Throughout most of the 1990s, the W140 S-Class served as Mercedes-Benz's flagship sedan. It was one of the last overengineered Mercs of the era and came with everything from double-paned soundproofed glass to rain-sensing wipers and an adaptive damping system. These are features that were mostly unheard of at the time, and while not all of these gizmos aged well, the W140's timeless styling has.

Florida-based tuner Renntech decided that this generation of the S-Class was the ideal base for its new project. Based on a 1992 Mercedes 600 SEL variant, the Renntech S76R has been the beneficiary of upgrades to its bodywork, interior, and powertrain - the result is a massively powerful full-size sedan that could surprise a more modern AMG S63.

Recognizing that this is still a luxury sedan, Renntech has kept the exterior updates to a minimum. The original car's clean lines have not been overwhelmed by a custom front splitter and tail lip, new side body claddings, and bespoke Renntech wheels. The forged aluminum wheels measure 19 inches and are shod in Michelin Pilot Sport 4 S tires.

There are 20 mm shorter springs that marginally lower the ride height, and the subtlest of trunk spoilers can be found at the back. Here, the custom exhaust ends in dual tailpipes, but again, it's not as visually shouty as a modern AMG E63's quad outlets.

In black paint, the S76R absolutely nails the sleeper look.

What a sleeper it is, though, because the S76R is one enormously powerful machine. The M120 V12 engine has been extensively updated by Renntech, with the 6.0-liter capacity increase to 7.6 liters. An ME-1 engine management system was installed and the tuner describes the programing of this unit as a "highly tedious and time-consuming process," which is what some of Merc's engineers must've uttered under their collective breaths when presented with the brief for this car.

The custom exhaust system has equal length stainless steel headers, quad mufflers, and sport catalytic converters. The combined result of these changes is 615 horsepower and 703 lb-ft of torque at the crank, way up from the original's 402 hp/428 lb-ft.

The current most powerful S-Class that Mercedes sells is the S580 which has a twin-turbo V8 making 496 hp and 516 lb-ft, so you'd be well advised to wait for the upcoming AMG variants before taking on the S76R.

Because of the large increase in power, the original four-speed transmission was replaced by a five-speed automatic, while an OS Giken limited-slip differential was included to help the big Merc retain traction.

Renntech then created a bespoke AMG carbon ceramic brake system that fits the W140 chassis. Six-piston front calipers are complemented by four-piston items at the back, giving the S76R the stoppers to match its immense power.

As with the exterior, Renntech didn't subject the S76R's interior to a gaudy color scheme like some other tuners. Black is the dominant color in the cabin, with diamond stitched leather upholstery, an Alcantara headliner, and more leather on the dash. While wood inserts were typical on original W140s, the sportier S76R comes with carbon fiber inlays.

At the back, the rear bench was thrown out in favor of individual, fully-adjustable rear seats. The sound system boasts new speakers and the ability to connect your iPhone to it.

President of Renntech, Hartmut Feyhl, spent over a decade working at AMG before that tuning firm became part of Mercedes-Benz. His influence on the S76R is evident; it's an incredible build and a fine example of Renntech's technical expertise and love for the Mercedes brand.