G-Force Junkie — Society Marbella

In the summer of 2022 I drove the Brabus 820, based on the latest Porsche 992 Turbo S. It proved to be hypercar fast both away from the line and accelerating from a rolling start, and I felt just short of uncomfortable with the g-forces it generated.

Fast forward to this summer and the Brabus Rocket R 900 “1 of 25” Limited Edition managed to make me feel seriously uncomfortable during a full- bore launch on grippy tarmac. The automotive equivalent of aviators G-LOC is something I have never experienced before in almost 40 years of performance testing, even with the likes of a 1,001hp Bugatti Veyron, and it was not a pleasant sensation. It was like a small gravity bomb had gone off inside my skull through first and second gears as the four fat rubber contact patches did their best to rip the tarmac away from Mother Earth. For a couple of seconds my brain was both shaken and stirred!

Today’s frontline jet fighters like the F-16 and F-35 are designed to pull 9g in combat manoeuvring, but the actual real-world limitation is the pilot’s ability to resist blacking out as blood drains away from the head under high G-forces. This is where training and a g-suit come into their own.

G-forces acting on the driver is an issue with modern racing cars as well. With
high downforce an integral part of the performance envelope that defines Formula 1 and Le Mans LMP cars, accelerating at over 2g, cornering at 3.5g,

and braking at 5g over race distance is very punishing to the human body.

Thus, an F-16 pilot sits fairly reclined compared to previous generation fighters, and if you have the opportunity to sit in a modern F1 race car you will be surprised to find that the seat is tilted back at nearly 30 degrees.
One of the reasons the Porsche 911 has always been the go-to daily driver sportscar for many is its sheer practicality. Apart from its ability to carry some luggage in the frunk and on the folded down rear seats (not GT models), you get in and out of a 911 like a normal car, avoiding the contortions required with low, mid-engine machinery. While the Porsche’s fairly normal upright seating position is very comfortable and allows you a much better view of your surroundings in traffic, it also means thathard acceleration and braking forces act on your body at almost exactly 90 degrees to the longitudinal axis. Thanks to the weight of its rear-mounted engine a rear-driven 911 has superior traction off the line than a front-engine rear-drive car, and an all-wheel-drive 911 simply has every advantage.

A big issue in an optimum launch situation is the balancebetween traction and slip. In launching an early rear-driven 930 Turbo, a modicum of wheel spin as you depart the line lets you avoid the clutch spin situation that does the drivetrain no favours. However, not all 4WD 911s are born equal, and the latest 992 Turbo S is the fastest accelerating 911 variant ever away from the line. Thanks to its near perfect mechanical and electronic pairing of engine, gearbox and the front and rear differentials, optimum acceleration off the line and from a rolling start is a given.

Brabus’ CTO, Jörn Gander explained how the indecently rapid Brabus 820 that I drove in the summer of 2022 had morphed into the Rocket R 900. “While the changes made to ramp up the air and fuel demands of the larger turbochargers to reach a reliable 820hp were a significant jump from the stock 650hp, finding an additional 80hp was more of an incremental process,” he said.

Since the 1980s Brabus has been a master of increased displacement engines,
and their first thought was to bore and stroke the 3.6 liter flat-six motor to 4.1 liter for the 900hp conversion. However, the small volume of potential sales versus the sky-high development costs did not make sense in view of the likely two-year remaining shelf life of the 992 Turbo S on which the Brabus car is based.

The rational alternative was to concentrate on improving efficiency by further refining the turbochargers, increasing boost pressure and optimising the control software for both the engine and the PDK gearbox up and downshift protocols within the closed loop system.

With nearly 50 years of expertise in forced aspiration Brabus were well placed to completely re-engineer the VTG turbochargers, and not one component from turbine wheel to uprated bearings was left untouched.

Back-pressure was reduced in the post combustion stage with a tailored Inconel exhaust system for low weight and efficient heat transfer. Incorporating the mandatory particulate filter, this helps to eke out the last few horses while delivering a more rousing soundtrack. Working with the factory valve control system, this gives you the choice of town friendly ‘Coming Home’ or open road ‘Sport’ modes.

The pair of bespoke Brabus VTG turbochargers used for the 820hp conversion feature larger compressor and turbine wheels within modified housings. To find the additional 80hp, or roughly 10%, the engineers directed their attention towards optimizing the profiles and geometry of the turbine and compressor blades for greater efficiency. In the old days engineers used a suck it and see process, but with today’s state- of-the-art computers and test equipment the development process is both very much faster and more accurate. However, even major manufacturers work with the research departments of major universities for some highly specialized areas of engineering, and it is not uncommon for Graduate, Masters and PhD students to end up joining these very companies after their time in academia.

Brabus has a good working relationship with the Engineering Department at Heidelberg University, and once again called on them for help with the Rocket R 900 project. The cutting-edge Computational Fluid Dynamics (CFD) equipment in the faculty can accurately predict airflow in and out of a given engine during the combustion process.

In this case the required target output of 900hp pointed towards the need for a 1mm increase
in the compressor wheel diameter along with slightly revised blade geometry. Once the new components were created and installed in the modified turbocharger housing, the ECU software was adapted to maximise the optimised airflow capability. The power increase Brabus was looking for showed up on the engine dyno printout, and on 1.9 bar of boost the 3.8 litre flat six makes 900hp at 7,200rpm and 1,000 Nm of torque at 5,100rpm. “To make full use of the extra 80hp we set out to fine tune the way the drivetrain responds by carefully calibrating the gearbox and clutch control software to the revised power and torque curves. The 900hp arrives at 7,200rpm, compared to the factory stock 650hp at 6,750rpm,” Gander explained.

“The PDK gearbox is mechanically strong enough
to handle the extra output, and we increased
the clutch clamping pressure to handle the extra power and torque and prevent the clutches
spinning during a full-bore launch on a grippy surface.”

Our test instruments recorded 2.5 sec for the 0-100km/h (62mph) sprint, with 200km/h (124mph) coming up in just 7.2 sec. Top speed is electronically limited to 340 km/h (211mph), due to the speed rating of the tires. While some other very quick cars might equal these initial acceleration numbers it was the sheer g-forces imposed by the lack of tire slip when using the Launch Control that delivered the aforementioned uncomfortable few seconds.

Form definitely follows function here in the way the wind-tunnel developed aerodynamics have been parlayed into a unique and distinctive Brabus house style that the Porsche world has never seen the likes of before. Jörn Gander explained that the Rocket R 900’s aerodynamic development proved to be one of the most demanding projects Brabus has ever undertaken. The wind-tunnel optimized Brabus Widestar carbon-fibre suit of clothes presents a greater frontal area to the air molecules than the factory fresh Turbo S. As well as moving the centre of pressure aft, the greater downforce created by the much larger carbon-fibre front spoiler produces a better overall aerodynamic balance for the tail-heavy Turbo S, changing the downforce ratio from the stock 80% at the rear to a more even 60%.

Behind the large front air intakes new pipework channels ram air to the factory radiators as well as the PCCB brakes. Bespoke carbon-fiber wheel arch liners with wind-tunnel derived vents pair with the distinctive alloy wheels to smooth airflow in the front wheel housings, speeding up its exit to enhance laminar flow past the doors into the large rear arch intakes feeding the intercoolers. Ride height plays a big part in aerodynamics as the car itself is actually the biggest spoiler of all if you think about it. The expert here is an ex-Dallara engineerwho helped Brabus optimize the final set-up for their coil-over suspension, which works
in conjunction with the factory PASM and front-lift systems.

The prominent carbon-fiber front spoiler that uses the factory mechanism to lower itself further into the airstream at speed
is matched at the rear by an integrated spoiler inspired by the unique arrangement first seen on the Group 5 Martini 2.8 RSR that debuted at the 1973 Monza 1000 km race.

On that first outing the RSR had makeshift aluminium spoiler panels riveted to the bodywork on either side of the Ducktail spoiler, these additional winglets giving rise to the ‘Mary Stuart collar’ nickname. By the time the Le Mans 24 Hours came around in June this wrap around spoiler had been properly integrated into wider rear wheel arches.

Taking its design cues from the Group 5 RSR, the Brabus rear spoiler, topped
off by a carbon-fibre Gurney flap, rises and tilts to produce similar downforce to the extendable rear wing on the stock Turbo S. Meanwhile, the extendable racestyle carbon-fiber front spoiler makes significantly more downforce than the factory chin spoiler. The wider wheel arches of the Rocket R 900 cover a set of 9.5J x 21 and 12J x 22-inch diameter Brabus Monoblock P forged alloys wrapped in 255/45ZR21 and 335/45ZR22 Continental Sport Contact 7 rubber. Tailor- made with unique offsets just for this car, these wheels feature carbon-fibre Aerodisks designed to help clean up the airflow, while their peripheral slots extract heat coming off the huge brakes when they are being used hard.

When fully unleashed the Brabus Rocket R 900 is a controlled explosion on four wheels with more than enough go to match its extrovert show. However,
it also has a softer side that allows you to cruise around town like the stock car.