Engine and gearbox
The engines used in the Z4 come from BMW’s latest modular range of units, the 20i and 30i models using a 2-litre single-turbo four-cylinder, while the M Performance M40i receives a 3-litre ’six, again with a single (twin-scroll) turbocharger.
The 20i delivers 194bhp between 4500 and 6500rpm and that’s backed up by a useful 236lb ft of torque from 1450rpm to 4200rpm. The same unit is found in the 30i but in a higher state of tune, developing 254bhp from 5000 to 6500rpm and 295lb ft from 1550 to 4400rpm.
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The lone six-cylinder in the range in the M40i ups the stakes considerably with 335bhp available from 5000 to 6500rpm and 369lb ft of torque between 1600 and 4500rpm. That torque figure is the same as an E39 M5’s V8 could muster, and is available over a much wider rev range, so it should be clear the M40i isn’t short of performance.
While the 2-litre four-cylinders in the 20i and 30i might not be the most tuneful four-pots ever made, they still sound relatively inspiring, certainly more so than in the 3-series where they’re rather muted. They’ll pop and bang on the overrun and on full-bore upshifts when in Sport mode, and while the sound is augmented through the speakers it does have a more natural tone to it than in other BMWs. They’re keen to rev and deliver when it comes to precisely measured throttle inputs.
The 3-litre in the M40i has a further layer of sophistication than the four-cylinder units and has all the trademark six-cylinder smoothness you’d expect. In Comfort mode though it can end up being almost inaudible as the gearbox is keen to change up at every opportunity, keeping the revs down and the soundtrack muted. To get the best from it, Sport mode needs to be selected, and then it becomes more inspiring with a sharper, significantly more sporting feel about it. With a wide torque band it feels fast almost no matter which gear you’re in.
There’s no doubt the Z4 would feel like a more engaging machine had BMW not decided to equip all models with the eight-speed ZF auto. No, doesn’t sound very sporting, does it? Eventually a six-speed manual is due to be offered in the 20i, but for a supposed sports car it would be good to have the increased level of interaction that a manual ’box brings to the driving experience. Yes, the auto will shift faster and be more efficient, but a three-pedal set-up would be far more engaging.
There’s nothing wrong with the eight-speed auto – it shifts imperceptibly when cruising and is fast enough – if not quite as fast as the best dual-clutch systems – when driving harder, it just feels like it’s much more in keeping with an executive cruiser rather than a sporting roadster.