With 425bhp from 5500rpm to 7300rpm and 405lb ft of torque between 1850rpm and 5500rpm, the twin-turbo six delivers 11bhp and 110lb ft more than the naturally aspirated 4.0-litre V8 that it succeeds.
An impressive 143bhp per litre — 38bhp per litre more than its predecessor through the effects of forced induction — also gives it the highest specific output of any series-production M division engine.
With Valvetronic variable valve timing and Double Vanos continuously variable camshaft timing tech, the six-cylinder unit revs to 7600rpm — 600rpm less than the old V8.
The new straight six, codenamed ‘S55 B30’, is billed as the most powerful series-production engine to find its way into the M3. It is also the smallest-capacity engine used by M division since the discontinuation of the first-generation M3 and its naturally aspirated 2.3-litre four-cylinder powerplant, the ‘S14 B23’, in 1991.
In combination with BMW’s efforts to lighten the M3 and M4 through the use of new lightweight materials, the added reserves improve the power-to-weight ratio of the M3 by 22bhp per tonne over its four-door predecessor to 280bhp per tonne. The M4 also improves by 22bhp per tonne over the outgoing M3 coupé to 284bhp per tonne. By comparison, the latest Porsche 911 Carrera S has 283bhp per tonne.
Channelling drive to the rear wheels is a standard six-speed manual gearbox. The Getrag-produced unit has been developed from scratch and is claimed to be 12kg lighter than the outgoing unit. Among its innovations is dry-sump lubrication, a double-plate clutch to handle the added torque loading, carbon-ceramic friction linings within the synchroniser rings and a blip function on downshifts. The individual ratios are also shorter than before.
Buyers will be able to choose an optional seven-speed M Double Clutch Transmission (DCT), again from Getrag, with remote paddle shifters. A development of the M5’s unit, it allows the driver to choose between manual and auto modes. Further features include launch control and a so-called smoky burnout setting, which permits a degree of wheelspin at low speeds. To help improve on-the-limit handling, it also receives stability clutch control, which opens the clutches when sensors detect understeer to reduce drive and bring it back on line.
Both gearboxes receive a stop-start function, brake energy regeneration and optimum shift indicator. Official figures reveal that DCT versions have a 40kg penalty, with the DCT-equipped M3 tipping the scales at 1560kg and the M4 at 1537kg.
In a new driveline development, the new M3 and M4 adopt a carbonfibre-reinforced plastic propshaft. Constructed as a single component with no centre bearing, it is claimed to bring a 40 per cent weight saving on the previous M3’s steel driveshaft while providing a reduction in rotating masses for better driveline efficiency.
The new driveshaft is allied to hollow output shafts at the rear within the Active M differential, which uses an electronically controlled multi-plate clutch to provide a varying degree of lock-up.
With an official 0-62mph time of 4.3sec in standard six-speed manual guise, the new M3 and M4 are 0.5sec quicker than their predecessors. The seven-speed DCT drops the 0-62mph time to 4.1sec. Top speed is limited to 155mph, although it can be raised to 174mph as an option.
Official figures point to a combined 32.1mpg in manual guise and 34.0mpg with the DCT gearbox. This results in CO2 emissions of 204g/km for the manual and 194g/km for the DCT. The figures for previous M3 saloon and coupé were 22.7mpg and 290g/km in manual guise and 25.2mpg and 263g/km in DCT form.