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Tuesday, February 19, 2008

Mercedes SLK Facelift: In Depth

Output up by 24 kW/33 hp with fuel consumption cut by around a litre

The increase in output brought about by the reengineering also adds to the decidedly sporty feel. Although the displacement remains unchanged at 3498 cc, the V6 now musters up 224 kW/305 hp at 6500 rpm - some 24 kW (33 hp) more than its predecessor. Torque has been upped too (+ 10 Nm) and now peaks at 360 Nm at 4900 rpm. 350 Nm is on tap from as low as 2000 rpm, resulting in prodigious pulling power and exceptional performance. The SLK 350 dashes from zero to 100 km/h in 5.4 seconds, reaching a top speed of 250 km/h (electronically limited).

Despite the higher output and high-calibre performance, the SLK 350 sports engine burns considerably less fuel. With the six-speed manual transmission on board, combined fuel consumption is just 9.5 litres per 100 km, a remarkable reduction of 1.1 litres per 100 km; if the 7G-TRONIC automatic transmission is fitted, the sports engine limits itself to a mere 9.2 litres of fuel for every 100 km on the combined cycle - 0.9 litres less than previously. The cut in fuel consumption also lowers CO2 emissions by 23 g/km to 219 g/km on the automatic model (manual transmission: 227 g/km, a reduction of 28 g/km).

The key technical data for the SLK 350 sports engine

Cylinder arrangement V6
Valves per cylinder 4
Displacement cc 3498
Bore/stroke mm 92.9/86.0
Compression ratio 11.7: 1
Output kW/hp 224 kW/305 hp
Rated torque Nm at rpm 360/4900
Fuel consumption combined l/100 km 9.2 - 9.5

The sports engine in the SLK 350 is based on the previous V6 unit . The high-revving drive unit can be distinguished visually by the large red "V" on its cover. The all-important technical modifications are not visible however, and only make their presence clearly felt when the key is turned in the ignition.

New pistons for higher compression ratio and optimised efficiency

As the cylinder charge is restricted in naturally aspirated engines, the engineers at Mercedes-Benz resolved to boost the six-cylinder engine's output by increasing the rev speed and raising the compression ratio to 1: 11.7 (instead of the previous 1:10.7). Enhanced, weight-optimised pistons with a curved surface have paved the way for the increase in compression ratio, which improves efficiency and therefore saves fuel, particularly at partial throttle. To guarantee seamless valve operation, however, deeper pockets had to be milled for the valve seats . Reinforcements in the area of the piston pins give them the strength required to withstand the higher forces being exerted on them.

Single-stage intake manifold ensures improved cylinder charge at high revs

As the flap-controlled intake manifold used previously does not enable a high cylinder charge at high engine speeds, it has been substituted for a single-stage intake manifold to overcome this drawback. In order to optimise the intake manifold's design, the engineers elected to construct it from plastic. On the one hand, the core melt-out technique could be deployed with this material, meaning a high degree of design flexibility. Secondly, as plastic is a poor conductor of heat the new intake manifold stays colder than a die-cast part. So, the intake air does not heat up as much, which has a beneficial effect on the cylinder charge.

To address the inherent shortcomings of the single-stage intake manifold at engine speeds below 4000 rpm, the camshaft position has been advanced by 5 degrees of crank angle on both the intake and the exhaust sides. Since the altered flow characteristics of the new intake manifold allow the intake valves to be closed earlier, there was no need for a "sharper" camshaft to boost output. The excellent running quality has thus been retained at both idling speed and partial throttle.

Lightweight valves with sodium cooling and modified valve train

Higher engine speeds furthermore necessitated modifications to the valves and valve timing. Lightweight valves made out of ultra-high-strength and high-temperature-resistant steel are fitted in the new sports engine. The valve stems for the exhaust valves have a 3.4-mm bore and are filled with sodium to cool them down.

The distinguishing characteristics of any high-revving engine also include the conical springs that are deployed for the valve timing. As oscillations build up in the natural frequency of conventional cylindrical valve springs once a certain engine speed is reached, such springs are no longer able to close the valves properly. Conical springs, on the other hand, are able to reliably close the valves even at high engine speeds thanks to their heavily dampened natural vibration characteristics. The increased installation lengths of the conical springs, a revised intake port for optimum cylinder charging, as well as minor modifications to the cooling jacket all meant that a new cylinder head casting was needed.

Additional technical measures include stiffer cam followers as well as a tri-oval chain sprocket. Rather than being circular, this component has a barely noticeable triangular form which helps it to effectively counteract the chain sprocket vibrations that tend to increase at engine speeds above 6300 rpm. The tri-oval shape generates inversely phased amplitudes, thus reducing chain vibration and, in turn, the forces acting on it. The vibration damper on the belt pulley has also been adapted to make allowance for the modifications.

The exhaust system has been retained without any technical amendments. And thanks to the new engine management system, the sports power unit is still able to run on 95-octane premium fuel despite the higher engine speeds and output.

Emotive soundtrack underlines high-performance credentials

The capabilities of the new sports engine are not only evident in its power delivery; it also lets its presence be known by emitting a deliberately sonorous sound that lends acoustic emphasis to its high-performance character without ever being obtrusive. Yet, as the driving noise above 140 km/h is no louder than on its predecessor, a high standard of motoring comfort still comes guaranteed.

All that has changed is the tone. It is the noise emitted directly by the engine and a modified air cleaner that strikes a somewhat more aggressive note. Compared to the other SLK models, the engine makes a far more forceful impression, especially on the driver. This is down to an emotion-charged soundtrack, which generates the sort of noise associated with a powerful sports engine when the driver comes off the throttle. On models fitted with the 7G TRONIC automatic transmission, the engine management furthermore triggers an automatic double-declutching function during downshifts. Quite apart from producing an exceedingly sporty sound, this has practical benefits too: as double-declutching equalises the rotational speeds of the crankshaft and transmission, load-alteration effects are reduced, and the gearshift becomes more gentle and harmonious. The sporty acoustics therefore go hand in hand with a boost for safety.

Following the facelift, the engine line-up for the second generation of the highly successful SLK-Class will be completed by three further variants:

The four-cylinder supercharged engine produces an extra 15 kW/21 hp of output (135 kW/184 hp) thanks to the use of a modified engine management system, a more dynamic turbocharger and enhanced pistons. At the same time, maximum torque has been increased from the previous 240 to 250 Nm between 2800 and 5000 rpm. This propels the SLK 200 KOMPRESSOR from 0 100 km/h in 7.6 s (automatic transmission 7.9 s) and on to a top speed of 236 km/h (automatic transmission 232 km/h). Fuel consumption on the combined cycle has dropped by 1.0 litre to 7.7 litres per 100 km (automatic transmission: by 1.0 litre to 8.0 l/100 km). At the same time, CO2 emissions in six-speed manual models have been reduced by 27 g /km to 182 g/km (automatic: a reduction of 25 g/km to 190 g/km).

The SLK 280 has also undergone further improvement with respect to fuel economy and therefore CO2 emissions. It continues to produce 170 kW/231 hp from its displacement of 2996 cc, delivering 300 Nm of torque between 2500 and 5000 rpm. It takes 6.3 s to accelerate from 0 to 100 km/h (automatic transmission 6.2 s) and its top speed stands at 250 km/h. Fuel consumption has been cut by 0.4 l to 9.3 l/100 km (automatic transmission: by 0.2 l to 9.1 l), whilst CO2 emissions have been reduced by 11 g to 220 g/km (automatic transmission: by 6 g to 216 g/km).

The familiar 5.5-litre V8 powerplant is still at work under the bonnet of the SLK 55 AMG, developing 265 kW/360 hp and a peak torque of 510 Nm. The 0-100 km/h sprint is over in 4.9 seconds, the top speed is electronically limited to 250 km/h. The SLK 55 AMG has a combined fuel consumption of 12.0 l per 100 km with CO2 emissions of 289 g/km.

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Related entries:

OFFICIAL: Mercedes-Benz SLK Facelift Unveiled
Mercedes SLK Facelift Keeps its F1 Nose


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