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Picture it this way... if the engine is running, the exhaust turbine is turning which means the compressor is also turning. If that is turning, it is moving air. This is the key, the air is ALREADY moving when it comes into the engine. It doesn't have to go into boost to affect efficiency. Anything that will increase velocity even slightly in the manifold can give better fuel distribution cylinder to cylinder which will make the engine more efficient which could equal better mileage.

Kevin


There are so many flaws in this I don't know where to start. First, yes the turbine is spinning. Why? Because of the restriction in the exhaust flow. That in itself is a give and take in fuel economy. The give.....a restriction means less airflow through the engine. Less airflow means less fuel, except,(here's the take) the "in cylinder" mixture gets tainted from the restriction, and weakens the combustion process.
There is also parasitic loss. There is no way to spin anything without expending energy. The energy in this case comes from the fuel.
Turbos and superchargers don't speed up airflow. They increase density. There is a throttle plate between the compressor and the intake manifold on most gas engines. The throttle plate plays a big role in manifold density. At idle and light cruise there is a vacuum in the manifold. The less vacuum, the higher the load on the engine. More load(more air) equals more fuel.
Bottom line is a turbo is an energy exchanger, a machine. You can't convert energy forms without a loss. You wont get more out than you put in.
A turbo can make an engine more powerful, but not without more fuel. The only thing that a turbo is intended to do is add air to an engine. If you add air, you need to add fuel. Adding fuel is not the path to economy.

"use it 'till it breaks, replace as needed"