BMW AG 3L Turbocharged DOHC I-6
By Bill Visnic WardsAuto.com, Dec 20, 2006 9:08 AM

Yeah, auto makers and their powertrain engineers have claimed for years some latest pairings of engines and turbochargers have “eliminated” turbo lag, the wait-to-accelerate feeling that forever has been the bane of turbocharged automotive powerplants.

Well, ah, BMW AG’s actually done it.

So sweet, so smooth, so willing, so immediate is BMW’s all-new turbocharged 3L DOHC I-6 that we’re convinced car enthusiasts will not know the auto maker’s hallmark inline 6-cyl. engine has forced induction.

The only way they’d really know, perhaps, is from the epic torque rush that signals turbo, rather than from the presence of turbo lag.

BMW, in fact, had quickly tired of its “standard” 3L DOHC I-6 taking a back seat to larger V-6s dealing out in excess of 300 hp. It called in forced-induction boffins from turbocharger expert Mitsubishi Heavy Industries Ltd., which helped engineer the new N54 twin-turbo setup, making the first BMW turbocharged gasoline engine since 1981.

With just three of the BMW straight-six’s cylinders dedicated to huffing exhaust past each turbine, the turbochargers can be sized for rapid response rather than to handle large exhaust volume.

BMW revives gasoline-turbocharging after 25 years with new 300-hp N54.

The other turbo lag-reducing factor is BMW’s fitment of its second generation of direct-injection gasoline fueling. DIG naturally plumps low-speed torque, which also helps rouse engine rpm while the turbos wind up.

The effect is electric: Apart from adding a substantial 45 hp to the standard 3L inline-six’s 255 hp, torque surges to a V-8-like 300 lb.-ft. (407 Nm) that effectively is available for just about the entire useful engine-speed range.

One of the few criticisms that can be leveled at BMW’s inline 6-cyl. layout is that, at 3L of displacement, torque output is relatively meager. Well, the turbos have solved that problem, thanks very much.

The thrust from this new turbocharged I-6 is captivating. With a much broader torque peak than any V-8 can deliver, the acceleration surge seems endless. The driver never tires of calling the torque out to play, thanks in large part to the deliciously precise throttle, a hallmark of all BMW inline sixes that only seems magnified with the twin-turbocharged variant.

The low-inertia dual turbochargers and direct injection – along with variable valve timing – seem to have been fated to join with the always-special straight six in a systems-approach design that, to our right feet and pants seats, have no appreciable shortcomings.

OK, it’s a shame the Valvetronic variable valve-lift system isn’t there to increase what already is startlingly good fuel economy, even when extracting the most from both turbos. And this variant of the standard N52 I-6 foregoes that engine’s exotic magnesium-aluminum block for just plain aluminum with iron cylinder liners.

But those are inconsequential nits. With its new turbocharged inline-six, BMW has developed what may be the perfect performance engine for this age. It’s that good.

bvisnic@wardsauto.com