Series turbochargers, made popular by the Mazda RX-7, use a smaller turbo to get moderate boost for quicker response and a larger unit to generate airflow necessary for big horsepower. Electronic turbos that use a small motor to initially spool up the turbo do basically the same thing--build boost more quickly for sharper response and lower emissions. Caterpillar uses series turbos on three of its larger road engines, the biggest a 15.2-liter I-6 that makes 134 pound-feet of torque per liter. We expect most future turbos to be variable geometry, electronically assisted, or series.
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