It is a limited (2600 units) version of 3 Series saloon built to qualify the homologation requirement for the FIA World Touring Car Championship (WTCC). The 320si uses a modified 4-cylinder engine (N45) different from the standard 320i (N46). Modifications include an increased redline to 7.300 rpm, a shortened stroke (by 2 mm), an increased bore (by 1 mm) and a higher compression ratio (11,0:1). The engine is mostly hand-built and does not have Valvetronic, decreasing the number of moving parts. Acceleration from 0 to 100 km/h (62 mph) takes 8.1 seconds and acceleration in fourth gear from 80 km/h (50 mph) to 120 km/h (75 mph) takes 7.7 seconds compared with a standard 320i which achieves them in 9.0 and 9.1 seconds respectively. Fourth gear acceleration is comparable to that of the 325i which achieves the 80 to 120 km/h (50 to 75 mph) sprint in 7.5 seconds. BMW WTCC works driver Andy Priaulx won two of his three consecutive World Championships in the 320si E90 as well as four other drivers achieving over 35 wins in the championship since the cars release.
The BMW 320si is used by a few teams in the British Touring Car Championship (BTCC). Colin Turkington won the 2009 BTCC Drivers championship using the BMW 320si.