The rugged G-Class may be the oldest model series in Mercedes-Benzs range as it has been constantly in production since 1979, but the Germans arent about to let it go yet as they continue to improve their most famous luxury off-road vehicle.

Our spies recently caught up with a couple of prototype models of the flagship AMG variant while testing in Europe. The G-Wagen SUVs were fitted with new exterior mirrors featuring the same design as the ones found on newer Mercedes passenger models and a modified lower front end with three large air vents.

Even though one of the vehicles spotted was wearing G55 AMG and V8 Kompresssor badges, it is believed that Mercedes-Benz is actually testing the 2012 G63 AMG.

The current G55 AMG packs a 5.4-liter supercharged V8 producing 507HP at 6,100 rpm and 700Nm (516.3 lb-ft) of peak torque from 2,750 rpm to 4,000 rpm, for a limited top speed of 210km/h (131mph) and a 0-100km/h (62mph) time of 5.5 seconds.

The new G63 AMG will likely make use of Mercedes-Benzs new 5.5-liter bi-turbocharged V8 that is not only more powerful, but also cleaner and more fuel efficient than the supercharged unit.

The same powerplant can be found under the hood of the newer Mercedes-Benz AMG models such as the CLS 63 AMG where it is rated at 525HP / 5,750rpm and 700Nm (516.3 lb-ft) of peak torque delivered between 1,750 and 5,000 rpm.