Volumetric Efficiency.

[Trevor]

Originally Posted by shotgunslade

There is a significant different between the behaviour of positive displacement compressors like the Whipple and the centrifugal compressors typically seen with turbochargers. A positive displacement device delivers a fixed volume of gas depending upon its speed of rotation. The actual mass flow will be dependent upon that volumetric flow and the pressure drop on the inlet side of the compressor. Pressure drop on the inlet side will reduce the density of the incoming charge and thus reduce the delivered mass flow. The mass flow is not significantly affected by the pressure drop on the discharge side. The main effect of restriction on the discharge side is to increase the power required to rotate the compressor. This increase in parasitic power is small compared with the increased power generated by the increased mass flow. So, the power increase of a positive displacement device is not very sensitive to pressure drop between the compressor and the engine, and dynamic tuning of that conduit has little effect. Mass flow through a centrifugal compressor on the other hand is dependent upon pressure drop on both sides of the compressor as the compressor generates a pressure differential without creating a fixed volumetric flow. Thus tuning and resonant design of the conduit between the centrifugal compressor and the engine can generate increased mass flow of air into the engine, by reducing the pressure against which the compressor is working. A centrifugal compressor can be deadheaded without destroying anything, because it cannot generate flow at a given speed of rotation against backpressure above a certain level,. A positive displacement compressor, if deadheaded, will either explode something on the discharge side, or will break the compressor drive.

Well spoken and spot on !

Even so and in accordance with the above, the effects of a resonant design originally intended for a naturally aspirated engine will not be at best when matched with a centrifugal type supercharger.

I am fully aware that certainly you with a shotgun and most others, will appreciate this obvious fact, but felt it necessary to re inforce the point in view of previous strange statements.

Cheers, Trevor.