gauge (TIT) for a turbocharged airplane. That seems counterintuitive, but for the problems I listed above with manifold pressure gauges it’s the only reliable way to determine whether engine is properly operating.
The throttle in a light training airplane like a PA-28 only controls the butterfly throttle valve in a carbureted airplane, or the fuel/air control unit in a fuel injected airplane. Engine tachometer only measures the rotational speed of the driveshaft and propeller