The sports car engine that powers Mercedes-Benz’s most luxurious sedan

What defines a sports car engine? Is it the sound it produces? The vibrations it makes? The way it delivers its power? In fact, it is all three combinations that make the engine feel intuitive. In the world of V-8s, there are two engine layouts, and they are determined by the type of vehicles they power. On one side we have the cross-plane crank, while on the other side we have the flat-plane crank.

Mercedes-Benz has just unveiled the facelifted S-Class for model year 2027, which the company claims is the most comprehensive mid-cycle facelift it has ever carried out. One of those extensive changes includes the V-8 engine, which replaces the cross-plane crank with a flat-plane crank. What gives? And why is this a big deal in the luxury sedan segment? Let’s take a look at what a flat-plane V-8 is to find out why it’s so important to put this engine in the S-Class.

Flat plane versus cross plane

The choice of going with the flat-plane or cross-plane configuration is not exclusive to V-8 engines, although it is most commonly used for this engine type. Four-cylinder engines are almost always planar, while cross-planes are the dominant crankshaft layout for V-8 engines. That’s why the new S-Class with a flat crank was a surprising decision.

Cross-plane cranks for most cars

To understand why they are called flat-plane or cross-plane, let’s look at the crankshafts of these engines. Starting with the transverse plane, the pins to which the cylinders are attached are arranged at 90 degree intervals around the axis. Viewed from the front, a cross-plane crankshaft resembles a cross (or an X), hence the name “cross-plane”. Based on its sound profile, the cross-plane V-8 produces a rumble, which is essentially the same as muscle cars, luxury sedans and SUVs with such an engine. They also allow larger engine displacements. That’s why every V-8 with a displacement of more than 5.5 liters has a cross-plane crank.

So why do most V-8s have a cross-plane crank? Well, they have the most balanced layout. Remember the pins that were placed at a 90 degree angle? That virtually eliminates vibration. The deep and throaty rumble? That’s because of the 90 degree intervals and the resulting firing order on each left (L) and right (R) bank is uneven. Also, the 90 degree angles are heavier and especially due to the longer stroke, these engines are known for their higher torque. This also means that V-8s with a cross-plane crank don’t rev very high. This is why all large and heavy vehicles with a V-8 engine tend toward the cross-plane crank layout.

Flat cranks for sports cars

On the other hand, a rarer type of V-8 engine features a flat-plane crankshaft. The largest supplier of flat-plane V-8 engines? That would be Ferrari, and as you can guess from the sound profile of their engines, they have a high-revving, high-pitched sound. With a flat crankshaft the pins are placed at a 180 degree angle, so when viewed from the front the flat crankshaft is simply flat, hence the name. Flat-plane cranks are lighter, which is why these engines respond better and can handle higher speeds. The shorter stroke also means it is not rich in torque, which is also why these engines require high RPM.

Due to the flat, unbalanced shaft design, vibration is more difficult to reduce, but that does not mean it is not possible. Furthermore, the flat layout guarantees a perfect RLRLRLRLRL firing sequence, resulting in that high-pitched sound. Thanks to this design, flat-plane crank V-8s do not have large displacements. The largest flat-plane V-8 ever fitted to a production car is the current Corvette Z06’s 5.5-liter LT6 V-8, which produces 670 horsepower and revs to 8,600 rpm.

The world’s first luxury sedan with a flat-plane V-8 crankshaft

In fact, this makes the 2027 Mercedes-Benz S-Class the world’s first luxury sedan with a flat-plane V-8. All luxury sedans, even the ones AMG made, all had transverse-plane V-8 engines, so let’s take a look at why they chose this engine configuration.

For compliance with emission standards

Speak with CoachMercedes-Benz says one of the main reasons for the switch to a flat crank is mainly due to compliance with emissions standards. A flat-plane crank is not necessarily significantly cheaper in terms of emissions than a cross-plane crank, but in the effort to reduce as many emissions as possible, all changes are welcome.

In this case, a flat-plane V-8 has better exhaust gas evacuation – a process in which burned exhaust gases are removed from the combustion chamber during valve overlap, introducing cleaner air into the mixture, resulting in cleaner emissions. In addition to the ride, there is a 48-volt mild-hybrid system, the so-called EQ technology from Mercedes-Benz. Together with the better exhaust gas removal provided by the flat-plane V-8, the new V-8 called M177 Evo will keep the V-8 alive at Mercedes-Benz.

What makes the M177 Evo special

The M177 Evo is the first V-8 with a flat-plane crankset since the M178 LS2 of the AMG GT. The M177 Evo, like the M177 to which it is related, still has a displacement of 4.0 liters. Emissions aside, Mercedes-Benz switched to a flat-plane crank because of the quicker response thanks to the lighter crankshaft, a feature of this engine we mentioned earlier. Additionally, it also has a higher rpm, but in this case adjustments to the firing order, the turbos, and other extensive changes result in a very wide torque curve, like what a cross-plane crank would produce. The full 553 pound-feet of torque (37 pound-feet more than the previous S 580’s M177) is available from just 2,200 rpm to 4,500 rpm.

Specifications Mercedes-Benz S-Class 2027

Consistent with this engine’s role for use in luxury sedans is the company’s decision to add balance shafts because, as we mentioned earlier, the layout of a flat-plane crankshaft V-8 means it has more secondary vibration. Don’t expect high-revving engine sounds here, as Mercedes-Benz has worked meticulously to reduce noise, vibration and harshness at every stage of this engine’s development. All in all, what we have here is a V-8 that combines the responsiveness and lower emissions of a flat-plane crank with the smoothness and torquey character of a cross-plane crank. Typical Mercedes-Benz ingenuity, perhaps even over-engineering, and we love it for that – until the warranty expires, of course.

Everything else that’s new in the S-Class

The 2027 Mercedes-Benz S-Class is getting what the company calls its most extensive mid-cycle facelift ever. In fact, more than 50 percent of the vehicle is new, even though this is still not a generational change. Some changes are welcome, while others are likely just change for change’s sake.

Technical overload

One of those includes the interior, which somehow tells us that Mercedes-Benz may not have learned yet based on consumer feedback. You see, luxury is characterized by fine materials and interesting textures, and there’s nothing visually interesting about a huge expanse of glass with three screens called the MBUX Superscreen. All this is powered by the MB.OS operating system, with a 12.3-inch driver display, a 14.4-inch infotainment touchscreen and a 12.3-inch front passenger display. On the plus side, the disappearance of beautiful materials on the dashboard has been somewhat compensated by more wood surfaces on the doors and on the center console.

The S-Class has always pushed the boundaries of technology, but the new round vents with “digital ventilation control” seem to be technology for technology’s sake. It basically means that the outer circular vents are now controlled electronically rather than simply by your hands. However, this allows the MBUX Virtual Assistant to control these vents. The AI ​​voice assistant relies on ChatGPT4o and Microsoft Bing Search. Of course there are plenty of optional extras such as executive rear seats with two touchscreens, an ottoman and a new camera for making video calls, a Burmester 4D surround sound system with 39 speakers (the standard sound system is a Burmester 3D system) and heated seat belts that work in addition to the seat heating.

A starry sky

If you love stars, and especially the three-pointed star logo, then you’ll love the 2027 Mercedes-Benz S-Class. The front features a 20 percent larger grille with an illuminated border and a pattern of the three-pointed star logo. Speaking of which: for the first time, the upright three-pointed star logo is also illuminated. In line with newer Mercedes-Benz models, the S-Class gets LED headlights and taillights with the three-pointed star as part of the lighting design. There are also new wheels, while the AMG Line version (shown in silver) adds sportiness to the full-size luxury sedan. Towards the rear, the LED taillights also feature the three-pointed star, which may be an exaggerated mark for this full-size sedan.

Sources: Mercedes-Benz, AutoCar, NCBI