When many think of compound turbocharging, they typically think of gutted-out race trucks rolling coal down the 1320. But it’s not quite as extreme as you may think.

Compound turbocharging is a method wherein two sizes of turbos work together to create more boost at any point on the throttle. This is particularly beneficial for diesel engines, which also generally run at lower RPM compared to gasoline devices. 

How Does Compound Turbocharging Work?

With a compound turbo system, there are two turbochargers arranged one after the other in series:

Primary (High-Pressure) Turbocharger

The smaller turbocharger takes the exhaust gasses first. This process provides an immediate boost at low engine speeds since it has rapid spool-up characteristics.

Secondary (Low-Pressure) Turbocharger

The bigger turbo is engaged as engine speed builds, dealing with increased exhaust flow to sustain and build boost at high RPMs.

It all starts with the turbocharger, which pulls in and compresses atmospheric air before sending it into the primary turbocharger for even more compression ahead of entering the engine. That compression stacking leads to increased boost pressures and greater engine efficiency.

Compound vs. Twin Turbocharging

It’s important to distinguish between compound and twin turbocharging systems:

Compound Turbocharging

Turbos are arranged in series, with one feeding the other to multiply boost pressure. This configuration is akin to connecting batteries in series to increase voltage, thereby increasing pressure in the system.

Twin Turbocharging

Turbos operate in parallel, each handling a portion of the exhaust flow independently. This setup is similar to connecting batteries in parallel to increase current, thereby increasing flow rate.

Both systems are designed to increase engine efficiency, but they are used for very different applications. Gasoline engines that run at higher RPM are more likely to employ twin-turbo setups; diesel engines needing more boost pressure at lower RPM tend toward a compound system.

To upgrade your factory 6.4L compound turbo system, check out KC Turbos.

Who Can Use Compound Turbocharging?

But compound turbocharging isn’t only for extreme performance or race cars. Some manufacturers have even put compound systems into production for increased efficiency and power delivery. An example is the compound turbo set up that the 6.4L Powerstroke diesel engine (2007-2010) came with from the factory.

The advantages of compound turbocharging:

• Improved Efficiency: By using two turbos to split the work up between them, it operates more efficiently across a wider range of engine speeds.
• More Power Delivery: Better, more responsive power throughout the RPM band for improved drivability.
• Reduced EGTs: Ideal for preventing rising EGTs that can be detrimental to engine life and performance under hauling/towing or other high-load applications.

To learn more about adding a compound system to your Duramax, Cummins, or Powerstroke, check out Pusher Intakes who specializes in these swap for nearly every year truck.

Considerations Before Implementation

Compound turbocharging provides a lot of great benefits, but it also does bring complexity to the engine system. Handled right, and executed properly — it will work fine. That said, such additional performance potential can quickly wear on engine parts if not correctly controlled.

In short, compound turbocharging is an advanced forced induction technique that marries the advantages of different sizes of turbos to achieve optimal performance and efficiency. This is particularly useful in diesel engines, where it means a smoother power curve and better behavior over a wider variety of conditions.

To learn more about compound turbocharging check out this helpful FAQ from Pusher Intakes, or for upgrading your 6.4L Powerstroke – check out this video from KC Turbos.