Automatic wastegate
Ignacio,
Looking back through this thread, I guess we all missed the 'real' meat of your question which was the operation of the 'automatic' wastegate.
Insofar as the 1973 through 1980 P337 series, the wastegate is controlled by oil-pressure which is varied through separate oil-line plumbing to a device called the 'VAPC' which is the acronym for Variable Absolute Pressure Controller, located on top of the engine. It is mechanically interconnected to the throttle body in such a way that upper deck pressure from the compressor section of the turbocharger is sensed by a vaccum sealed aneroid mechanism, similar to your home barometer, that regulates or modulates the amount of oil pressure in an oil-line from the outlet side of the wastegate actuator to the controller and to a scavenger pump back into the engine's oil supply. That's a simplified explanation.
Manifold pressure is maintained at whatever throttle position you select by the VAPC sensing compressor discharge pressure at the throttle valve inlet, which is called 'upper deck pressure.' The set-up is such that the controller setting varies proportionately to the amount of power selected by the throttle through the mechanical linkage to the throttle arm.
When the engine is started, the oil pump located on the lower back side of the accessory case builds pressure and there is a capillary tubing attached to the bottom of the accessory case in the sump area from where oil is drawn into the wastegate inlet chamber which controls a valve and ultimately a mechanical butterfly inserted in the exhaust pipe.
At idle throttle there is very little compressor discharge pressure, so the VAPC is allowing all of the oil flowing through the capillary tubing to the wastegate, through the outlet side up through a larger oil return line to the VAPC and back into the engine, thus the wastegate is 'open'. Now, when the throttle is advanced, the throttle body calls for more fuel to be injected, the engine driven fuel pump increases fuel pressure, the compressor discharge pressure momentarily decreases as the air enters the engine and the engine begins to accelerate. As the air pressure decreases, the VAPC senses the decrease and closes off the oil pressure in the return line from the wastegate and oil pressure builds in the wastgate chamber which closes the valve and thus repositions the butterfly to a more closed position. There is also a spring that holds the wastegate 'open' and the oil pressure must build to a sufficient level to overcome the resistance of the spring.
As the engine RPM increases more exhaust is diverted from the exhaust pipe through the exhaust turbine end of the turbocharger and the increased exhaust pressure blowing on the turbine increases the compressor speed (remember they're on the same shaft). As the turbine speed increases, more compressor discharge pressure is created by the impeller at the ram-air end of the shaft until it balances with the manifold pressure and throttle setting selected. As the airplane climbs the VAPC senses the lower pressure changes and closes the wastegate butterfly further by restricting the oil flow until eventually reaching what is known as "critical altitude" where the system reaches equilibrium. The 'automatic' part of the wastegate is actually the VAPC sensing differences in upper deck pressure which in turn repositions the wastegate butterfly.
I've simplified this as much as possible, and it's a lot easier with pictures to show the flow of things, but I think you'll get the basic idea. Climbing to higher altitudes, the VAPC 'automatically' senses lower air pressure and continually closes off the oil pressure in the return line from the wastegate, but descending to lower altitudes is not automatic... you must continually reduce throttle settings as you descend as air pressure increases in the lower atmosphere.
Since you're new to this realm, what I suggest is you order a copy of a POH (Pilot Operating Handbook) from from one of the Cessna dealers for a 1978 P337H and go through the "Airplane & Systems Descriptions" in addition to the other tabbed areas on "Limitations", "Normal Procedures", "Performance", etc. The reason I suggest the 1978 version is because they come with the original Cessna cover and they're assembled such that the punch holes are placed on an extra width margin which won't impair the printing and your reading. The 1978 manual is more refined than earlier versions and you'll find that with the exception of 4724 maximum ramp/taxi weight, all other parameters are the same as the 1973-1980 versions of the P-model.
Hope this helped you udnderstand the 'automatic' wastegate. It's really the VAPC that's the heart of the entire system.
SkyKing
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