Twin-Turbocharging: How Does It Work?

Who wants one turbo when you can cram two in there? Here's how it can be done...

Turbochargers have been the holy grail for power gains for many decades, stressing engine blocks to their very limits through additional horsepower and heat output. Whether your car has a turbocharger stock or has been modified with new injectors and a manifold to accommodate one, the fast spinning turbine blades have often been the go-to for petrolheads seeking that beloved choo-choo.

But if a fairly sizeable chunk of additional power isn’t quite enough to quench your thirst, twin-turbocharging could be the answer. With legendary cars like the Mazda RX-7 and Ferrari F40 having not one but two turbochargers at their disposal, it’s time we had a look at how twin-turbocharging works and the different types available on the market.

Parallel twin-turbos

This is about as standard as twin-turbocharging gets, using two turbochargers of the same size to work together to force air as quickly as possible into the cylinders. The exhaust gasses recycled to the turbos are split equally between the two but usually combine again in a common inlet before entering the cylinders.

The benefit of this simplistic system is the potential for much less turbo lag than from one large turbocharger doing all the work. In V-engines, each turbocharger is generally assigned its own bank of cylinders, instead of one large turbocharger having to force air through convoluted plumbing to make its way around the engine bay to the required cylinders. The lack of lag also occurs due to the convention to use slightly smaller turbochargers when parallel twin-turbocharging, replacing one large turbo that will have larger vanes. This makes the spooling process much easier for the incoming air.

To keep these benefits while balancing the need for power, the general rule is that parallel turbos should be set at relatively low boost pressure to reduce turbo lag, but with the combination of the two turbines creating ample power.

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 gasses 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 therefore 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, relying on the primary turbo to eradicate any lag lower down. Car modifiers can also go pretty crazy with a sequential system, varying the ratio between the small and large turbocharger 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 up 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 compresses the air further. The final boost pressure in a staged system can be much larger than a normal 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

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

Used famously in BMWs of late including the M2, twin-scrolling has made the art of turbocharging much more efficient in terms of both packaging and performance, giving four-cylinder engines the capabilities of much larger capacity six-cylinder engines of a previous generation.

The future

Other ways of improving the capabilities of twin-turbocharger setups have been engineered very recently, with the most extreme coming from Audi with its SQ7 performance SUV. The Range Rover Sport SVR-rivalling barge uses a standard twin-turbocharger system, supplemented upstream by an electric compressor. Designed to pre-compress air straight from the intercooler, the electric fan spins at up to 70,000rpm to further aid the boost pressure of the air that eventually reaches the cylinders.

Although Audi claims this effectively eradicates lag, one should be cautious about applying such a component to their own turbocharged system as many aftermarket ‘electric turbochargers’ are simply electric fans that will do nothing but restrict the exhaust gas flow to the turbine vanes.

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 the suck, bang and blow of any internal combustion engine.

Every petrolhead must surely dream that someday they can turn up at the local meet and pop the bonnet to expose a pair of gleaming turbochargers the size of their own head, gaining the admiration and jealously of every V-TEC-loving Civic fanboy strolling past. So keep dreaming and be patient - there’ll be an unmolested Supra out there for you somewhere, I promise.

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