The basic versions of the 800 used two 2.0L 16-valve developments of British Leyland's stalwart O-Series engine, dubbed M-Series. The 820e, with single point injection, and the 820i with multi-point injection, i.e. 4 injectors. The top versions used a Honda designed V6 unit in 2.5L capacity. Initially, only a saloon body was offered; a liftback version — referred to as a fastback — became available in 1988.
Later, a diesel version of the car was launched in 1990 using a 2498 cc engine from Italian company VM Motori, which was related to the slightly smaller engine used in the 2400 SD Turbo model of the Rover SD1, and Range Rover Turbo D.
The Sterling badge was used in Europe and most global markets to denote the top saloon luxury version and the Vitesse badge used to denote the top fastback sporting version. The Vitesse became available at the same time as the 2675 cc Honda V6. Both of these top of the range models were initially only available in the UK with the V6. In some European markets, in particular Italy, the 2.0 litre petrol was badged as Sterling and later available (in turbo form) as Vitesse to avoid the punishing duties that made engines over 2.0 litres unviable for volume sales.
Towards the end of Mark 1 production the Vitesse had nearly as many "luxury" features as the Sterling (for example, electric seats). There was also a brief run of just over 500 820 Turbo 16v cars using a turbocharged version of the M-Series developed with help from Tickford, leading to this model often being referred to as the "Tickford Turbo". Utilising such enhancements as sodium-filled exhaust valves and Mahle forged pistons the car produced 180bhp (134kW), although there is much speculation about this figure being severely held back by the electronics as not to step on the toes of the 177bhp (132kW) V6 engined Vitesse model as well as to preserve the reliability of the gearbox. In reality the engine was capable of 250+hp while still preserving the realiability and drivability.