What is it? A limited-run homologation special of BMW’s world touring car. A mere 2600 will be made available worldwide, and only 500 will be coming to the UK. Based on the 320i M Sport, the suspension, gearbox, bodywork and interior are all carried over unchanged. The areas that have been tweaked include the wheels (unique 18s), brake discs (now 300mm all round), and most significantly the engine, which is hand-built specifically for the 320si with more power and a very different character from the regular version. It may be stretching the point to call this a latter-day four-cylinder M3, but it’s well on the way. The 320si engine is no mildly tweaked version of BMW’s regular 148bhp 1995cc unit. It has been engineered to rev higher; the bore is up by 1mm to 85mm and the stroke down by 2mm, to 88mm, while the induction and exhaust valves are all bigger, too. The compression ratio is up from 10.5:1 to 11.0:1, cylinder liners are aluminium alloy rather than cast iron, and there’s no Valvetronic induction system. The cylinder head is cast alongside BMW’s F1 engines. But it takes lifting the bonnet to unearth the most exquisite part of all: a carbonfibre cam cover. The fact that this saves 10kg and lowers the centre of gravity is valuable on the race car, and largely irrelevant on the road car. What matters here is that it looks as cool as hell. The upshot of the changes is that the 320si’s engine develops 171bhp at 7000rpm. The 0-62mph time falls from 9.8 to 8.1sec and the top speed is up by 6mph to 140mph. What’s it like? At idle the engine sounds normal, gears engage with regular 320i precision and the clutch has the same weight and progression as usual. This might be a race-derived engine, but the package is pure 3-series. There is fulsome urge right round to 7300rpm, and it is linear and responsive, although nothing like as urgent as the fastest hot hatches. It’s best above 4000rpm, where it’s surprisingly raucous, too. Should I buy one? At £25,000, this car costs £415 less than a regular 320i M Sport, so will no doubt sell out in weeks. It’s a shame there’ll only be 2600 examples. A junior M3 could be something very special indeed.