BMW Z4 vs Porsche 718 Cayman (2018) twin test review

Published: 29 December 2018

► New Z4 roadster is here
► Munich vs Stuttgart
► BMW takes on Porsche

Why would BMW allow us to bring a Porsche 718 to the launch of the new Z4? Because they love this magazine, or because they have a particular fondness for this author? Dream on. They rolled out the red carpet for our white rival from Stuttgart because they're confident – very confident – that for its third generation the Z4 has finally evolved into a proper sports car.

The first Z4 was an abomination from the Bangle House of Horror. The second-generation car was a pretty yet dynamically flawed retractable hardtop aimed squarely at the Mercedes SLK; hardly the most dynamically sparkling target on which to set your sights. The new Mk3 is a totally different animal. Codenamed G29, it laps the Nordschleife in 7min 55sec (faster than the just retired M2, if a tad tardier than the new, more focused – and M4-engined – M2 Competition) and blitzes 0-62mph in a remarkable 4.6sec, putting itself on par with the M4 cabriolet.

So the Z4 M40i is itching to go head to head with the £50k sports car benchmark. But which Porsche 718? WLTP emissions testing temporarily made both base models hard to come by (although normal supply has now resumed) but, bar its coupe bodystyle, our white PDK-equipped non-S Cayman is the perfect sparring partner. The Porsche is £3k cheaper and 40bhp down on the Bavarian but an S would be too fierce, and £3k more. (Plus Porsche will charge you a couple of thousand pounds for a PDK gearbox and the Sport Chrono pack, both of which you need to go up against the well-equipped Z4.)

718 PDK gearlever

In fact, the new Z4 so spookily splits the two Porsches on price and power you suspect it's no coincidence. But the Cayman, not the Boxster? Put it this way, if the BMW can stick with the benchmark coupe, it really will have become an altogether more serious proposition.

'When the board gave the project the green light they told us to make it a co-operative venture (with Toyota) and to make it a good one; a proper sports car,' says project leader Michael Wimbeck. We had a 718 for benchmarking early on. I showed it to the board members, had them compare it to our car, and pointed out that, while it was inferior for noise and comfort, that this also gives it character.

'The new Z4 is purer, more dynamic and more progressive 4 than the outgoing model. There is no better roadster to explore empty B-roads early on Sunday morning than the new Z4.'

Going to ground

It's Monday lunchtime, not Sunday morning, as we head out of Lisbon, and the rain is falling in dense, billowing sheets. Visibility becomes a guessing game as we head for the coast, and the first serious downpour in five dry months has coated the surface with what feels like liquid soap.

Grip is soon an illusion, roadholding a series of broken promises, and anything other than the tenderest of touches on either pedal risks sending us into the sea. Still, at least I'm awake now.

Racing up and down the zig-zagging hillsides in second and third gear feels like riding the freshly honed cutting edge of a curved dagger. Get it wrong, and brace yourself for the worst. Get it right, and you're rewarded with the satisfaction of steering inputs, gearchanges and throttle adjustment working in harmony and shrugging off the weather. In such moments do true sports cars make themselves known.

Initially we compare the Porsche and BMW at considerably less than full throttle, enjoying the fact that both are happy to be driven relativelyslowly and have their share of comfort and convenience.

BMW Z4 interior

Inside, the Z4's cockpit is usefully spacious, if burdened with an awful lot of instrumentation and switchgear and a busy, fat-rimmed steering wheel. The most button-heavy zone is around the gear selector. Here we find the latest version of the iDrive controller, four driving programme keys labelled Sport, Comfort, Eco Pro and Adaptive, the DSC switch and the toggle that opens and shuts the roof in a brisk 10 seconds. (It's firmly closed right now, but a brightness beyond the distant clouds gives cause for hope.)

The dashboard of the Cayman is positively olde worlde when pitted against the BMW's clever colourful displays, the practical controller and the ergonomic multi-functionality of the Z4. The differences between the two MMIs look insurmountable at first, but the novelty soon wears off and you adjust to whatever you're in. The BMW has the more clearly legible displays, the more logically arranged direct-access buttons and the more advanced voice control system. But as far as I'm concerned both cars would benefit from a large digital speedometer, a
bigger tacho with a visual and audible redline-approach warning, and a large read-out to tell you which gear you're in.

High and (a little bit more) dry

Come early afternoon the rain eases off and strong winds begin to blow the blacktop dry. We spend all afternoon swapping between the duo on some truly great roads dotted with blind corners, heart-attack descents and cinemascope vistas. The police must be busy helping drenched cats out of trees, because they're certainly not out here measuring how much we're exceeding our fun allocation.

With the cornering grip back in full force, the BMW gets a chance to really shine, and show off the merits of front-mounted straight-six versus the Porsche's mid-mounted boxer four.

Although our Z4 has more power and more torque, it weighs a substantial 1535kg versus 1440kg for the Cayman. Fitted with the seven-speed PDK 'box and the Sport Chrono pack, the mid-engined two-seater can accelerate from zero to 62mph in 4.7sec. The more powerful Z4 M40i will do the same job in an even brisker 4.6sec, but it is governed at 155mph where as its rival maxes out at 170mph.

Z4 vs 718 cornering

The BMW engine delivers 335bhp between 5000 and4 6500rpm; 369lb ft of maximum torque is available from just 1600rpm. The boxer needs 6500rpm to put down 295bhp, and it maintains its torque peak of 280lb ft between 2150 and 4500rpm. At 35.8mpg against 38.2mpg, the Cayman's boxer four is slightly thirstier than the Z4, and has to work hard to keep up with the luscious straight-six. The Porsche's optional seven-speed twin-clutch gearbox is more than happy to help. Nonetheless, as we press on, speeds rising as the tarmac turns from dark grey and shiny to light grey and grippy, the BMW starts pulling away from the Porsche in small but quantifiable increments.

When we first drove an early Z4 on the BMW proving grounds in Miramas, the roadster impressed with a variety of talents. It worked well as open-top GT, it would perform as sports car for road and track, and it blended the relaxed mastery of a cruiser with the hardcore handling of a bruiser. Six months later, on this historic patchwork turf north-west of Lisbon, the M40i passes its baptism of fire in the company of a serious rival.

The nub of our comparison is a demanding 10-mile stretch. Up in the Z4, down in the Cayman. Then up in the Cayman, down in the Z4. Both our test cars are on 19-inch wheels, and the direct comparison reveals that the Porsche does not ride quite as well as the BMW in Sport mode.

The Z4's throttle response, transmission set-up, dampers, steering weight and the electronically controlled diff lock can be tweaked by the driver in five different steps. Sport + is okay on the track, but Sport is more compliant and thus better suited for the open road. As far as the transmission goes, the most inspiring mix is hyper-precise manual downshifts followed by automatic upshifts.

In Comfort, the Z4 can be a little mushy and undefined, while in Sport + the ride feels notably more brittle, the steering a tad heavy, and the diff lock trades in smoothness for bite. The six-cylinder engine oscillates between a low-rev murmur and a high-rev roar. Even with the roof down, the driver feels comfortable and secure, has a commanding view despite the low seating position, is protected surprisingly well from rain by the large laid-back windscreen, and from the passing storm overhead by the tacky removable deflector wedged between the fixed head restraints.

With a shorter wheelbase than its predecessor, the new Z4 feels every inch the compact drop-top with sporty DNA. The Cayman on the other hand looks and feels like the baby brother of the 911. The BMW is easier to drive most of the time. It flies a straight line as unerringly as a migrating goose, it decelerates with the elegance and efficiency of a landing swan, and it follows the road as accurately as a buzzard trailing its lunch. It's an emotional and exciting piece of kit, but at the same4 time it builds your confidence thanks to its ability to carry high turn-in speeds, with phenomenal front-end grip and a rear axle that does your bidding accurately and effortlessly. And on low-friction terrain, the 335bhp two-seater loves to lapse into power-on or lift-off oversteer. On dry roads, though, grip, traction and roadholding are beyond reproach.

Z4 vs 718 tracking

By late afternoon the Porsche 718 has proven itself every bit as competent and as challenging as the BMW. Its steering is lighter and even more responsive, its grip is easy to read and a joy to exploit, and its 95kg weight advantage adds a useful and very welcoming dash of agility. On a quick third-gear downhill dare, the Porsche is in fact every bit as fast as the more powerful BMW, because it allows you to brake a little later and to step back on the gas a little earlier, with ABS, ASR and DSC acting as hands-on referees to keep you from the evil-looking rocks waiting at the roadside.

Any Porsche weaknesses the Z4 can exploit? Well, yes. The Cayman's brakes are grabbier (if ultimately a tad stronger), while the uncompromised roadholding needs a smooth surface to really shine; it's less happy than the BMW to rough it on sub-prime blacktop. The Porsche's flat-four engine also relies on high revs to deliver, and fails to muster quite the same linear punch as a six. Plus of course it sounds more like an air-cooled Beetle on steroids than a detuned M1.

What sets the roadster apart from the coupe is the Z4 M40i's new variable-rate sports steering, which boasts a reassuring meatiness around the centre position, more feedback when turning in, and a quicker rate of response during rapid changes of direction. Shortening the wheelbase and extending the overall length compared to the Mk3 version does not seem to make much sense until you actually drive the 2019 Z4, which is less twitchy at speed yet more switched-on through the twisties. (When it comes to the BMW parts bin, the Z4 steals the 3-series' front bulkhead, brake calipers and rear axle, though the axle is reinforced here and bolts to stiffer mounting points.)

Of course, there are cheaper and less powerful Z4s to be had, and more powerful and pricier Porsches. And let's not forget the Jaguar F-Type, Audi's TT and the brilliant Alpine A110, not to mention the Supra, Z4's Japanese twin.

But there's plenty of time in which to test those cars. What of these two, right now?

Z4 vs 718 static

Z4 vs Cayman: verdict

The Cayman has the nicer steering; lighter, quicker, more involving. Its handling encourages and allows a slightly wilder approach, letting you get closer to front-wheel lock-ups under braking, compelling you to actively fight ruts and ridges that momentarily deflect the trajectory, and approaching the limit with a live-wire connection between steering, throttle and rear suspension.

What the Porsche lacks in this company is an extra helping of torque to whiplash the car forward with a vengeance.

Points in favour of the Porsche include quantifiably 
less body fat, a sportier overall set-up, a rowdier Sport+ 
calibration, the aforementioned extra bit of brake bite and a loftier redline. Having said that, four cylinders and 2.0 litres simply don't burn the same fingernail-curling fireworks as a rasping 3.0-litre six.

At the end of the day the Z4 is the faster car – and more often than not it matches the Porsche for tactile excellence and transparent interaction to boot. If you're comfortable with the extrovert exterior design and the over-styled cockpit, the Z4 rewards with a legendary engine, a remarkable chassis and an impressive, involving and invigorating turn of speed. How marked is the BMW's transition from poseur to pro? Munich should have badged its new roadster Z5.

BMW Z4 vs Porsche 718 Cayman

BMW Z4 M40i
> Price from £49,050
> Engine 2998cc 24v twin-turbo straight-six, 335bhp @ 5000rpm, 369lb ft @ 1600rpm
> Transmission 8-speed paddleshift auto, rear-wheel drive
> Performance 4.6sec 0-62mph, 155mph (limited), 38.2mpg, 168g/km CO2
> Weight 1535kg
> On sale March 2019

> Price from £46,074
> Engine 1988cc 16v turbocharged flat-four, 295bhp @ 6500rpm, 280lb ft @ 2150rpm
> Transmission 7-speed twin-clutch auto, rear-wheel drive
> Performance 4.7sec 0-62mph (with Sport Chrono Package), 170mph, 35.8mpg, 180g/km CO2
> Weight 1440kg
> On sale Now

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By Georg Kacher

European editor, secrets uncoverer, futurist, first man behind any wheel