The days are numbered for the epic 6.2-liter M156/159 V-8 that has energized one admirable “63”-badged Mercedes-Benz AMG product after another for the past few years. Chalk up the engine’s death to the unrelenting drive for efficiency. While most of AMG’s “63” models have switched to a twin-turbocharged 5.5-liter V-8, the thunderous, M159-powered SLS AMG GT’s replacement, the 2016 AMG GT sports car, is getting a new, even smaller twin-turbo V-8. We recently got first-hand experience with this new engine, code-named M178. Here is what we learned.
So far, we have not yet seen the exterior of the AMG GT fully disrobed—the final interior, however, has been revealed—but we can tell you that the new M178 V-8 is, in essence, a double-throw-down version of the turbocharged 2.0-liter four-cylinder that powers the CLA45 AMG compact sports sedan. Casting two banks of cylinders with a common crankcase and rotating the assembly to a north-south orientation yields a 4.0-liter (3982-cc) V-8 that’s ready, able, and anxious to fill the departing V-8’s big and gloriously loud shoes.
Mercedes Gets a Little Hot-V Action
Lively throttle response and a passion for the red-colored end of the tachometer are just a few of the charms that made the naturally aspirated M156 V-8 so endearing. The twin-turbocharged M178, on the other hand, boasts output curves that are fatter at the bottom and a different sound and feel while still upholding the M156’s exemplary performance standards. At least, that is AMG’s intention as posited by Christian Enderle, engine and powertrain development chief at Mercedes-AMG.
The M178 is Mercedes-Benz’s first “hot-V” engine, meaning its two BorgWarner turbochargers are positioned between the cylinder heads instead of outboard of them. While General Motors toyed with reversing the traditional location of intake and exhaust valves in 2006, other manufacturers such as Audi, Bentley, BMW, and Ford have successfully used the hot-V layout for years.
Stuffing the turbos in an engine’s V makes sense because it concentrates exhaust heat and momentum to spin the turbochargers’ turbine wheels with utmost efficiency. Other benefits include shorter, simpler plumbing and less exhaust heat lost ahead of the catalytic converters. Of course, there are downsides: cradling turbos in the V increases engine height, and dealing with residual heat when the engine is shut off immediately after a hard run is a challenge. AMG engineers addressed the latter issue by creating what they call a “wind tunnel” attached to the underside of the hood to duct cooling air to and past the sizzlingly hot turbos. There also is a heat shield to keep them from boiling the hood’s paint.
Don’t Call It a Square
Another fundamental engine-design change between the naturally aspirated M156-159 and the successor M178 is a revised bore-to-stroke ratio. Born on the racetrack, the M156 V-8 is a classic oversquare design; in the high-performance SLS Black Series, the engine had a large bore (4.02 inches) and the shortest practical stroke (3.72 inches) to support a 7900-rpm redline. The M178 retains most of the old engine’s stroke (3.62 inches), but its bore is significantly reduced (to just 3.27 inches). Although the reduced bore provides less room for valves, gorging the pistons with up to 17.4 psi of boost remedies breathing concerns. More significantly, reducing the bore diameter yields a chamber with a better height-to-width ratio to support more efficient combustion.
Like the M156, the M178 gets dual overhead camshafts operating four valves per cylinder with variable intake and exhaust timing. To reduce friction, the valves are opened with finger followers equipped with rollers (in contact with the cam lobes) and hydraulic lash adjusters. Bosch piezo-electric direct fuel injectors also are new, and they deliver five squirts of fuel per combustion cycle.
The cast-aluminum block is a closed-deck design to provide the desired strength. A special Mercedes-developed process called Nanoslide coats the aluminum bore surfaces with a thin, porous coating of iron for improved wear resistance with minimal friction. A dry-sump lubrication system allows the engine to be mounted as low as possible in the chassis. Longitudinal packaging is improved as well, with Mercedes claiming that the overall engine length is trimmed 3.5 inches compared with the M156.
All of these tricks combine to help the M178 produce 479 lb-ft of torque at a mere 1750 rpm and 503 horsepower at 6250 rpm—at least when fitted to the 2016 AMG GT. The low torque figure should translate to excellent drivability, and since the AMG GT is heavier than a Porsche 911, it’ll need all the power it can muster to take on its home-turf competition. Although the M178’s power drops off at high rpm, AMG set the redline at an entertaining 7000 rpm.
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Opening the AMG GT’s hood reveals the petite new V-8 to be partially obscured by bulky air-filter and intercooler housings, while the two turbos are hidden beneath a heat shield. At least you’ll be able to easily revel in the engine’s soundtrack, because AMG imbued it with assertive pipes. Tapping a cockpit switch sets the exhaust system’s trio of baffles to serenade mode (our term), and the resulting music is bass-heavy with sharp snorts and barks accenting up- and downshifts.
There’s a clear familial link to AMG’s other V-8s old and new, and we’re thinking that the AMG GT’s rousing soundtrack and its two-cylinder advantage versus Porsche’s 911 are what will convince AMG fans—and folks in the market for a roughly $100,000 sports car—that the decision to drop the M159 wasn’t so bad after all. Need a bit more practicality than an expensive two-door sports car offers? Fret not, for a different version of the M178, dubbed M177, will power the next-gen C63 AMG.
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