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To reduce the power output (at any given engine speed) from the maximum possible, you still have to be able to trap a reduced mass of air in the cylinder.
That is the purpose of throttling; it reduces P in P.V = m.R.T in the cylinder. When the P of the air that fills the cylinder is reduced, the equation balances by reducing m on the other side (T ~constant).
The inlet manifold is merely a reservoir of low pressure air from which the engine can draw. The throttle is the means used to maintain the desired pressure in that reservoir of air.
It is a variable sized orifice which allows just enough air to pass through it to match what is being drawn out of the reservoir by the engine. It's not possible to allow air into the reservoir at a higher rate without also increasing the air pressure in the reservoir.
The pumping work that costs fuel economy is due to the pressure difference between that within and that outside the cylinder.
If you don't (want to) reduce P in the cylinder, in order to reduce m and regulate power output you have to do something else; either reduce V or increase T. (Lets leave out combinations of all three).
Increasing T of the air in the cylinder, with the same pressure on each side of the piston, might work if it didn't causes issues with engine knock.
Reducing the V of the air in the cylinder is possible by varying the valve timing. Ideally you would shut the inlet valve immediately after the desired mass (volume) of air had entered the cylinder. There wouldn't have to be any work done until after the inlet valve shut. After that point the work done is recovered on the compression stroke, following the exactly the same line on a PV diagram as it does when throttled.
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