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Perhaps in the real world it works better in some way, but if you just examine the thermodynamic cycle, and consider a turbine that works purely off a pressure drop, the piston has to supply most of the energy powering the turbine. The turbine has some losses, so at the end of the day we don't have much of an efficiency improvement.
If you draw out a P-V diagram for yourself you'll see what I mean. It's very important that the turbo collects the kinetic energy of the exhaust pulse wave to end up more efficient than a plain old supercharger (acting as an impulse turbine). If the exhaust manifold and turbine housing are not optimized, you have a definite loss in efficiency. The point of twin scroll appears to be faster spooling, but dividing the exhaust pulses improves its function as an impulse turbine (and also acts to scavenge exhaust gas from cylinders better), which is probably why BMW put a twin scroll on a 1.6L mini that only makes barely over 200hp.
What's more is as I said the turbine is always sitting in the exhaust stream, and the only method to relieve backpressure is to vent with the blowoff valve. However the blowoff valve itself has a large pressure drop. A supercharged engine with bypass valve or clutch simply acts like a naturally aspirated engine at low loads while the same engine with a turbocharger will always experience additional backpressure as described. I don't doubt for a moment that under heavy load a turbo is more efficient, it's just a turbo has some disadvantages a supercharger does not have, and that the advantage is not as big as people might think.
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