Many automakers advertise their cars as race-inspired. Usually, it’s meaningless ad copy. Today’s race cars are highly specialized devices with increasingly little relevance to their road-car counterparts. But, Porsche can credibly say its new hybrid 911 is race-bred.
The problem faced by the engineers developing the 911’s new T-Hybrid system was the same one faced by a lot of racing teams a decade ago. In 2014, Formula 1 and LMP1 introduced new engine regulations with limits on fuel flow. More specifically, they regulated how much fuel an engine can use in an hour. For upcoming Euro 7 emissions regulations, the EU is cracking down on fuel enrichment. The problem in all cases is, how do you use less fuel while still maintaining good performance? Answer: By implementing a very clever hybrid system.
For the new 911 Carrera GTS, Porsche developed a system that pairs a new 3.6-liter flat-six with an electric motor attached directly to the crankshaft and a large electric turbocharger. Both are driven by a small 1.9-kilowatt-hour battery mounted in the trunk where the 12-volt starter battery used to live. Conceptually, this is very similar to an F1 powertrain, and it also carries some influence from Porsche’s Le Mans-winning 919 LMP1 car.
Porsche’s aim with the T-Hybrid was to make a system that allows the engine to run at Lambda = 1, the ideal air-fuel ratio of 14.7:1, as much as possible. (In America, the term “stoichiometric” is frequently used to describe Lambda = 1 running.) Per a 2020 article in the magazine Race Engine Technology, Porsche ran the 919’s V-4 at Lambda = 1 for its 2015 victory at Le Mans. And by the end of that car’s career in 2017, it was developed to run super lean Lambda values of up to 1.3, or 19.1 parts of air for every one part of fuel.
Then-F1 chief technical officer Pat Symonds explained in a 2022 presentation that the sport’s turbo V-6 hybrid power units routinely run at Lambda = 1.3 or 1.4. It’s worth noting that Porsche actually had an F1 engine in development at the turn of the last decade, in time for 2021, but the program was scrapped. So, the company has a lot of experience in lean-combustion engines, at least in motorsport.
The T-Hybrid system is closer in concept to a Formula 1 powertrain than the 919’s. The 911’s electric turbo is essentially a Motor Generator Unit-Heat (MGU-H), while the motor attached to the engine itself is a Motor Generator Unit-KERS (MGU-K).
An electric turbocharger is a lot like a conventional turbocharger, except that on the shaft connecting the turbine and compressor wheels, there’s an electric motor. In the 911, the turbo’s motor makes 14.7 horsepower on its own and it can either spin up the turbo to compensate for lag as exhaust gasses build up, or brake the turbo. That allowed Porsche and supplier BorgWarner to ditch the traditional wastegate and use the motor as a generator when limiting boost.
Essentially, you’re recovering energy that would otherwise be lost to heat, hence the letter “H” in MGU-H. In the new 911 GTS, the turbo has its own power inverter, so the energy it recovers by braking the turbo can be sent back to the battery or directly to the motor sandwiched between engine and gearbox. Jorg Bergmeister, the Porsche factory racer who helped develop the car, says that on the Nürburgring’s long Dottinger Hohe straight, the turbo is recovering energy to power the other motor, while still boosting the flat-six.
“This is the next step in turbocharging,” says Matthias Hofstetter, head of powertrain for the 911. Porsche engineers half-jokingly say this turbo is sized for something like a diesel truck, and with an 83-mm compressor wheel, it’s probably not far off. One of the main goals with the T-Hybrid system was to develop something as light as possible—hence why Porsche didn’t go for a plug-in system with a larger battery. Hofstetter explains that using one large turbocharger saves a lot of weight compared to using two smaller ones, but that creates its own problem.
“We had to make it electrified because you know the exhaust [coming from the opposite side of the engine] is very weak,” he says.
In the 911 GTS, the turbo is stuffed right up ahead of the passenger-side bumper, at a 45-degree angle from the cylinder head. And even if you were to, in theory, mount this single turbocharger more centrally, it would still be laggy, given its large size. That plus the ability to recover otherwise-lost heat energy from the exhaust, boosting thermal efficiency significantly, made using the E-Turbo obvious.
“For us and for me, it's an extremely good solution because when you're sitting in the car, you don't feel if it’s one turbocharger or two turbochargers,” Hofstetter says.
In the 919, Porsche used a traditional turbocharger mounted atop the engine, but it also employed what is effectively half an MGU-H in the exhaust, an electric motor attached to a turbine wheel in the exhaust that generated electricity to be stored in the battery, and deployed by an electric motor at the front axle. Hofstetter tells Motor1 that while no engineers from the 919 program came over and worked on the 911, the two teams consulted each other.
The 3.6-liter flat-six itself is a totally new design, though it still belongs to the 9A engine family Porsche debuted way back in 2009. In an era of downsizing, Porsche upsized to 3.6 liters, with a bore of 97 mm and stroke of 81 mm. The bore-to-stroke ratio is the same as the 3.0-liter twin-turbo used in the base 911 Carrera, which Hofstetter says provides ideal fuel burning. Compared to the 3.0-liter, Porsche flipped the position of the spark plug and fuel injector, which helps with fuel atomization.
There’s still a traditional 12-volt electrical system in the 911 GTS, but since the motor between engine and transmission works as both a starter and generator, there’s no need for a traditional alternator. So, the 3.6-liter has no belts. Accessories like the A/C compressor and water pump are powered by the high-voltage system, which helped make the engine smaller despite its larger displacement. It’s a little narrower, which allowed Porsche to upsize the rear tires from 305 mm to 315 mm, and it’s shorter. On top, Porsche mounts the intercooler from the 911 Turbo, the motor inverter, the A/C compressor, and a DC-DC converter.
Another race-inspired detail is the finger-follower valve train, which eschews traditional bucket-and-shim tappets for rocker arms to actuate the valves. They are more typically used in high-revving engines—like the 911 GT3’s 9,000-rpm flat-six—to prevent valve float, but Hofstetter explains that they’re employed here to help reduce overall engine width. Plus, the system has less friction than a traditional valvetrain. Porsche also uses variable valve timing here, but not lift.
You can’t talk about the engine without talking about the motor bolted directly to it. While a lot of other performance hybrids have a motor in the same position, Porsche’s is a lot less powerful, and there’s no clutch between it and the crankshaft. That means there’s no all-electric drive mode—and if there was, the 1.9-kWh battery would offer very little range—and its impact in general is more subtle. But Porsche’s system is far lighter than anything else on the market.
The motor makes 53.6 hp and 103 lb-ft of torque on its own and it’s used to help compensate for turbo lag at the low end, and at higher engine speeds, to provide for more linear power delivery. It also does some regenerative braking, taking some of the load off the rear brakes, though the 911 still uses a traditional brake pedal that only connects to the hydraulic braking system.
Though this is an oversimplification. Essentially, every time the driver hits the throttle pedal, the car interprets this as a torque request, and depending on a number of different factors, engages the turbo, electric motor, and engine in various combinations to give the driver what they want. In Sport Plus mode, the electric turbo spools up under heavy braking; when the driver gets back to power, the turbo is braked as the car hits its target boost pressure and the energy recovered can get sent directly to the motor or to be stored in the battery. And so on and so forth.
What’s remarkable is that the system is likely doing something different just about every single time, yet as a driver, you have no sense of this.
“You don’t feel it,” Hofstetter says. “Our control units manage everything. We manage the complete net of the car, and it decides what to do. More turbocharger, more e-motor, or more battery.”
The hardware here is complicated, but the software might be an even bigger challenge, as the car simply now has more ways to respond to a torque request. You can watch a power-flow display in the center console, but it’s honestly baffling, so quick is the handoff between all the different elements of the hybrid system.
Porsche doesn’t play up the motorsport connection super hard here. In a presentation, it features the 919 on a Powerpoint slide with its other past hybrid road and race cars, but that’s it. The automaker doesn’t use the terms MGU-H or MGU-K, even if they accurately describe the major hybrid components.
Yet the connection is there. A friend has long said automotive engineers do their best work when painted into a corner. That’s true of the people that developed F1’s turbo-hybrid powertrains—which, no, don’t sound as good as V-10s but are inarguable engineering marvels—and LMP1 systems like the 919’s V-4. And it’s true here. Faced with the need to reduce emissions and fuel consumption, Hofstetter and everyone else working on the 911 came up with something extraordinary.
This is racing technology for the road.
