Brabus increase the bore from 98 mm to 99 mm so just 1 mm more with new forged pistons at the same compression ratio. The stroke increase is where most of the displacement comes from at 96 mm versus the stock 90.5. This is courtesy of new rods and a new forged billet crankshaft.
On top of the internal engine changes the turbochargers are upgraded with new larger units and new manifolds. A conservative gain of 265 horsepower is stated and BenzBoost has no reason to doubt it. Yes, they also modify the transmission to cope with the increased power and torque. Brabus is likely capable of getting much more power out of this but you will pay handsomely for it. They even state the torque of over 1000 pound-feet is purposely kept down to protect the drivetrain.
Eventually the rest of the aftermarket will catch up and surpass this but as of right now this is the only documented built motor M157 setup with upgraded turbos. The increased displacement is icing on the cake. Brabus is also likely capable of sticking this motor in whatever Mercedes chassis they can get it to fit into.
Now for someone to somehow get a dynograph...
The new engine is based on the latest Mercedes 63 AMG eight-cylinder four-valve engine. For the increase in displacement to 5,912 cc (361 cu. in.) the BRABUS motor engineers brought their entire know-how amassed in the more than 35 years of the company's history to bear. Numerous engine components had to be newly developed and the engine block, cylinders and cylinder heads had to be extensively re-machined in the company engine-building shop.
Increasing the eight cylinder bores to 99 millimeters (3.89 in.) allows the use of special forged pistons with modified surface contours to achieve an even more efficient combustion. They are connected by billet piston rods to the precision-balanced billet crankshaft with a stroke of 96 mm (3.77 in.).
The two special BRABUS turbochargers with larger compressor unit and modified exhaust manifolds are of course also a major factor in the enormous increase in power by 195 kW / 265 hp (261 bhp) over the stock engine. Other performance-enhancing upgrades on the exhaust side are the metal catalysts with 75-millimeter-thick (2.95 in.) downpipes and the BRABUS stainless-steel sport exhaust whose pipe connectors have a diameter of 76 millimeters (3 in.).
The BRABUS installations on the intake side are immediately apparent upon opening the hood: in addition to the fiery-red BRABUS design accents on the engine, the BRABUS Gold Heat Reflection sheathing for the intake and charge-air tubes right away catches the eye. This measure also contributes to the high-performance experience provided by this engine, as does the newly designed BRABUS 850 intake manifold for the eight-cylinder four-valve engine. It includes a larger air filter box with enlarged intake cross section, which gets its air supply via a special air duct in the hood. Also part of the package is the carbon-fiber intake pipe with larger diameter, which was positioned behind the new BRABUS fascia directly exposed to the relative wind.
Special mapping for the engine management system programmed on the BRABUS high-tech engine test rigs and during road tests coordinate all high-performance components perfectly. This benefits not only the gigantic power yield, but also the outstanding throttle response and refinement of the engine across the entire rev range.
The performance data of the BRABUS 850 6.0 Biturbo high-capacity engine are correspondingly impressive: the V8 produces its peak power of 625 kW / 850 hp (838 bhp) at a low 5,400 rpm U/min. The equally potent peak torque of 1,450 Nm (1,069 lb-ft) is limited in the car to 1,150 Nm (848 lb-ft) to protect the powertrain as a whole. It is available steadily on a plateau between 2,500 and 4,500 rpm. BRABUS exclusively uses high-performance lubricants from technology partner MOTUL. A BRABUS limited-slip differential with a locking rate of 40 percent is available as an option.
In conjunction with the modified AMG SPEEDSHIFT MCT 7-speed sports transmission the BRABUS 850 "iBusiness" delivers a performance only attained by a very few full-blooded and significantly lighter sports cars. Sprinting from 0 – 100 km/h (62 mph) in 3.5 seconds is just as unrivalled in this class of vehicles as is the electronically limited top speed of 350 km/h (217 mph). The transmission either sorts the gears automatically or allows drivers to shift manually using the BRABUS aluminum shift paddles on the steering wheel.