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Turbochargers contain two turbines (think two pinwheels) on a common shaft. One turbine is plumbed into the exhaust system and the other into the intake system. Hot exhaust gas flowing at high velocity is routed through the exhaust side turbine and spins the turbo at high speed. On the other side of the shaft another turbine, the compressor, is spinning and compressing incoming air that is then forced into the engine.
Auto manufacturers are beginning to have a love affair with turbos for smaller engines. This is due to the efficiency inherent in the turbocharger design. To some degree, the energy to drive the turbo is “free.” The engine is already producing hot, fast flowing exhaust gasses, so capturing some of that exhaust energy to drive a compressor and increase the power production of the engine does not draw a lot of parasitic energy out of the engine. Thus, small engines can produce the power of an engine the next class or size up, yet retain much of the fuel efficiency of the small engine.
In race and hot street engines, when spooled up, turbos can make some awesome horsepower numbers. In 2011 in the Pro Mod classes where you can run turbos, superchargers, or nitrous, twin turbo race cars were the ones to beat.
But turbochargers have a downside: heat, plumbing complexity and lag. You want to locate the turbocharger as close to the engine’s exhaust ports as is practical, which means they’re usually located under the hood. There’s extra exhaust plumbing needed to route exhaust gasses first to the turbine and then somewhere to exit the car. All of this extra tubing introduces heat and can be complicated. And for large engines, like Hemis, you really want two turbos, which just doubles the plumbing issue.