Mercedes-Benz Drops 422-hp Naturally Aspirated Version of New 5.5-liter V8 into 2012 SLK55 AMG

Mercedes-Benz has just pulled the wraps off of the V-8 that will power the 2012 SLK55 AMG. The naturally aspirated 5.5-liter V-8 makes 422 hp and 398 lb-ft of torque in the hot SLK, and will be paired to a traditional torque-converter seven-speed automatic gearbox—not Benz’s MCT transmission, which swaps in a clutch pack in place of the torque converter.

This means that the SLK55 will have significantly less power than do its brethren packing the twin-turbocharged version of this engine, but the output is still an improvement of 62 hp and 22 lb-ft from the old 5.4-liter V-8 in the last-gen SLK55. Engineers at AMG tell us that there was just no room for the turbochargers in the SLK, and we imagine that the company is hesitant to give the lowly SLK power that’s comparable to the upcoming SL63, which will use the twin-turbo motor.

Like its force-fed brother, the naturally aspirated 5.5 will feature a stop/start system as a fuel-saving measure. But the new mill actually will go much farther toward reducing consumption by featuring cylinder deactivation—a first for any production AMG engine. Cylinders 2, 3, 5, and 8 will deactivate depending on speed and engine load, but we’re told the engine will be able to run up to 3600 rpm in four-cylinder mode; if a changeover is required, it takes just 30 milliseconds to fire up the cylinders again.

To accommodate the cylinder deactivation—both the process of deactivating and reactivating cylinders, as well as running in four-cylinder mode—the company had to make several modifications to the engine. For one, the traditional slushbox was chosen as the transmission because it can handle the gear changes more smoothly, and with four cylinders popping on and off, an effortless change to a much higher or lower gear is necessary.

The new engine will also be paired to a new exhaust system which features one flap on each side of the exhaust. The flaps will close most of the way when the engine is in four-cylinder mode so that the car runs more quietly—we imagine the four-cylinder exhaust note isn’t exactly a Symphony in Burning Octane. At 2000 rpm, the flaps begin to open progressively depending on how hard you’re mashing the gas pedal.

Between the start/stop system, cylinder deactivation, and some other minor tweaks, Benz and AMG say that the SLK55 with the new engine will use 30 percent less fuel than did the last SLK55. That car was rated at 14 mpg city/22 highway, so figure on a highway rating in the high 20s when the EPA feeds the new SLK’s data into the government supercomputer.

And in the event that all this talk of stop/start and cylinder deactivation strikes you as completely antithetical to the cause of AMG cars, worry not: The SLK55 will allow drivers to completely shut off the system with the flick of a switch.