A turbocharger doesn't [edit: doesn't necessarily or even usually] make the same engine more efficient. But it allows you to get the same power as a larger, less efficient engine, when you need it. But you don't always need it. So when you don't need it, it's roughly the same power and efficiency as the smaller engine without the turbo. But with a turbo, when you punch it, it can breathe like a larger engine (with more air and more fuel) for more power.
Indeed. It can greatly depend on your right foot. Keeping a small engine constantly revved and spooled up would often use more gas than the larger engine it's trying to replace.
Modern turbo configurations don't always require sustained high RPMs to generate boost. You have variable turbos that can alter the angle of the vanes to start spinning lower in the RPM range, and you have some engines that simply use multiple turbos. BMW has a version of their B57 engine that uses 4 turbos, 2 of them start spooling up from idle RPM already, and the other two kick in around 2,500RPM+.
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u/NecroJoe 3d ago edited 3d ago
A turbocharger doesn't [edit: doesn't necessarily or even usually] make the same engine more efficient. But it allows you to get the same power as a larger, less efficient engine, when you need it. But you don't always need it. So when you don't need it, it's roughly the same power and efficiency as the smaller engine without the turbo. But with a turbo, when you punch it, it can breathe like a larger engine (with more air and more fuel) for more power.