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Keeping the rotating assembly may make for a balanced motor in terms of rotational/oscillating mass. However, you'll never account for the fact that your power strokes will now become unevenly spaced.
Tell me this. Why can a smaller displacement motor generally create the same amount of power than a larger motor while doing so at higher efficiency? This is a comparison of a smaller motor and a larger one at the same power level.
It's because of pumping work. An engine is a pump. It's pumping air from intake manifold pressure to exhaust manifold pressure. Any time you pump from low pressure to high, you have to input energy. The difference is that the smaller engine, in order to keep up at a lower power level is running at higher manifold pressure. Therefore in terms of pumping work, gas engines are more efficient when they are working closer to their maximum volumetric capacity. So a 10L engine working at 10% throttle is less efficient than a 1 L engine working at 100% in terms of pumping work. This is one reason why it's more efficient to "shift early and use more throttle" in manual cars. Delta P is smaller.
This is one of the reasons for EGR. By introducing non-volitile air, you can cut back the fuel to air ratio, raising intake pressures at a given power output. Pumping work falls. Delta P is smaller.
This is one of the driving forces behind BMWs efforts to eliminate the throttle plate and use variable timing and duration intake valve actuators. They get to keep the intake manifold near zero vacuum and control the charging of the cylinders with the intake valve. No pumping losses because they are changing the charge volume rather than the charge pressure of the cylinder. Delta P is zero.
If you totally deactivate cylinders your motor is going to shake and wrattle like crazy. If you deactivate cylinders and still leave the valves active, you're going to put extra oxygen into the exhaust and cause the computer to dump extra fuel into the active cylinders - serving only to reduce economy, throw a MIL, and burn up the catalyst.
If you want to do this, remove the pistons and rods from the undesired cylinders. Redesign a new cam and crank around a rebalanced firing order. Reprogram the computer, and drive off into the sunset.
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