2015 BMW M3 and M4 swap cylinders for turbos

Slicing two cylinders off BMW's hot M-Series mid-size machines has had a remarkable effect - not only does it boost performance but it also slashes fuel consumption. Replacing the 4.0-litre V8 with a bi-turbo 3.0-litre inline six, the BMW M3 sedan and M4 coupe arrive in September in their fifth generation with lightweight cladding and race-tuned drivetrain.

They were shown at last week's Geneva motor show - though, strangely, set at the back of the stand to give prominence to the i8 and the front-wheel drive Touring hatchback. The M3/M4 six-cylinder replaces the V8 introduced in the 2007 E90 model, returning the cylinder count to that of the E36/46 that started as a 210kW 2.0-litre.

Model designations now pick up BMW's ostensibly clearer coding of odd numbers for sedans, evens for coupes and convertibles - hence M3 for the sedan and M4 for the two-door. BMW Australia says it's champing at the bit to get the pair. No prices or final specs are available though it's unlikely they'd be an increase on the existing models.

The current entry-level Pure coupe costs $131,945 as a dual-clutch automatic. Developed for the M3 and M4, the new bi-turbo engine pumps 317kW and, more importantly, 550Nm of torque - up 40 per cent on the outgoing V8-engined M3.

It will be available as a six-speed manual or a seven-speed dual-clutch automatic, with the latter being as quick as the manual but more frugal.

The optional dual-clutch auto includes an integrated launch control and a "smoky burnout" function to allow some rear-wheel spin while the car is moving at low speeds. The BMW also engages the clutch when the car is understeering, to return it to the cornering line. The dual-clutch versions claim 8.3L/100km (8.8L for the manual), down 25 per cent on the current model.

The 0-100km/h sprint takes 4.1 seconds in both variants, down from 4.6 seconds. Both cars have carbon-fibre roof panels and components to reduce weight. The coupe, for example, is only 1497kg which is down about 80kg on the existing two-door M3.

For the first time, the track-specification cooling setup keeps the engine, transmission and turbochargers at the optimum temperature. Externally, the cars carry the wider stance of the donor models and add subtle aerodynamic aids to maintain downforce and ease fuel consumption.

The M3 has a larger rear spoiler than the boot lip of the M4. Both have a smooth underbody and large front spoiler, both succeed in reducing lift equally at the front and rear axles.

BMW M Division head of development Albert Biermann says air is channelled through the grille, then the oil cooler to create a Venturi effect that reduces front-axle lift and improves steering feel. Even the distinctive side mirrors, unique to these models, are on twin stalks to reduce drag.

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