The rear wheels were driven by a traditional Lamborghini V12 engine mounted longitudinally with a mid-engined configuration. For better weight distribution, the engine is pointed backwards; the output shaft is at the front, and the gearbox is in front of the engine, the driveshaft running back through the engine's sump to a differential at the rear. Although originally planned as a 5-liter powerplant, the first production cars used the Lamborghini Miura's 4-liter engine. Later advances increased the displacement to 4.8 liters and then (in the Quattrovalvole model) 5.2L with four valves per cylinder.
All Lamborghini Countaches were equipped with six Weber carburetors until the arrival of the 5000QV model, at which time the car became available in America, and used Bosch K-Jetronic fuel injection. The European models, however, continued to use the carburetors (producing more power than fuel-injected cars) until the arrival of the Lamborghini Diablo, which replaced the Countach.
In 1985 the engine was improved again, bored and stroked to 5.2 litres (5,167 cc) and given four valves per cylinder (quattrovalvole in Italian). The carburetors were moved from the sides to the top of the engine for better breathing—unfortunately this created a hump on the engine deck, reducing the already poor rear visibility to almost nothing. Some body panels were also replaced by Kevlar. In later versions of the engine, the carburettors were replaced with fuel-injection.
Source: Wikipedia (original)