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Note: There are important considerations to the twin turbo concept given today’s technology as will be discussed more in Chapter 6, Designing a Turbo System. Multi-stage (or compound) turbo systems are a very exotic method of turbocharging and are usually reserved for extreme applications such as tractor pulling or similar high-performance applications. However, Caterpillar manufactures a 550-hp truck engine that uses two turbos staged together to reach higher boost levels and air density than could be durably achieved using only one turbo.

Each compressor design is trimmed to flow the amount of air compatible to an engine’s air requirement based upon that engine’s cubic inch displacement, volumetric efficiency, and operating RPM. A compressor that’s too small restricts the engine and limits horsepower. Likewise, a compressor that’s too large requires more power to spin than the turbine is capable of developing and thus will not spin fast enough to supply the airflow and pressure that compressor would otherwise be capable of developing.

B. The air is then compressed, which increases its density—or amount of mass per unit of volume—by the turbo’s compressor section, and is discharged through a boost tube (2). C. Most of today’s turbo systems use an aftercooler, also known as an intercooler or charge-air cooler, to receive the hot, boosted air as it leaves the compressor. The cooler removes the excess heat, which allows for further air density improvements while also cooling the intake air charge (3). D. Another boost tube then routes the air from the cooler’s discharge side to the engine’s intake manifold where the air is routed to the intake valves and it enters each cylinder (4).