Twin-Turbocharging: How Does It Work?

Turbochargers have been the holy grail for car modifiers for many decades, and stressing engine blocks to their limits with a twin-turbo setup has the potential to unlock ludicrous power gains. Whether your car has a turbocharger from standard or you’ve fitted an aftermarket system, the fast-spinning turbine blades have often been the go-to for petrolheads seeking extra horsepower.

Legendary cars such as the Mazda RX-7 and Ferrari F40 have used twin-turbocharging, so let’s look at how twin-turbocharging works and the different types available on the market.

How does twin-turbocharging work?

A twin-turbocharger works by using two turbochargers of the same size to force air into the engine’s cylinders to add extra horsepower. The exhaust gases are recycled, split between the two turbos, and usually combined in a shared inlet before entering cylinders. This forces a greater volume of air into the induction chamber, allowing the engine to create more powerful combustion strokes. This type of setup is known as a parallel twin-turbo system.

Which is better: single turbo or twin-turbo?

Both systems have their merits, but the benefit of a twin-turbo is that it has the potential for reductions in turbo lag compared to one single turbocharger doing all the work. The twin-turbo delivers lower boost pressures to reduce turbo lag, but the combination of the two turbines creates plenty of power.

In twin-turbocharged V-shaped engines such as a V8 or a V6, each turbocharger is usually assigned its own bank of cylinders instead of one large turbocharger forcing air through complex plumbing to make its way around the engine bay to the required cylinders. Each turbo can spool up more quickly, and directly supply its bank of cylinders without too much pipework to travel through. Reduced lag is also possible using two slightly smaller turbochargers when parallel twin-turbocharging, replacing one large turbo that will have larger vanes.

However, there are also a few other types of twin-turbo setups, which create power gains using slightly different methods.

Sequential twin-turbos

This setup uses two different sizes of turbochargers; a small-vaned turbo for low exhaust gas flow at lower engine speeds and then a much larger second turbo to take over once it’s had a chance to spool up.

A compression valve sits in front of the large turbo, making sure that all of the lower energy exhaust gases produced at the bottom end of the rev range are isolated to the smaller turbocharger to maximise power delivery at a rev range once useless to most single turbocharger setups. As the engine speed rises, the compression valve is opened slightly, allowing the larger turbine to begin to spool. The valve is then triggered to open fully at a set volume of airflow, allowing the secondary turbo to maximise its efficiency.

Sequential turbocharging takes away virtually all of the downsides of single turbocharging and supersedes a parallel setup, as the secondary turbo can be set to extremely high boost with the primary turbo eradicating any low RPM lag. Car modifiers can also go pretty crazy with a sequential system, varying the ratio between the small and large turbochargers to create some truly scary power deliveries. Think MkIV Toyota Supra, and you’ll be able to visualise possibly the greatest platform for sequential turbocharging.

Staged turbocharging

Using the same principles as a sequential setup, staged turbocharging uses a ‘stepped’ process to build air compression to extremely high levels before entering the engine’s cylinders. Starting with a small turbocharger, the air is passed directly to a slightly larger turbo, which further compresses the air. The final boost pressure in a staged system can be much larger than a standard twin-turbo system but is fairly catastrophic when it comes to lag. This is why it is generally used in diesel engines with high compression ratios and low rev ranges.

Twin-scroll turbos

You could opt for a twin-scroll turbo to save the hassle of using two turbochargers. This system is effectively two turbos crammed into one casing, with the exhaust manifold strategically split between the engine’s cylinders. In a standard single-scroll turbocharger, the exhaust pulses converge before and inside the turbocharger, creating erratic and turbulent airflow. The twin-scroll system allows the exhaust pulses to be kept separate and enter the turbocharger through their own inlets.

Twin-scrolling has become increasingly popular in modern cars, and it has made the art of turbocharging much more efficient in both packaging and performance. As a result, even four-cylinder engines fitted with twin-scroll turbos can achieve the power figures of a single-turbo six-cylinder engine from a decade ago.

Whether twin-turbocharging is just a dream that will never come to fruition through your stagnant project car or whether you’re a lucky owner of a car that happens to have it as standard, it’s an insanely cool way of turning up your engine’s performance.

Have you twin-turbocharged your car? Or do you prefer the character of a laggy single-turbo system or even natural aspiration?