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Why the BMW M4 is better than the M3 V8 (2013)

Wed, 25 Sep 2013

It’s no exaggeration to say that the turbocharged engine in the upcoming BMW M3 (saloon) and M4 (coupe) models is one of the most important engines ever produced by BMW’s M Division. No, it’s not the first turbo lump the performance off-shoot has built – you'll probably know we’ve already had the X5/X6M mutants plus the brilliant 1M and M5 and M6 – but it is the first turbo ever to slot in a production M3, the car that personifies BMW’s iconic performance sub brand, which itself has always been defined by high-revving, naturally aspirated engines. All of this explains why M’s engineers have cut their teeth on other engines first before plucking up the courage to develop the M3/M4 turbo while turning their backs on the outgoing E92 M3’s exceptional V8. BMW has given us the lowdown on the new engine at a technology workshop just outside Munich ahead of the car’s 2014 launch.

The details on the new BMW M3's twin-turbo engine

Rumours of a V6 triple-turbo circulated early in the M3/M4's development but the production unit is a 3.0-litre straight six with two turbochargers, and an over-square bore of 89.6mm and 80mm stroke. It shares nothing other than peripheral ancillaries with BMW's other straight-six petrol engine, but the familiar design has allowed BMW to retain the production infrastructure to build it, saving money to reinvest back into other weight-saving measures without drastically increasing the price of a relatively cost-sensitive performance car. This means that the M3/M4 range should still retail from around £60k when it goes on sale next year.

You said something about saving weight?

The weight-saving measures aren’t limited to the body and chassis, but the engine components themselves. With two fewer cylinders to lug about, it's perhaps not surprising that this engine is lighter than the V8 it replaces, but its 10kg saving is nonetheless impressive given the extra weight added by twin turbochargers, the intercooler and its associated pipework. The crankcase is 2kg lighter and is made from forged steel, helping torsional rigidity, while the magnesium sump saves another kilogram.

Is the engine really better than the amazing E92 M3 V8?

The straight six was developed at BMW’s Preussenstrasse facility in Munich, where development versions have undergone 400 hours of endurance testing, but it’s built at the Steyr, Austria, plant. Outright horsepower hasn’t increased dramatically at 424bhp to the old car's 414bhp, and it’s delivered lower down the rev range too at 7500rpm, but the power curve remains similarly linear. The big difference comes with that turbocharged torque plumping up the lb ft from 295 to over 369lb ft, and remaining constant from 1800 to 4750rpm, before tapering off so that the torque at peak power is similar to the V8’s. The extra torque alone would guarantee a significant increase in performance, but the M3 and M4 models are around 80kg lighter than their predecessors, weighing 1500kg, and insiders promise a similar leap in lap time as the outgoing V8 E92 posted over its 3.2-litre predecessor, the E46. The new car, codenamed F80/F82, is significantly more eco-friendly too, ducking under 200gkm of CO2 to compare favourably with the V8’s 263g/km.

The turbos run at a maximum of 1.25 bar boost but often less, the electronic brain only cranking up to that level of pressure under high operating temperatures or at high altitude. It means the power and torque figures will remain consistent in the vast majority of conditions. As Vice President of engineering, Albert Biermann, explains, that means a standard M3/M4 will often be as fast on track as a tuned car with a claimed additional 50bhp but no extra leeway to continue to develop that power as operating temperatures soar.

What's the engine like on the move?

From passengering in an M4 with ex-F1 man and current DTM star Timo Glock (more on that and other tech details in the November issue of CAR), we know that the sound of the new engine is a more muffled, deeper rumble than the rattly metallic rasp that defined previous naturally aspirated straight sixes, but the charge to high revs remains fiercely intense. The thunderclap that follows the dual-clutch gearbox’s shifts will be familiar to anyone who’s driven the latest M5/M6, while BMW promises that the small, fast-acting turbos (smaller than those in the 1M, in fact), together with flow-enhanced manifolds and an intercooler that sits adjacent to the inlet tract help to deliver the instantaneous responses that M fans know and love. Timo seemed pretty happy too.

Bring on that first drive early next year.

By Ben Barry