First Drive Review
In an automotive world obsessed with the newest of new vehicles, midcycle styling updates don’t generally garner much attention beyond the initial press announcement. But when the visual changes are accompanied by a boost in horsepower, well, consider our interest piqued. Such is the case with the 2009 Mercedes-Benz SLK350, which, thanks to a number of improvements to its V-6 engine, makes 32 more horsepower than before, up to an even 300, and eight more pound-feet of torque, now listed at 266. As with the 2008 model, a seven-speed automatic transmission with a manual mode operated via the shift lever or steering-wheel paddles is the only transmission for the SLK350. The SLK also has a new steering rack across the range, which consists of the 355-hp SLK55 AMG and the rechristened 228-hp SLK300, née SLK280.
It’s Still a 3.5-Liter, but Not As We Know It
Underneath the hood, the updated “sports” engine is discerned by the red stripes on the engine cover. The 3.5-liter displacement is unchanged; in fact, it’s almost the same engine as before. The power increase comes from an increased compression ratio of 11.7:1 (up from 10.7:1) and a 7200-rpm redline, an improvement of 1000 revs. The compression ratio is achieved simply with newly designed pistons, but the extra revs required even more changes. A new, single-stage intake manifold made of plastic improves airflow at higher rpm. Lighter valves with sodium-filled valve stems and conical valve springs also help the high-rpm performance. Finally, the timing-chain sprocket has a slightly triangular geometry to reduce vibrations above 6300 rpm. A new air-filter design has changed the engine tone to sound more aggressive even though the exhaust is unchanged. The last SLK350 we tested—back when it was available with a manual—did 0-to-60 mph in 5.4 seconds. The 2009 SLK350 has a slightly taller final-drive ratio, but the horsepower increase should be good for a 0-to-60 sprint close to 5.1 seconds. That’s just a couple of ticks behind the Porsche Boxster S, which starts at $56,560 for a 2008 model, or $6285 more than a 2008 SLK350. Mercedes claims the pricing will be similar when the ’09 goes on sale in May.
Direct Steering with a Variable Ratio. Huh?
Besides acceleration times similar to those of the Porsche, the 2009 SLK now has a similar steering rack as well, and this is a good thing considering Porsche’s reputation for excellent steering. Mercedes calls it a direct-steer system, and it works by using a variable-ratio steering rack. At the center of the rack, which corresponds to small movements of the steering wheel, the ratio is lower, or more indirect. As the steering angle increases, so does the steering ratio, so less input is needed to turn the wheels at extreme angles. The result is good straight-line stability combined with easy maneuverability in sharp corners.
