AMG is a German aftermarket firm that for decades has been modifying Mercedes cars for maximum power and performance. It also comes with upgraded leather and wood interior, a bit more aggressive body styling than a regular M-Class, bigger tires and brakes and all that power and speed. Cylinder bores are larger in diameter than normal, and the magnesium intake manifold has a wider cross-section to supply more air to the bigger engine.
Camshafts are a new, lightweight, modular design and there are higher-tension valve springs, too.
All these modifications crank up the horsepower to 342 at 5,500 rpm, and torque zooms to 376 foot-pounds at 3,000 rpm. The RX does not offer a V8. The ML55 is the heaviest of all these vehicles with a curb weight of 4,861 pounds. With the standard five-speed, electronically controlled automatic transmission marshalling the power, the test ML55 zoomed forward from a standstill. It wasn't a neck-snapping sensation, because the transmission seemed to manage the power smoothly.