New 422hp AMG V8 Gets '4-Pot' Mode

Cylinder deactivation system promises fun and economy for SLK 55

By Chris-R / Thursday, 21 July 2011 / Loading comments

The 4 (or 8) cylinder SLK 55 AMG

PH got a privileged invite to Affalterbach yesterday, where head of AMG powertrain development Fritz Eichler pulled the wraps of the company's new M152 engine - a normally aspirated variant of the 5.5 litre M157 'biturbo'.

The engine is notable for its F1-derived cylinder deactivation system and 'stop-start', which means the new SLK 55 AMG (in which the engine will make its debut at the Frankfurt motorshow in September) should combine supercar performance with class-leading efficiency.

With all cylinders firing the M152 engine produces a heady 422hp at 6,800rpm and a whisker under 400lb ft at 4,500rpm. Yet coupled to the latest version of AMG's existing 7 speed torque converter auto (chosen because it works best to smooth out the transition between 4- and 8-cylinder operation), the SLK 55 is capable of up to 33mpg on the combined cycle. That's an increase in efficiency of 30 percent over its predecessor, while CO2 emissions have also been reduced to 199 grams per kilometre.

Fritz Eichler and the new M152

The four-pot mode only works in transmission mode 'C', and apparently the switch from 8 to 4 cylinders and vice-versa is seamless. If you want to accelerate hard from 4 cylinder 'eco' mode, the switch to 8 cylinders takes just 30 milliseconds, so it happens entirely imperceptibly while the gearbox kicks down. (They say.)

Cylinder shut-off works between 800rpm and 3600rpm, by cutting fuel to cylinders two, three, five and eight. The valves on the deactivated cylinders remain closed on the compression stroke, so there's a kind of spring rebound effect from the compressed fresh air within the cylinder which allows the load change losses to be reduced. (Unless Herr Eichler lost us on that point, which is entirely feasible...) A 'centrifugal force pendulum' has also been introduced into the transmission to eliminate nasty four cylinder vibrations.

In case you were wondering, the system can't be introduced on the M157 itself, (although the majority of its components are identical), because AMG hasn't yet worked out how to seamlessly integrate the turbos - although you can bet they're working on it.

Getting the lowdown on the new unit

You might also be wondering what a four cylinder AMG engine is going to sound like? Well, we can't answer that yet, but the company says it has been working hard to make sure it doesn't offend expectations. To that end the SLK 55 AMG will be the first AMG model to feature variable flaps in its exhaust system, specially tuned to eliminate some of the treble notes and amplify the bass during 4-cylinder operation. Under full load the flaps in the silencers are opened, so you can enjoy the traditional V8 thunder.

Speaking at the engine's unveiling, AMG chairman Ola Kallenius was keen to impress on us the importance of technology such as this to the AMG brand.

"We want to be the driving force in performance systems going forward, but when we do something new we have to be true to the AMG gene and at the same time we have to be efficiency leaders.

"We like to say this is our first '99 gram' CO2 car, but with a one in front," he quipped, "because with an efficiency gain as great as 30 percent it feels like a 99 gram car for the performance sector."

As long as it doesn't drive like one, we'll all be happy...