2015 BMW M235i: A Benchmark Powertrain
June 11, 2015
Our long-term 2015 BMW M235i has a powertrain that works. I don't mean "works" as in "is functional," but as in "the operation of which was obviously fussed over by people who know what they're doing."
The M235i's 3.0-liter turbocharged and direct-injected inline six-cylinder is not new. This engine family ostensibly started life as the N54, a twin-turbo mill, then transmogrified into the single-turbo, Valvetronic-havin' N55. Yet despite its tooth length, it's just so darned good.
But it's not without some character flaws.
First, the good. With this engine, you get its creamy smooth inline-6-ness which, if you've never experienced, there's really something to it. It's a trait unique to this configuration, much as flat-sixes sound like a ripsaw, and crossplane V8s go lub-lub-lub. Now imagine that straight-six sewing machine characteristic mated to a seriously stout — and broad — torque curve.
Wait, that's too clinical. Try this: the M235i freakin' rips. I unceremoniously laid into the right pedal near the end of a 15-minute drive with two jaded car nerds aboard and both uttered words to the effect of, "Dang! What's this thing powered by again? Dang!"
It sounds great, too. Well, it sounds great at full whack. Really great. Mostly intake honk, a little exhaust, and the only time you hear the turbos is when the compressor bypass valves do their muted sigh during gear changes or when you lift suddenly. At part-throttle, it just sounds good.
Let me reiterate. This is a fast car, and the ZF 8-speed gearbox is complicit to the M235i's sense of urgency, particularly when you select 'Sport' via the little rocker on the console. When you do so, gear changes speed up, downshifts are rev-matched, throttle gain increases, and the transmission shift schedule gets more enthusiastic about holding gears and downshifting.
It's all good stuff, but none of those words really do justice to just how refined this transmission is. It's calibrated so well. The way it takes up from low revs with so little torque converter slop, minimal shift shock (looking at you, Chrysler nine-speed) and just plain crispness. Eight gears are probably too many when you've got an engine with this much torque, but it works better than expected.
I do have a couple of beefs with the M235i's engine, though — more specifically, its throttle calibration. There's a distinct throttle lag when you tip in from zero throttle, whether from a standstill or from a roll. It's not turbo lag. It's a bit of absolutely nothing, while the Congressional committee inside the ECU bickers amongst themselves, debating how much torque you requested a split-second ago (are they using an abacus? A Speak-n-Spell?).
It then opens the throttle to a degree commensurate to that torque request, but in the meantime, you've dipped the pedal even further. And because it's Congress and nothing happens until the committee implements an emergency deadline measure, ultimately you leave the line looking like it was your first time driving a car, ever. So BMW, fix your stupid throttle response.
My other issue is with its stop-start function. It's woefully abrupt during stops. Mainly because it stops the engine before you've actually reached a halt, which makes for a very lurchy stop about three-quarters of the time and, again, makes it appear to your passengers as if it's the first time you've driven a car. Ever. The restarts are smoother, though not particularly hurried.
The elastic throttle response is more irritating than the stop-start thing, but on balance I have to imagine other manufacturers, while benchmarking the N55, wonder how on earth they're going to top the way this engine just oozes competence.
Jason Kavanagh, Engineering Editor