New cars: Mercedes SL

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Geneva star

What is it about some cars which enables them to avoid the fate of obsolescence? Where most go through the successive phases of novelty, acceptance and then senescence, a few seem able to pass smoothly from being up to date to being, if not “classics, then at least outside fashion.

Mercedes’ current SL sports-car has had this good fortune, passing almost two decades on our roads without losing its position as the height of many people’s automotive aspirations. Finally, at the Geneva Show, the company has revealed its handsome successor. We had a chance to examine the technical aspects of the car just beforehand, though driving impressions will have to wait.

Visually the new SL breaks little new ground, but presents an obviously Mercedes face to the world. A broad sloping SL grille protrudes between the wrap-around lamps, and the bonnet line sweeps up to the screen and again to the rear deck where the hood is recessed flush. Separate lower-body mouldings and smooth bumpers help this largish car to look slim, and the traditional ridged rear lamps maintain the SL rear view. The hard-top is particularly elegant, but the car looks equally good with the hood down or up.

As before the range embraces six and eight-cylinder engines, but the model structure is novel. The intermediate sizes (350 and 450SL) have gone; instead there are two 3-litre sixes, a 12-valve and a 24valve, delivering 190 and 231 bhp. When the SL is launched the top model will be a 5-litre V8 of 326 bhp, but the long-rumoured V12 is crystallising slowly, not to be revealed until the early 1990s.

Starting the range is the single-cam 12-valve six, broadly similar to the 3-litre unit in current Mercedes saloons with some cam and head flow changes to reduce emissions and improve output: torque now tops at 185 lb ft in catalyser form. The intermediate model is also a 3-litre of the same bore and stroke, but this time with four valves per cylinder and twin cams with variable timing on the inlet side. This provides for zero valve overlap at idle, lengthens the inlet period on acceleration and then cuts it back again at high revs, so that the torque curve is rounded out, idling is smooth and pollution reduced.

The operating principle is similar to, though much more sophisticated than, Alfa Romeo’s VIVT system, in that the camshaft can twist relative to the chain-wheel which rotates it, thanks to a collar moving along a steep spiral thread on the shaft. That collar is moved by engine oil pressure controlled by the injection system electronics. It is a simple way of having one’s cake and eating it too, keeping the benefits of four-valve technology while pretty well disposing of the drawback of poor low-speed torque. In the new engine, called SL/24, the torque is above 178 lb all the way from 3000 to 6500 rpm, peaking at 271 lb ft. However, the system will not be added to the four-cylinder in the Cosworth 190 2.5/16.

If these figures are inadequate, the proven S-class V8 has also gained the new 32-valve head and variable cam timing, upping its power to 326 bhp and the torque to 321 lb ft. Although the mechanisms are the same, the valves are at a narrower 38° than the SL/24, keeping down the width of the quad-cam unit.

Both 300SL/24 and 500SL engines share the new KB 5 injection system which unlike most current layouts is separate from the ignition system, though linked to allow for the usual management functions. This is claimed to give greater brain power and to cope better with any sensor failures. As an indication of how clever it all is, it will remember what the conditions (the altitude, for example) were when the car was parked so that start-up will be as near perfect as possible. In addition power outputs are the same whether or not a catalyser is fitted.

A four-speed autobox is compulsory wear on the 500 and optional on the other two. In addition the 300 SL/24 buyer may replace the normal five-speed manual shift with a closer ratio sports box. Performance according to model ranges from 139 to 156 mph, with the 500 taking 6.2 seconds to hit 62 mph.

All convertible manufacturers insist that their open car is very rigid, but looking at the SL body-in-white (the trade term for a bare but primed shell) the massive sills and axle-bearing structures stand out, and the company is justly very proud of the results of some USA testing. There is a Federal requirement for roll-over strength which is waived for open cars; the SL, however, substantially exceeds the saloon requirements. Distortion is also reduced to saloon levels.

Passive safety always figures largely in Mercedes products, and the SL has a number of unusual or unique features. ABS is standard, airbags for driver and passenger can be specified (at the expense of the glove-compartment), and the system also tensions the seat-belts. These height adjustable belts are built in to the seats, with an electric interlock to ensure that the seat-back is locked, and all movements are electrically adjustable with three memories. The seats themselves have magnesium alloy frames which absorb strain and help to strengthen the shell against lateral impacts, while an overlapping tongue at the base of the door reduces the risk of intrusion.

The most novel safety feature of the SL is the flip-up roll-bar. A fixed bar spoils the looks of a full convertible, so the SL driver has the choice of raising the padded bar by hydraulic means or leaving it down. But should the car be involved in a crash, the spring-loaded bar is disconnected from the hydraulic pistons, flipping up and locking in place in a third of a second. This action is triggered by impact sensors and depends on deceleration of more than 4g or a tilt of more than 26°, as long as one of the rear suspension arms is on droop, proving that the wheel has left the ground.

Hood operation is powered, of course, using extremely high pressures so as to allow very small actuating cylinders hidden in the screen rail to lock the roof. There is no manual input at all; as long as the button is held down (to avoid trapping fingers) the roof retracts in 30 seconds and conceals itself with a neat cover. During this complex operation, which involves 15 cylinders and 11 solenoids, the roll-bar and side windows are lowered, returning automatically to whatever position the driver left them in.

Watching the fabric roof unfold is slightly disconcerting, because against all the rules of physics it expands in all directions including width to overlap on all sides the hole it has just emerged from. Draughting the inter-action of all those links must have kept the computers busy for days.

The whole roof structure is tensioned with steel ropes which act on rubber seals to isolate and quieten the interior, and internal stiffening prevents it ballooning up. Two-piece construction makes for easier replacement should the fabric be damaged. Although the drag when open is 30% up on the closed figure, the old SL was worse with its hood up than the new one is open. Extra comfort comes from a detachable screen of see-through fabric which clips to the rollbar and stops any draughts down the collar. This also helps keep heated air in the cockpit. Mazda has had the same idea for its RX7 convertible, and so far both companies are describing the feature as unique.

Automatic climate control incorporating an ultra-effective pollen filter is standard, and it makes allowances for whether the roof is open or closed. It also runs for up to 30 minutes with the engine off.

A light-weight (74 lb) aluminium hardtop is standard issue with every SL, and this also locks in place with the push of a button once it is positioned. Synthetic materials were considered for the hard-top, but although these can be made strong enough, they were rejected because they are not able to stand up to the abrasion of an overturned car sliding on its roof, and Mercedes’ figures show that cars turn over in some 10% of accidents.

Interior comforts are abundant: the seat position memory also alters the steering column height and all mirrors, including the interior. The outer pair of fascia vents is controllable remotely, and heated air is fed behind the door panels so that they do not chill the elbows.

As well as the doors, which unlock automatically in the event of a crash, the central locking secures the glove compartment and other interior stowage, while there is an option of remote control for this which also closes the side windows, using an infra-red key which changes its combination every time it is used.

Mercedes’ multi-link rear suspension is employed for the SL, the various bushing movements being designed to cancel each other out and avoid toe-in changes. At the front the uprights pivot on a transverse arm with an inboard spring and a vertical strut outboard. On the base car this is a simple damper, but for dearer models yet more sophistication is on offer. We have already seen the automatic traction control (ASR) and auto-locking differential (ASD) which will be available along with ABS, but to complete the acronymic quartet there is now ADS — Adaptive Damping System. This is aimed at reconciling the desire for a soft ride with firm control of body movement, and switches between four degrees of damping according to a sophisticated variety of incoming information. Damping forces increase with overall speed, cornering force, steering input, sharp acceleration or braking, extra weight and sudden wheel deflections as over potholes or rough surfaces.

Coupled to this is an automatic level control which maintains ride-height at all four corners, until 75 mph is exceeded, when it lowers the body by 15mm to decrease lift and drag. Below 50 mph the driver can also add 30mm ground clearance for rough tracks.

Non-ADS models rely solely on the coil springs, but ADS cars actually spread the weight between the coils and gas pressure in the dampers; this is much more sophisticated than the hydropneumatic system of the S-class.

To accomodate sufficient braking power the alloy wheels are of 16in diameter, and the front discs are ventilated. All models are equipped with ABS, and the 225/55 tyres are of ultra-high-speed ZR rating. When we finally see a V12 version, it will be no faster at the top end — the same 156 mph to which the 500 is electronically governed, as the V12 will be. Officially this is for the sake of the tyres, and there is certainly a justification for this, but 156 mph is of course 250 kph, a sensitive speed in a country where environmental concerns are very pressing.

The prices released in Geneva were less frightening than expected: 300 SL will reach Britain first, in September, at about £40,000. When the multivalve models will arrive has not yet been determined. GC

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