Do we all agree that at 200bhp cars by and large become interesting, that by 300bhp things are beginning to concentrate the mind, and that once you have 400bhp propelling a long ton everything starts to feel really quite rapid?
Good. Now we’re sitting comfortably.
All three can currently be had for under £20,000 (less than half their price new), which considering the performance on offer looks a bargain.
Add rock-solid build quality, plus practicality sufficient to mean that they can be daily drivers, and we’re on to something worth investigating.
Mercedes has been doing roadsters built and imbued with the same sense of solidity as its saloons since 1955, with an evolution of styles though the various SLs, Pagoda and Panzer-wagen, before arriving at the C-class-derived R170 SLK of 1997.
That was replaced in 2004 by the R171 with its 30mm longer wheelbase and ‘F1-inspired’ nose designed by Steve Mattin. It was facelifted in 2007, and powered by everything from a 1.8-litre 160bhp supercharged ‘four’ through to a 3.2 V6 and 5.3 V8, the most powerful version hand-built by AMG. All had a folding hardtop marketed as the Vario-Roof.
Like the R107 and R129 before it, you get the feeling that the SLK doesn’t want you to know too much about what’s going on under the skin; it just gets on with it.
The steering is light and a little dead but the grip from the 225- and 245-section Contis is massive, and the V8, buried under all-cloaking beauty covers, is astonishingly free-revving for a big-capacity motor.
Prod is, er, prodigious but despite being the weightiest car here it’s not the heaviest carbon-emitter: that honour goes to the BMW. Surprisingly, the Mercedes is not the biggest here, being only 8mm less stubby than the Z4, while the Cayman is almost a foot longer.
Aside from that marvellous motor, where you find endless amusement in the SLK is in its appointments. As well as the retracting roof – as long as the boot isn’t too full – there are heated seats and a pair of ‘air scarfs’: warm-air blowers in the head rests, aimed at the back of your neck to ensure you stay nice and toasty with the top down.
Then there’s the gearbox, a seven-speed auto offering endless permutations of confusion. There’s the stick, for Park, Reverse, Neutral and Drive, as usual. Once the lever’s in D, slapping it right or left gives you up or downchanges, and you can achieve the same with buttons on the back of the wheel.
A further switch in front of the gate gives three modes: Sport, Comfort or Manual, with a digital readout between the main gauges to let you know where in all of this you are. This is very definitely a driver aid because the SLK doesn’t seem to care which ratio it’s in. It swaps cogs smoothly in all modes, holding the gears much longer in Sport, but doesn’t blip on downshifts like a Ferrari.
Column-mounted paddles that apparently came attached to faster software were an option. There’s so much torque from the motor – as owner John Cummings says, “it likes hills” – that once you’ve got bored with playing tunes on the transmission it’s just as quick to leave it in auto and watch the world spool by, as rapid as you like, in perfect smoothness.
It may not talk to you much, the Merc, but you feel you can trust it to make the decisions while you enjoy the scenery.
The Cayman was criticised at launch in 2005 for being a second-generation, Typ 987 Boxster with a roof instead of an expensive retractable top, yet costing £4000 more. It’s a more subtle device than that, though. The extra rigidity conferred by that handsome teardrop lid stiffens the chassis and makes for a very together car.
There are some similarities to the 911, of course – the motor is the same as a 997’s, and the steering feels similar to a 997 or 964; not as talkative as an early 911 but better than the deadened all-wheel-drive 996. No surprise because the 987 platform shares a lot with the 997, itself a heavily updated 996, but with the engine in front of the gearbox rather than behind it.
The change of the six-speed Getrag is about on a par with a G50 3.2 Carrera, even though it has more linkage. It’s slick for a Porsche, and the ratios are flatteringly closely stacked.
What is the same as a 911 is the linear power delivery from the water-cooled M97 powerplant. The turbine-like smoothness common to all flat-six Porsches appears devoid of dips or spikes, making it feel less powerful than it is, although with only 251lb ft – the same as a modern 2-litre turbodiesel – it’s noticeably less torquey than its rivals.
It’s happy revving to 7000, with the sweet spot past 5200, although it’s a little softer, less urgent, once beyond the power peak at 6250. In 911 style, it feels unburstable and able to sustain those revs for longer than you can.
Where it’s unlike a 911 is in its singular stability. There’s none of the shoulder-writhing that you feel from behind you in the rear-engined car, and it appears less sensitive to throttle changes mid corner – although some of that might be down to Porsche’s very benign stability control (benign as in, you don’t notice it).
Owner Jeremy Laird says that in trackday situations it responds well to a little brake trail on turn-in to bite the front and unsettle the rear, and that it gives much more feedback on 17in wheels with 205-section front boots than on the standard 18s. He fitted 17s in order to run winter tyres and preferred the feel so kept them on, with taller rubber than standard.
The slightly plasticky feel to the cabin comes as a surprise, although that’s mostly down to the cheap-looking leather wrapping the wheel. The rest of it is much better quality, and well screwed together (most of them by Valmet in Finland).
In terms of practicality, as well as storage under the hatchback there’s a decent front boot. Getting at the motor for any more than topping up oil and water (via a flap in the rear compartment) involves half an hour with some tools, though.
The Z4M coupé filled a gap left when the much-missed Z3M was retired in 2002.
Jackknife yourself in (it’s the lowest of our trio) and even with its slight HR Giger overtones there’s no doubt that you’re in a BMW even before you start it, thanks to the clearest instruments. Dark plastics – complemented on this one by a rare gloss-black ‘piano dash’ – contrast with light leather that doesn’t look as if it will wear very well.
But all that is irrelevant when you fire it up. The B32 version of the legendary iron-block S54 has that deep-chested boom beloved of E9 CSL lovers.
Unlike the old single-cam M30, though, it manages to combine this with astonishing revvability, pulling hard and strong right round to the 8000rpm redline with a sonorous yowl. Apparently, BMW has had to electronically replicate this for the latest M3.
The Z4M is not the lightest here but feels it. It is the quickest – both subjectively and on paper – and, like the devil sitting on your shoulder, its slightly deviant tendencies are always inviting you to play.
The proximity of your behind to the rear axle – it’s Caterham-like in that respect – adds confidence, as does the chunky though communicative hydraulically assisted steering.
With its short wheelbase it is very tail happy but it does look after you when it decides to step out, rather like an M3. We didn’t try turning off the Dynamic Stability Control on greasy January roads, but if you want the full one-armed paper-hanger effect, just press the Sport button. Throttle response sharpens noticeably, so when pushing on the driver always has something to do and can’t quite relax.
Even with the clever variable-clinch M-diff helping as much as it can by optimising traction, this is one you need to be in front of, although you’ll always have brakes, even when it’s wet. When it senses rain the Z4M periodically kisses the discs with the pads to dry them, so there’s no lag if you hit the pedal.
Only the gearchange disappoints slightly if you are used to ’02 slickness; the six-speeder is precise, but notchy.
So, which one? Depends what you want. They are all faster than you need and, thanks to competition raising the game, there’s not a duffer.
All can be had for roughly the same: £20k for a decent one, or £5k less if you take a gamble. All return 25mpg in real life, and much less if you boot them.
At £555 a year (£325 for any registered before March ’06), the Porsche is marginally the least painful to tax; the others are in the full-cholesterol top-guzzler band and sting you for £570.
The SLK will go full stonk and ripple tarmac or simply meander, and looks after itself, whatever you do with the gears. The slightly nervy BMW wants to play all the time, although its constant demands could get wearing. The Porsche just packs in the miles as the super-sharp and rock-steady device that it is.
There’s no ‘best’ until you decide which way your focus drops, although on paper the Merc has it for its performance and versatility.
The BMW is basically a hooligan, and with its Manta-ray envelope packing plenty of muscle looks as if it’s drifting even when it’s standing still.
The no-frills and slippery Porsche is for getting places fast and fairly discreetly, as the owner’s several Alpine trips attest. And the deliberately detailed Mercedes is for being seen in, with the added option of dropping the roof without compromising comfort, but just doesn’t involve you as deeply.
Ultimately, it’s all about how naughty and brave, focused or laid-back you feel.
Images: Tony Baker. Thanks to the Red Lion at Chelwood Gate
MERCEDES-BENZ SLK55 AMG
- Sold/number built 2005-’11/9541
- Construction steel monocoque
- Engine all-alloy 5439cc V8, Bosch ME 2.8 injection
- Max power 355bhp @ 5750rpm
- Max torque 375lb ft @ 4000rpm
- Transmission seven-speed auto, RWD
- Suspension coil-spring struts front with two lower links, multi-link rear
- Wheels and tyres 225/40 and 245/35x18
- Length 13ft 51/2in (4099mm)
- Width 5ft 101/2in (1794mm)
- Height 4ft 23/4in (1287mm)
- Weight 3473lb (1575kg)
- 0-60mph 4.9 secs
- Top speed 155mph (limited)
- Mpg 23.5
- Price new £51,975
PORSCHE CAYMAN S
- Sold/number built 2006-’09/n/a
- Construction steel monocoque
- Engine all-alloy 3387cc 24v four-cam flat-six, Bosch ME injection
- Max power 291bhp @ 6250rpm
- Max torque 251lb ft @ 4400-6000rpm
- Transmission six-speed manual, RWD
- Suspension coil-spring struts and wishbones all round, anti-roll bars f/r
- Wheels and tyres 235/40 and 265/40x18 Length 14ft 3in (4340mm)
- Width 5ft 103/4in (1800mm)
- Height 4ft 31/2in (1305mm)
- Weight 2955lb (1340kg)
- 0-62mph 5.4 secs
- Top speed 171mph
- Mpg 27
- Price new £44,250
- Sold/number built 2006-’08/4275
- Construction steel monocoque
- Engine iron-block, alloy-head 3246cc 24v dohc ‘six’, BMW/Siemens MSS 54 management system
- Max power 338bhp @ 7900rpm
- Max torque 269lb ft @ 4900rpm
- Transmission six-speed manual, RWD
- Suspension coil-spring struts front, multi-link Z-axle rear
- Wheels and tyres 225/45 and 255/40x18
- Length 13ft 5in (4091mm)
- Width 5ft 10in (1781mm)
- Height 4ft 2in (1268mm)
- Weight 3241lb (1470kg)
- 0-60mph 4.8 secs
- Top speed 155mph
- Mpg 23
- Price new £42,245