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In the halcyon days of turbocharging, Porsche, Mazda, Subaru, and Toyota all offered cars with twin sequential turbos. Now only Bugatti does it.
On one of the strangest, coolest days I've ever had on this job, I drove a Porsche 959. The car belonged to the Porsche Museum. Its minders threw me the keys for a quick rip on the roads around Sonoma Raceway without supervision.
The whole experience felt bewildering, thanks in no small part to the singular driving experience of the 959. Its 2.85-liter flat-six owed much to Porsche’s dominant 956 and 962 prototypes and used two turbochargers for a (likely underrated) output of 450 hp and 370 lb-ft of torque. The turbos operate in sequence. Below 4000 rpm, exhaust gasses from all six cylinders spin the right-hand-side turbocharger; beyond 4000 rpm, exhaust gasses from the left-hand cylinder re-route to its corresponding turbo.
Paul Frere's The Porsche 911 Story has a fabulous explanation of the 959's complicated plumbing system, but all we really need to know here is that the system reduced turbo lag significantly. If the driver went to wide-open-throttle at 2500 RPM, the 959 would reach its maximum 14.2 psi of boost in two seconds, whereas with a simple parallel twin-turbo setup, the same would take 6.5 seconds. Out around Sonoma, the 959 felt like nothing else. Below 5000 rpm, it feels like a pretty quick old 911; above, it's astounding, with an almost violent surge to its 6500-rpm power peak. A couple colleagues also drove the 959 that day. All of us spoke in hushed tones about what happened when that second turbo was going.
Yet, the next time Porsche offered a twin-turbo car, the 993-generation 911 Turbo, the 959's sequential system was abandoned. Porsche never revisited the concept. Few automakers have.
The principle behind a sequential turbocharging system is fairly basic. A large turbo may provide the desired peak power, but it takes a long time to spin up a big turbo, leading to big-turbo lag. Twin turbochargers operating in series—where one feeds the next—have the same problem. Two turbos operating in sequence theoretically offer the quick spool of a small single turbo, with enough oomph to deliver lots of air, and thus, horsepower, boost at higher engine speeds.
The 959 proved influential, especially on Japanese automakers. The late Eighties was the bubble era in Japan, when automakers had practically unlimited budgets to engineer whatever the hell they wanted. (Fun aside: Nissan actually bought a 959 for benchmarking while it was developing the R32-generation Skyline GT-R. It did not copy the 959's sequential turbocharging, though the GT-R drew lots of inspiration from Porsche's variable all-wheel drive system.) Mazda went with sequential turbochargers for the third-generation RX-7 and in the three-rotor Eunos Cosmo. Subaru did the same with various (mostly JDM) Legacy models. The most famous sequential-turbo equipped Japanese car, though, is the Toyota Supra, with its twin-turbo 2JZ-GTE. All these systems use the same basic principle as the 959—exhaust gasses are routed to one turbocharger at low RPMs, then around 4000 rpm some of those gasses are diverted to a second turbocharger.
So, does sequential turbocharging work? Yes and no. At the time, these setups were a huge technological step forward, and we were extremely impressed at the time, but by today's standards, sequentially turbocharged cars feel odd. You wouldn’t call them super laggy, but the introduction of the second turbocharger is never 100-percent seamless. It's almost as if there's a momentary flat-spot in power delivery, then wham! Off to the races.
That's not the main issue, though. Sequential turbocharging requires all sorts of convoluted plumbing and valving to manage the path of exhaust gasses. This introduces costs related to both engineering and manufacturing, plus a lot more failure points. The 959 cost so much to develop, it nearly broke Porsche. For the Japanese automakers inspired by the 959, the extra money to devise complicated systems like this dried up when the Japanese asset price bubble burst in 1992, shortly after Mazda and Toyota had introduced their sequentially turbocharged cars.
Plus the performance gains of sequential turbocharging were questionable. Sure a 959 feels like a huge step forward from earlier 911 Turbos, yet a 993 Turbo, with its simpler parallel turbocharging setup—one turbo for each bank—doesn't feel like a step back. "The low inertia of the turbine and compressor wheels and their small diameter ensure a quick response," Frere wrote of the 993. "This was much quicker than was the case in the Porsche 959 (which, in theory, should have been almost immediate), though it must be remembered that the 26 percent larger capacity of the [993 Turbo's] 3.6-liter M64 engine helps."
"It's all about how much power we can make with the least amount of warranty claims, the least amount of costs," says BMW tuner and engine builder Steve Dinan. "It's more than what the technology is worth. All that technology still works, it's just that the incremental gain is so small and the cost is so high, the cost-benefit ratio isn't there."
Most building 2JZs for big power ditch the complex sequential plumbing in favor of either one big turbo or two smaller turbos operating simultaneously. "I'm sure there's some level of engineering you could do to optimize a certain power level using twin sequential turbo setup," says legendary engine builder Stephan Papadakis, "but the complexities in engineering, and the complexities in the exhaust system with valves and the control of the valves, turbochargers of slightly different sizing… there's much simpler ways of netting at least as good performance."
Papadakis Racing exclusively uses single turbos in its 2JZ builds, as simplicity and reliability in its motorsports-prepped engines is more important than low-end response. Many still build twin-turbo 2Js, but it's rare you'll find one with the sequential setup intact. Most tuning the RX-7's twin-rotor ditch the sequential turbo setup for the same reasons, and because the Mazda system is unreliable.
It's also worth noting that turbocharging fell out of fashion for production road cars by the end of the Nineties. They only returned in a major way in the 2010s, as automakers realized that downsized, turbocharged engines offer superior performance on increasingly strict emissions and fuel-economy tests. By this point, both internal-combustion and turbo technology more specifically evolved to make sequential turbocharging undesirable beyond the complexity it brought. Lighter materials for the turbine and compressor wheels mean turbos spool up faster, and sophisticated engine controls help develop more low-end power, negating the need for large single or twin turbochargers.
BMW did briefly experiment with sequentially turbocharged diesel straight-six and V-8 engines with as many as four turbochargers, but one imagines these died for the same reasons of cost and complexity. Still, automakers are still introducing new technology to help reduce lag and improve response in turbocharged engines. Perhaps the most common is twin-scroll turbochargers, which use exhaust manifolds designed to blow the turbine in pairs, feeding more energy into the turbocharger and back to the engine. More exotic is variable-vane turbochargers, which use flaps to effectively change the shape of the turbine wheel, helping improve performance across a wide powerband. These are used in diesel engines, while Porsche is the most prominent booster (sorry) of variable-vane turbos in gas engines.
A more recent development is the electric turbocharger, which connects an electric motor to the common shaft between the turbocharger’s turbine and compressor. The electric assist provides virtually full boost pressure in an instant. Mercedes-AMG is using these on the new C43 and C63, while the brand's 53-series straight-six models use an electric auxiliary compressor. This compressor functions like a turbo, but uses an electric motor to spin the compressor wheel, rather than exhaust gas. The electric auxiliary compressor can route boost into the engine's intake directly, or push air through the exhaust-gas turbocharger.
There is still one automaker using sequential turbocharging, for now—Bugatti. For the Chiron's W-16 engine, a simple valved system routes boost to one turbocharger per cylinder bank until 3800 rpm. At that point, the valve opens and the third and fourth turbochargers get to work. According to Bugatti test driver Andy Wallace, max boost can be reached by the primary turbochargers on each bank, but as the revs climb higher, the 16-cylinder needs more air, hence more turbos.
It's telling that the only car that uses sequential turbocharging these days costs well over $3 million. Soon, too, the W-16 will end production, because even Bugatti is not immune to tightening regulations.
Perhaps Bugatti's next production engine will stick with sequential turbocharging, but for the vast majority of cars going forward, if you're going to have two (or more turbochargers) it just makes far more sense for them to operate in parallel. More often than not, the simplest solution is the best.