Are turbocharged engines really more fuel-efficient?

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The energy used to run the compressor in a turbocharger comes partly from the wasted exhaust energy (part comes from the exhaust pumping work), so the energy needed for boosting is partly free. The turbo does create extra back-pressure, so that does have penalties (parasitic loss and worse knock resistance at high power). The energy used to run the compressor in a Supercharger comes 100% from the crankshaft, so it is all parasitic loss and all of it contributes to higher cylinder loading and worse knock resistance. The net result is that turbochargers can provide more boost more efficiently than Superchargers.

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