If you attended this year’s Geneva Motor Show, you could be forgiven for thinking that the Mercedes-Benz SLS AMG and Porsche Cayenne are the most-tuned cars on the planet. However, where the SLS show cars on various stands featured body styling that covered the spectrum from the sublime to the ridiculous, the gray car on the Brabus stand stood out for throwing down a power and speed challenge to the major league supercars. Dubbed the Brabus 700 Biturbo, this car has exactly what it says on the tin: 700 hp from a twin-turbocharged version of AMG’s 6.2-liter powerhaus V8.
Because most supercar enthusiasts are not interested in SUVs, they would not automatically connect the 700 Biturbo to the Brabus ML63 Biturbo developed in 2009. But this existing engine meant that the groundwork was already laid, so the project was not a major undertaking. Since the AMG V8 in the SLS is fundamentally a tuned, dry-sump version of the V8 in the ML63, the issues simply revolved around adapting the existing ML twin-turbo system for the SLS. In practical terms, this installation was actually easier on the SLS since the engine has more space around it than in the ML. This provides better airflow for the intercoolers, and allows the use of a more efficient exhaust system.
Step one is a complete engine strip down. The major internal change is swapping out the standard pistons for a set of forged Mahle Motorsport pistons that reduce the compression ratio from 11.0:1 to 9.0:1. This allows Brabus to use a modest 0.9 bar of boost with no fear for the reliability and longevity of the motor.
The Brabus ML63 Biturbo requires a relatively complex bespoke exhaust manifold due to packaging issues. This has one section serving three cylinders and the turbo, with the fourth pipe joined by a balance pipe.
In comparison, the SLS manifolds are very straightforward, with long, equal-length primary pipes made from high-nickel-content cast iron to withstand temperatures up to 1,200 degrees Celsius. The basic turbocharger is based on a Garrett unit, which is then heavily modified by Brabus. The scroll housing is Garrett’s, modified to relocate the oil pipe exit point from its bearing housing. Inside, the turbine wheel and compressor wheel are both bespoke units.
Brabus’ Deputy Development Chief Jorn Gander explains: “If you take a turbocharger from the manufacturer’s catalog, you will find that the innards rotate clockwise. However, in a twin-turbo arrangement, we found that the airflow in the system is only optimal if the turbochargers are mirror-imaged; otherwise the flow characteristics of two clockwise-rotating turbochargers create pulses that adversely affect response times.
“So we have the innards of the left-hand turbocharger specially made with the blades that rotate counter-clockwise to balance things out. The tricky part on both units was getting the oil flow to the bearing housings just right. We put a lot of effort into optimizing the oil pressure and size of the feed lines.”
The twin water-to-air intercooler system is basically half of the setup used on Brabus’ most potent V12 Biturbo engines, which use four such intercoolers. With their own self-contained water circuit independent of the engine’s cooling system, these efficient intercoolers throw off heat far more effectively than an equivalent air-to-air system.
Other than the pistons, the rest of the engine internals are unchanged. The standard cylinder heads are well machined, and in any case, force-aspirated engines are less sensitive to head porting than their naturally aspirated counterparts.
Downstream of the turbos and exhaust headers there are special catalytic converters. The rest of the exhaust system is made from titanium, the same as the normal Brabus SLS exhaust. With four 84mm outlets, this system saves 26 pounds and weighs 40 percent less than the factory stainless steel equivalent.
One useful feature of the Brabus exhaust is its pneumatic sound flap system that allows you to vary its voice at the touch of a button. In its quiet setting, the system cuts significant decibels off the factory standard level. On full noise, the system is louder than stock, thus providing the best of both worlds.