BMW’s Best Six Is…A Diesel

BMW six-cylinder engines have long been a byword for silky smoothness and zingy performance; the elastic surge of acceleration that gathered intensity as the tach needle swung past 4000 rpm, accompanied by that velvety snarl from under the hood, meant “The Ultimate Driving Machine” tagline was more than just clever advertising shtick. But not every BMW six has been a home run. Remember the M20 eta engines? These torquemeister motors were designed to lug, not to rev, in the belief that the higher gearing they could pull would deliver vastly improved fuel efficiency. The etas were redlined at 4750 rpm, and sort of drove like diesels, except they were way smoother, and with lighter pistons and conrods, revved faster. Confused BMW drivers spent the whole time pounding the steering wheel in frustration as the etas smacked the rev-limiter just when they were expecting them to hit their stride.

BMW’s relatively new N55 straight six, now the mainline engine for the U.S. market, is no eta. But it’s most definitely not the sweetest six BMW has ever made. The N55 is technically impressive, boasting Valvetronic variable valve timing and an innovative twin-scroll turbocharger. It’s more efficient that its predecessor, yet delivers potent performance. So what’s the problem?


The problem is the N55 is almost impossible to drive smoothly in stop-start traffic. There is a strange hesitation in the power delivery when you accelerate from a standstill – almost as if the turbocharger has to spool – and then a big surge of torque arrives all at once. It’s the single most annoying characteristic of our N55-powered 535i GT long termer.

I’m still not sure whether this is purely a calibration problem; whether the BMW engineers haven’t quite got the new ZF eight-speed auto trans to have a civilized conversation with the engine when you nail the gas or vice versa. But after driving the new BMW 535d in England, I suspect the problem might be the N55. Because BMW’s new 299 hp 3.0-liter turbodiesel hooked to the eight-speed ZF may just be one of the finest all-round passenger car powertrain combos in the business.

This engine is the third, and most powerful, version of BMW’s N57 common rail, direct injection, twin-turbo oil-burner, developing its 299 hp at 4400rpm, and a hefty 442 lb-ft of torque from just 1750 rpm. A low inertia small turbocharger delivers near instant grunt off idle, while a larger second turbocharger ensures volumetric efficiency at higher engine speeds.

Compared with the N55, the N57 delivers pure, seamless thrust right from standstill – even when you’re easy on the gas pedal – the ZF swiftly shuffling through its eight ratios. It’s perfect for the cut-and-thrust of London, where you’re constantly hustling for the smallest advantage in the traffic. The gobs of instant grunt mean rolling response on the freeway is impressive, too – just wiggle your big toe, and the 535d jumps to attention, allowing you to plug that gap in the fast moving stream of Benzes, Jags, and Range Rovers heading for the Home Counties on a Friday afternoon.

The N57 growls like a contented lion when you tickle the throttle, and there’s a slightly granular quality to the feedback from the engine room, but otherwise you lose nothing compared with the N55-powered 535i. At about 5.6 seconds the 535d’s claimed 0-60 mph time is virtually identical. More impressively, you’ll spend less at the pump, as BMW claims the 535d will return 36.3 mpg on the combined Euro cycle. We saw a best of 36.7 mpg for a 100-mile motorway run, and a worst of 23.5 mpg after 67 miles of stop-start running around the greater London area. Total consumption for our 1032 miles in the car was 30.2 mpg (all figures U.S. gallons). When a 13.8-gallon fill costs just over $120, such math is more than merely academic.

Part urban assault weapon, part trans-continental cruiser, the 535d is the best of the new 5-series models I’ve driven so far (the 550i I drove during our Car of the Year evaluation last year was spoiled by its slow, clunky, and noisy six-speed manual transmission). And most of the credit goes to the N57 turbo-diesel. It’s BMW’s best six.

Angus Mackenzie on March 16 2011 5:00 AM 

[Source: blogs.motortrend.com]