Only One Tri-Turbocharged Engine Was Ever Mass Produced

The turbocharger has been around since 1905, when Swiss engineer Alfred Buchi patented a ‘turbine compressor’ that provided an internal combustion engine with forced induction. Since then, the humble turbocharger has evolved from a crude and basic turbine into a highly advanced piece of performance kit.

In some instances, up to four turbochargers provide additional thrust for a high-performance engine, virtually doubling the horsepower. Like Bugatti’s amazing Chiron W16, a supercar capable of more than 300 mph.

Twin turbochargers are far more common, and many cars now use this configuration. Production cars with three turbochargers are far less common – only one mass-produced engine with three turbochargers does service in a handful of BMW models.

Under Pressure?

Alfred Buchi may have patented the concept of a turbine compressor for an internal combustion engine in 1905, but it apparently took the Swiss engineer another decade to create the first working prototype. Even after all that lead time, the world’s first turbocharger proved finicky and unreliable.

Buchi’s turbocharger revolved around use in airplanes, and was supposed to compress the air fed to the engine at higher altitudes, ensuring engines did not lose power at great heights. In 1920, the Packard-Lepere biplane set a new altitude record, exceeding 33,000 feet – a feat previously thought impossible. It did so thanks to a Packard turbocharged and supercharged Liberty L-12 diesel engine, producing 450 hp.

Major Rudolph Schroeder, chief test pilot of the US Army’s engineering division, flew the plane to a world-record height of 33,114 feet. Schroeder suffered some emergency at that altitude, possibly due to a lack of oxygen after an equipment malfunction.

Facing oxygen starvation, Schroeder apparently put the plane into a steep dive, losing 30,000 feet before pulling up. After safely landing, he was whisked away in an ambulance, requiring medical attention. The theory and efficiency of the aeronautical turbocharger had been proven beyond doubt.

Yet it would require another 42 years before the first turbocharged production car hit the road. That car was the Oldsmobile Turbo Jetfire. The Jetfire’s 3.5-liter V8 breathed through a Garrett T05 turbocharger, and the engine had 215 hp. The so-called ‘Turbo Rocket-Fluid’, a complex cooling system using a mixture of distilled water and methanol, kept the turbocharger running cool.

Unfortunately, the Jetfire’s turbocharged engine proved woefully unreliable, the turbo and engine picking up issues, combined with iffy performance. It didn’t end there. The suspension was apparently horrendous, combined with a steering system that made an oil tanker’s steering seem sharp and responsive.

It was a gallant and visionary effort, but after just two years on the market, the Jetfire was discontinued, with around 9,000 cars made (and most of those recalled back to the factory).

It would require some silly-fast 1970s racing cars to really get the turbocharger ball rolling.

Porsche Gives It A Boost

In the early 1970s, Porsche’s 917 turbocharged racing car changed the world forever. The hugely successful 917 racing cars won the Le Mans 24 Hour race, and dominated virtually every race it entered.

By 1973, the Porsche 917/30 variant, featuring an enlarged 5.4-liter version of the flat-12 engine, came with twin-turbochargers, and produced up to 1,580 hp. The 917/30’s domination of the American Can-Am series was so complete, spectators lost interest, eventually leading to the formula’s demise by the end of the 1974 season.

On the flip side, the experience Porsche’s race engineers gained from the 917 project had a direct impact on its development program for its road car engines. In 1974, Porsche revolutionized the performance car realm with the introduction of the erstwhile 911 Turbo. The first-generation 930 Turbo had a three-liter, turbocharged flat-six engine that produced 260 hp. It only needed 5.5 seconds to complete the 0-60 mph sprint, and could reach 155 mph.

The golden age of the turbocharger had officially arrived, thanks to Porsche’s innovative use of the technology, combining blistering performance with everyday usability. It was even reliable.

Three’s Company

Modern turbochargers are highly advanced and efficient, manufactured from exotic materials such as magnesium and titanium. Variable geometry turbochargers, twin-scroll turbochargers, anti-lag systems, twincharger systems (a combination of supercharger and turbocharger) and electrically assisted turbochargers have changed the face of combustion engine performance.

Twin-turbocharged gasoline engines have become the norm in the majority of high-performance cars. Car companies like McLaren, Ferrari, Lamborghini, Aston Martin, Porsche, Ford, Nissan, Maserati, BMW, Mercedes and Audi all feature twin-turbocharger technology in their finest performance machines.

On the other end of the scale, Bugatti employs no less than four turbochargers to power its 300-mph Chiron, which produces 1,500 hp.

So what’s the story with three turbochargers? Surely some rookie BMW apprentice punched in the incorrect numbers on a calculator when they designed this lob-sided engine?

One, Two, Three… Boost!

BMW’s N57 3.0-liter straight-six common-rail diesel engine was introduced in 2008, and for many years served in almost every model in the BMW inventory. It originally came with one turbocharger, was later upgraded to twin turbochargers, and finally, with BMW’s famous M department getting in on the act, the N57S version with three turbochargers was introduced in 2013.

The M engine was deployed in the 550d xDrive, the X6 M50d and the X5 M50d. The three-liter engine produces 381 hp and 546 lb-ft of torque. The 550d xDrive sprints from 0-60 mph in just 4.6 seconds, and top speed is limited to 155 mph. The same car can also reach a claimed 44.8 MPG, ensuring it offers the best of both worlds… not only is it fast, it also sips very little diesel.

But why three turbochargers? And how do they work? Firstly, all three turbochargers are different in size with varying boosting pressures, and each has a unique job. Two of the turbos are comparatively small, high-pressure units with variable turbine geometry, working in conjunction with a larger, low-pressure turbo.

One of the small turbochargers activates at engine speeds just above idle. Thanks to its low inertia, it spins up without delay at the slightest prod of the throttle pedal, providing compressed air to the combustion chambers from the lowest engine revs. As the engine speeds increase, the exhaust gas flow also reaches the larger turbocharger, which gets a piece of the action from as low as 1,500 r/min. This means the peak torque of 546 lb-ft is available between 1,500 and 3,000 r/min.

When the driver is really in a hurry, and pins the throttle pedal to the carpet, a vacuum-modulated exhaust flap opens up in a split second, diverting extra gas flow past the first small turbocharger, which is already in the game, to reach the large, low-pressure turbo. The third turbocharger, which is integrated into this bypass line and also has a low inertia and spools up very quickly, now also gets a piece of the action, adding even more charge pressure.

Now, with the larger turbo boosting at full capacity, and the two smaller ones doing their part too, the full 381 hp is unleashed.

This process is precisely controlled by the engine management system, ensuring the optimal pressures are applied as required. Also, if the large turbocharger is operating at maximum revolutions, a vacuum regulator opens a wastegate valve to relieve the pressure and avoid exhaust back-pressure (which is detrimental to performance, and the engine, in the long run).

Furthermore, the supply of fresh air is meticulously regulated through pneumatically activated flaps. At low revs, a bypass flap sends the air directly to the smaller, high-pressure turbocharger, spinning it into action right off the bat. If the engine revolutions are below 2,700 r/min, a change-over flap prevents the air from reaching the third turbo to prevent unnecessary fluctuations in pressure.

The power is sent to all four wheels via an M-tuned eight-speed automatic transmission, with a manual shift option. BMW’s M division also specially tuned the xDrive system.

We Want More… And More!

If you find the BMW X6 M50d’s stock 381 hp and 546 lb-ft just, well, bleh, you can always head to a company like German tuning company G-Power. The company offers an ECU upgrade and exhaust upgrade that hikes power to 455 hp and torque to 641 lb-ft.

G-Power’s engine remapping also disables the top speed limiter, so this 3.0-liter triple-turbo should be able to reach about 170 mph. You can also throw on some lightweight Hurricane RR forged alloy wheels.

Of course, the stock triple-turbocharged straight-six engine offers plenty of power in stock trim. BMW’s most powerful diesel engine ever is no longer with us… when the N57S engine was replaced by the B57 in 2015, BMW introduced a quad-turbocharged version… and the triple-turbo N57S was done for.

But the world is all the better for the fact that it exists, and that BMW’s M division had the vision to create such a technological marvel.