Video: Mercedes’ “Backwards” Twin-Turbo Hot-V Engine Explained

We come across some really unique items as we scour the web to bring you the freshest engine news possible, and Mercedes’ “Hot-Inside-V” engine definitely falls into the category of unique, as discussed in this video from Engineering Explained. Traditionally, a V-configuration engine has the intake ports towards the centerline of the engine, and the exhaust ports pointing towards the outside of the engine. A centralized intake plenum feeds the intake ports, and individual exhaust manifolds usher out the spent gasses. Whlie there have the been an insane number of plenum design theories over the years, they have almost always fed from the center, and exited out the sides.

This twin-turbocharged 4.0-liter V8 engine from the AMG GT-C, however, is the exact opposite. The intake ports feed from the outside of the engine by two separate intake systems, and the exhaust ports are on the inside of the engine’s valley – hence the “hot inside-V” nomenclature. Each exhaust manifold has a turbocharger mounted to it, and both turbos are forward-facing, residing in the valley of the engine. The system utilizes two separate air-to-water intercoolers to keep the intake charge cool, along with a separate radiator for each intercooler. This configuration essentially isolates each bank of cylinders into its own four-cylinder engine.

Looking down on the top of the engine, you can see the two turbochargers neatly nestled in the valley between the two cylinder heads.

Looking at the engine from the front, the first advantage of the design is immediately obvious; it’s a compact, narrow setup. Normally, a twin-turbo V8 engine is a packaging nightmare, with all the hot-side and cold-side plumbing, not to mention the turbochargers themselves. The compact dimensions also allow for the engine block to reside entirely behind the front axle centerline, improving vehicle dynamics. In addition to the space-saving aspect of the compact engine layout, is the fact that all of the intake and exhaust tubing runs are kept short and efficient, keeping turbo lag to a minimum, and maximizing boost response.

Looking at the engine front the front, the compact aspect of the “Hot-Inside-V” design is readily-apparent.

What is not so apparent, as EE’s Jason Fenske explains, is the engineering benefits found in keeping all the “hot” parts bundled together. Keeping the intake charge cool is always a high-priority for the performance of any engine, and by running the intakes to the outside of the engine, it keeps them away from under-hood heat sources. Also, by keeping all the hot parts together, it retains energy and increases the efficiencies of the turbine side of the turbochargers. Cognizant of creating excess under-hood temperatures, Mercedes has designed an intricate under-hood airflow system.

This particular engine is rated at 550 horsepower and 502 foot-pounds of torque. What is really interesting, however, is that peak torque is carried from 1,900 rpm to 5,750 rpm, and peak horsepower carries from 5,750 rpm out to 6,750 rpm, with a maximum engine speed of 7,000 rpm. With such a wide, usable powerband, we’re definitely wondering if this unique engine configuration will catch on in the years to come.

About the author

Greg Acosta

Greg has spent over a decade in automotive publishing as Senior Editor of Race Pages magazine. In his free time, he is a firearms instructor and volunteer in the police armory.
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