Turbocharger starts producing boost only above a certain rpm (around 1200-1500rpm) due to a lack of exhaust gas volume to overcome the inertia of rest of the turbo propeller. Power suddenly increases after that particular rpm when turbo propeller starts spinning. So power Vs. rpm curve of a turbocharged engine has a steep increase in power at boost threshold rpm. There have been many advancements in technology to reduce boost threshold rpm below idle speed rpm of the engine, so as to virtually eliminate the boost threshold.
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