The Mercedes-AMG E53's Mild-Hybrid Straight-Six Is Fascinating

A 3.0-liter six coupled with an electric motor, a twin-scroll turbo, and an electric compressor sounds too complicated for its own good. It's not.

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Chris Perkins

The Mercedes-AMG E53 has one of the most interesting engines of any car on sale today. Its 429-hp 3.0-liter straight-six might not be a full AMG-designed monster like the brand’s 4.0-liter twin-turbo V-8, but with a twin-scroll turbo, electric motor, and an electric compressor, it makes the most of today's available tech.

Mercedes returned to straight-sixes two years ago with the M256, essentially two cylinders grafted onto the company's 2.0-liter four. All of the company's gas passenger car engines are pretty similar, each with a bore of 83.0 mm and a stroke of around 92.0 mm (the M256’s is 92.4 mm).

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Mercedes-Benz

In both regular Mercedes and AMG-tuned guises, the M256 has an "Integrated Starter-Generator" (ISG) bolted to its crankshaft. This is an electric motor powered by a 48-volt electrical system with its own 0.9 kWh battery. The ISG produces 21 horsepower and an impressive 184 lb-ft of torque, helping to start the engine almost instantaneously and providing extra muscle. The ISG also powers the water pump and A/C compressor, which means there are no belts at the front of the engine. That’s great for packaging as it helps keep length in check, which is always a concern for straight sixes.

Mercedes-Benz Sechszylinder-Benzinmotor M256
 Mercedes-Benz six-cylinder engine M256. Engine cross section
Mercedes-Benz

Both the regular version of this engine—which we get in the US in the CLS and GLE 450—and the AMG-tuned variant have a single exhaust-driven twin-scroll turbocharger, though the one in the AMG is slightly larger. The biggest difference is that the AMG unit gets an "electric auxiliary compressor."

Think of it like an electric turbo. Or a supercharger. An AMG product planner told me the company calls it a compressor because it behaves like both. The car’s 48-volt system powers an electric motor that spins a turbine up to 70,000 rpm in an instant for a maximum of seven pounds of boost pressure.

There’s a valve in the system that directs the denser air generated by the compressor—it can either feed the engine directly or feed the exhaust-driven turbo to reduce lag.

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Chris Perkins

Torque defines the E53. Mercedes says there’s a maximum of 384 lb-ft, which is available from 1800 to 5800 rpm, but it feels like it has more. The ISG muscles the car along with authority at low revs, while the combination of the electric compressor and exhaust-driven turbo take care of everything beyond with no perceptible lag.

The E53’s digital gauge cluster can display a boost gauge, and it’s a lot of fun to watch it go from zero to full in an instant. It won’t help you figure out what’s generating the boost though, as it seems to switch between showing levels from the ISG, electric compressor, and turbocharger. In fact, it’s hard to suss out what the engine is doing at any given moment. The switch between the various power-adding appendages happens seamlessly. You're just left with the sensation that the E53’s engine is a lot bigger than 3.0 liters.

But it doesn't feel downright quick in the way you’d expect from a car with an AMG badge. Blame a 4519-pound curb weight, . Out on the highway, the E53’s power is ample, with effortless passing afforded by all its torque-boosting accoutrements. Really, this is more of a comfortable GT car than anything else.

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Chris Perkins

On the highway, you might get good fuel economy if you put the E53 in Eco mode. Some Mercedes products will decouple the engine from the transmission during coasting, allowing it to sit at idle, using less fuel. The ISG allows the E53’s engine to shut off entirely for brief periods, but as soon as you touch the throttle, it springs back into life, exactly in the right gear at the right RPM.

The EPA quotes the E53 Coupe as having a 28-mpg highway fuel-economy rating, and that’s entirely believable. According to the car’s trip computer, I averaged 26.4 mpg on a 30-mile trip at an average speed of 37 mph. That included a few attempts at stretching the E53’s legs.

My only real gripes with the M256 is that it doesn’t rev as freely nor sound as good as BMW’s B58 3.0-liter straight-six, but those are minor. The M256 in AMG guise is a remarkable piece of engineering. Mercedes had high expectations to meet in its return to straight sixes, and the M256 blows right past them. Now, our focus turns to Jaguar Land Rover, which recently announced a similar motor for its cars. It’s up against a good one.

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