Quote:
Originally Posted by serialk11r
Except, a turbocharger is always creating a non-trivial amount of restriction in the exhaust, and on throttle plate controlled engines this always translates into pumping loss. Electronic wastegates can reduce this slightly though.
Furthermore, positive displacement superchargers are always equipped with bypass valves and twin screw type superchargers sometimes have electromagnetic clutches, so in most driving conditions the parasitic load is very small.
Thus, most people who add superchargers seem to report identical fuel economy, while people adding turbochargers usually see reduced fuel economy. Under full load though, a well designed turbocharger system is certainly more efficient.
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Actually, if you remove the muffler like I did, the restriction is quite trivial under light loads, and parasitic loss is far lower under boost than that supercharger. I wouldn't rely on fuel mileage reports from power junkies; that said I get 29-30 mpg on a factory turbo engine swap in my 280Z, despite the low compression ratio (7.4:1), and that was better than the original n/a engine in the car. Best I ever got on the original engine was 25mpg, same speeds, same trips.
I'd be willing to bet turbos are more efficient than a clutched supercharger system across all operating conditions. But I've not seen data if there is any.