Can anyone tell me where the MAP sense line is tapped from on these "big" engines?
My thoughts are that the MAP gauge should be sensing the Blower output.
From the Pratt & Whitney "Manual of Engine Operation" (1949, relates to all Pratt radial engines): "In practice this pressure is not measured at an intake port, but at the rim of the supercharger collector. It is known as manifold pressure, or absolute blower rim pressure (abrp), or even manifold absolute pressure (map), and is expressed in inches of mercury (in. Hg.).*"
So yes, it senses blower output, one would think.
The MAP is used as a (but not THE) engine power measuring value, hence measuring the blower output will be largely useless to the pilot as variations in throttle setting will not alter the blower output. Only RPM change or (on some) vane angle change will do that.
The throttle valve is
before the supercharger, so variations in throttle setting will change the amount of air allowed into the blower and as a consequence change the output pressure of the blower (= MAP, see above). Obviously RPM changes will also change blower output (MAP).
And PS, this is not just paper wisdom. They let me fly DC-3's every now and then (Pratt & Whitney R-1830's), and every time I move the throttles the MAP (= blower output, see above) definitely changes - a lot. When I move the propeller pitch levers, the MAP also changes - a little.
And before anyone asks, no, I'm not gonna try to switch off the fuel to one engine next time to see what happens
