Don't disagree with anything said. Backpressure plays a role in the deal too (e.g., number exhaust valves, header/manifold/cat). Each engine is engineered out for a specific task. I guess I was thinking about the example more literal. The plane is on the ground with the chocks on no aero forces on prop, the throttle valve on carb is fixed at some position and the only thing being done by the pilot is toggling the prop pitch from fine to course to increase the thrust load and lower the rpms. If the rpms on the supercharger pump are directly proportional to turns on the driveshaft (i.e., the belt drive gearing is a fixed ratio), what would make the manifold pressure increase if the rpms of the engine and supercharger are both proportionally decreasing from the increased prop load and fuel being metered is unchanged? I think what blackdog is saying makes sense. Backpressure on the exhaust side holds the air in the intake manifold longer for that particular engine at that range of rpm/load change and fuel rate. Another thing to consider, the supercharger being belted to the driveshaft. I guess is a variable load on the engine. It takes energy to pump air. So while the prop is increasing load to lower engine rpms, the lower rpms on the supercharger pump are decreasing load (still assuming fixed fuel meter). That's intuitively what I was thinking about above where cranking that supercharger pump at certian rpm range with the increase in load that it brings to do that work doesn't pay off verses normal aspirated. Jeez, what a brain drainer this one is.
05-30-2011, 07:58 AM
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