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    • A technical look at the new 408 hp Mercedes 3.0 turbo M256 inline-6 engine with an electric compressor

      Mercedes is taking a page out of BMW's book and building a modular inline engine family. Why? To cut costs of course. If you maintain the same bore and stroke the engine architecture can be stretched from three, to four, to six-cylinders meaning many parts will be shared.


      The M256 is the first Mercedes inline-6 engine to be produced since the M104 ended production in 1999. The M256 is a very different take on the I6 from the M104. Why? For one, direct fuel injection.

      Mercedes is all about efficiency and keeping costs low with this motor. It features 10.5:1 compression, a turbocharger, an electric compressor, and energy recovery.

      The M256 will replace the M276 across the lineup and the starting point will likely be the S-Class and the upcoming Mercedes-AMG E50 coupe. Here is how it stacks up against the V6:

      Engine M 256 M 276
      No. of cylinders/arrangement 6/in-line 6/V
      Displacement per cylinder cc 500 499
      Displacement cc 2999 2996
      Cylinder spacing mm 90 106
      Bore mm 83 88
      Stroke mm 92.4 82.1
      Bore/stroke 1.11 0.93
      Connecting rod length mm 140.5 148.5
      Rated output kW/hp over 300/408 245/333
      Peak torque Nm over 500 480
      Compression ratio 1: 10.5 10.5


      The displacement is roughly the same but the M256 specifically targets 500cc per cylinder which is for the modular design. They go with small cylinder bores but a much longer stroke to hit 2999 cc's. This is a heavily undersquare design while the M276 was oversquare. Do not expect a high revver.

      The bore spacing is also tighter. This is due to an I6 being inherently longer than a V6 so in order to prevent a negative impact on weight distribution Mercedes is keeping the engine more compact. That is also why Mercedes removed belt-driven components from the front of the motor. The tighter spacing can be a negative for tuning at higher power levels depending on the block strength.

      Power and torque are both up. The M256 produces 408 horsepower and over 368 lb-ft of torque. Mercedes likely won't have much trouble getting more power out of this motor and neither will tuners.

      Why? Because the turbo does not need to be sized for low end response. That is where the electric compressor comes in. This is not a conventional twin turbo motor. The 48 V electric system is designed specifically to feed this electric compressor.

      Mercedes calls it an electric turbo but that does not make any sense as exhaust gases are not involved in turning the compressor wheel:

      Quote Originally Posted by Mercedes
      Its performance is as dynamic as that of a V8, as the new six-cylinder in-line engine comes with an especially intelligent form of turbocharging: assisted by the ISG at start-off, the electric auxiliary compressor (eZV) guarantees immediate high torque when driving off and accelerating, bridging the time before the large exhaust turbocharger cuts in. The electric turbocharger accelerates to 70,000 rpm within 300 milliseconds, ensuring an extremely spontaneous reaction from the engine. The result is a dynamic engine response with no turbo lag.

      The M 256 is the first member of a new family of premium petrol engines that have for the first time been systematically designed for electrification from the outset. The 48 V electrical system serves not only high power consumers, such as the water pump and air-conditioning compressor, but also the Integrated Starter-Alternator (ISG), which also supplies energy to the battery by means of highly efficient energy recovery. The ISG dispenses with the need for a belt drive for these components. This not only reduces the overall length of the engine and its complexity, but also paves the way for new, efficient control possibilities. The still existing 12 V system supplies power to consumers such as lights, cockpit, infotainment and control units.
      The idea is for the electric compressor to help move the car at low rpm while the conventional turbo spools. Mercedes believes they completely eliminate turbo lag this way but manufacturers have made 'lagless' claims before. It certainly will be interesting to see how close they get.

      With the electric compressor wheel spinning to 70,000 rpm in only 300 milliseconds it likely will be hard to perceive any turbo lag at all. The question from a tuning perspective will be, can this compressor be overspun? Does this mean fitting a larger turbocharger won't come with a lag penalty?

      Tuning will be far more complex but it likely also will not have the same traditional negatives where one has to sacrifice quick spool for top end power.

      There still are many details set to come but it certainly will be interesting to see how the Mercedes aftermarket adjusts to this new engine design.




      This article was originally published in forum thread: A technical look at the new 408 hp Mercedes 3.0 turbo M256 inline-6 engine with an electric compressor started by Sticky View original post
      Comments 6 Comments
      1. subaru335i's Avatar
        subaru335i -
        Sweet! Do they mean that there is an electric motor in the middle of the compressor and turbine like in the F1 split turbo? That would regenerate power from the turbine when boost isn't needed and spin the compressor to reduce lag.
        That would make sense with them calling it an "electric turbocharger". It wouldn't make much sense to have a separate traditional turbo and electric compressor.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by subaru335i Click here to enlarge
        It wouldn't make much sense to have a separate traditional turbo and electric compressor.
        Well that's what it is...
      1. subaru335i's Avatar
        subaru335i -
        Alright, thats why I phrased it as a question. Seems like having a separate auxiliary compressor is expensive and another thing to break but there must be reasons why they are doing it.
        I was just hoping the split turbo technology from F1 was trickling down to road cars because IMO it is so elegant.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by subaru335i Click here to enlarge
        Alright, thats why I phrased it as a question. Seems like having a separate auxiliary compressor is expensive and another thing to break but there must be reasons why they are doing it.
        I was just hoping the split turbo technology from F1 was trickling down to road cars because IMO it is so elegant.
        No it's fine I get what you're saying it's just this is not that advanced yet still very advanced. Basically, it's all about eliminating turbo lag and keeping costs down.
      1. Bowser330's Avatar
        Bowser330 -
        Eliminating turbo lag so MB can use a larger turbo than normally appropriate to keep the power curve up till redline.
      1. Sticky's Avatar
        Sticky -
        Click here to enlarge Originally Posted by Bowser330 Click here to enlarge
        Eliminating turbo lag so MB can use a larger turbo than normally appropriate to keep the power curve up till redline.
        That's exactly it but now how about going with an even bigger single in the aftermarket? Potentially pretty cool, right?