Founded by master technician Holger Mohr, who represented Kleemann in Germany for many years, IMSA also had its own Lamborghini tuning programme. Today however, Holger sees his future in an aftermarket personalisation segment whose goal is to maximise the inherent all round potential of super sports cars. The RXR One project is the vanguard of this new direction.
“The idea for this car came out of a specific request from one of my existing Lamborghini customers,” Holger explained. “This owner had reached the stage in life where he needed a more practical supercar for use as a daily driver, but was not willing to give up the performance of his tuned Lamborghini.”
RXR stands for Road X-tra Racing, and the brief was to marry the performance of a major league mid-engine supercar with the inherent practicality of a traditional front-engine, rear-wheel-drive GT car like the AMG GTS on which the RXR One is based.
“The AMG GTS was the best car on which to base this project as it already ticked all the boxes in terms of practical packaging, its front-mid-engine, transaxle gearbox layout, and race-style double wishbone suspension.”
Having set the specification with IMSA, the owner then agreed that other like-minded people should be offered the chance to share this incredible concept, hence the idea of a limited edition car built to each owner’s colour and trim specification.
IMSA approached the transformation of the AMG GTS into the RXR One with a four-pronged design and engineering strategy: Weight reduction, aerodynamics, power increase, and chassis upgrades.
Replacing substantial sections of the bodywork with carbon-fibre panels, plus weight reduction in other critical areas results in a saving of around 300kg, bringing the DIN standard weight to a shade under 1,300kg. This makes for a power-to-weight ratio of 1,118hp/ton!
Designed with the help of a state-of-the-art computer programme, the autoclaved carbon-fibre bodywork of the IMSA RXR One achieves the optimum balance between drag, downforce, and a purposeful appearance.
The larger carbon-fibre wheel arches extend the car’s width 50mm per side (100mm overall) in front, and 55mm per side (110mm overall) at the rear. In comparison, the factory AMG GT R is 57mm wider overall front and rear than the GT S, but is only 15kg lighter. The wider arches of the RXR One are optically and functionally linked by the deep side-skirts that enhance the ground effect generated by the cars completely flat bottom.
Other bespoke carbon-fibre body components are the complete front bumper/spoiler assembly. The bonnet, roof, boot-lid, and rear wing are other major components made from autoclaved carbon-fibre.
The nose section contains air intakes for the intercooler and oil cooler, while the bonnet features a ram intake for turbocharger cooling, as well as venting for the main radiator. The race-inspired complete flat bottom and air outlets for the front and rear wheel housings help to reduce high-speed aerodynamic lift. At the rear, airflow departing from the completely flat underbody is controlled by the large carbon-fibre rear diffuser, which features deep boundary layer fences.
The AMG M178 family 3,982 cc twin-turbo, dry sump V8 motor has been comprehensively re-engineered through lightening and balancing the factory steel crankshaft and mating it to bespoke forged pistons with forged H-section connecting rods.
A bespoke intake system running from the front of the nose individually feeds each cylinder bank via modified turbochargers, a twin-row intercooler system, and larger throttle bodies. The cylinder heads are ported, polished and gas-flowed and exhaust gases escape via a bespoke Inconel exhaust system, which helps to lower backpressure. The resulting reduction in exhaust gas temperatures allows the use of higher boost pressures and output is 860hp with 1,000Nm of torque.
To reliably handle this massive output the gearbox was re-engineered using a competition grade clutch pack featuring two additional clutches, a forged intake drive shaft, high-pressure valves, and an independent oil cooling system. The rear differential also has its own oil cooler.
“Dissipating heat quickly is paramount on short racetracks in particular where you do not build up a lot of speed but are constantly accelerating and braking so that the engine heat soaks,” Holger explained. “Then the thermal protection protocols in the ECU mapping will start to dial back the power to preserve the engine. However, if you get on top of the heat dissipation issue then you can maintain full power throughout.”
The solution was to return to basics and re-design and re-dimension the engine, gearbox and differential cooling systems to deal with the extra heat generated by the greatly increased engine output.
For starters the radiator was swapped for a more efficient unit with a big cooling fan. “We changed the way the whole cooling system works, making it both simpler and more effective in airflow terms,” said Holger. “It is more like a racecar system now, and even the two turbochargers now have their own independent water-cooled circuit.”