Cadillac does it. Porsche does it. Audi does it. Even Mercedes and BMW. It’s a Hot V turbocharger. Why go against the norm when it comes to snail placement? This is why, and how it works.
With a conventional turbo, or pair of turbos, the turbocharger is hung off the side of the engine. Perched down there at the end of the exhaust manifold. It’s an easy place to put it, and it’s generally a safe place to put it.
But there are some serious drawbacks. Sticking the turbos outside of the engine footprint makes the engine take up more space. On an already wide engine, it can make fitting a turbo into the engine bay nearly impossible. Much like how the LS swap is more popular than Ford and Chrysler swaps due to the narrow cylinder heads that engine uses.
If you’re going this route on a V-cylinder engine, you need a pair of turbos. Most of the time. There are a few models over the years that have only used one bank of cylinders to power the turbo. Notably, the Saab 9-5’s 3.0L V6 went about it this way. The other way to mount a single turbo is to mount it further away from the cylinder head. Usually way at the back of the engine. That takes the sacrifice to drivability that turbochargers already make and then makes it even worse.
That’s because the further the turbo is from the exhaust valve, the more boost lag you get. It takes time for the engine to start pumping more air once you open the throttle. It takes even more time for that air to start spinning the turbos. Move the turbo further away from the exhaust valves and it takes longer to make boost. Hence more turbo lag.
Turbochargers also thrive on hot air. The hotter the air is, the more quickly it flows toward equilibrium. That spins the turbo more quickly, giving you more boost sooner. Move the turbo further from the exhaust valve and the exhaust starts to cool. That lowers efficiency.
So turbos are great, but they suck for packaging. And if you put them in the logical place, they’re less efficient.
Enter the Hot V.
That puts the turbos in between the cylinder heads, instead of outside of the engine. They’re extremely close to the exhaust valves, they don’t expand the footprint of the engine, and you can easily use a single turbo for a V engine.
BMW pioneered the system on the N63 V8, and Mercedes-Benz made good use of the system on the AMG GT’s 4.0L V8. The cylinder heads are effectively reversed. The air is sucked in from outside of the V and exhausted into the middle. Mounted to those cylinder heads are a super-stubby exhaust manifold. Or in some cases, the exhaust manifold and the turbo housing are one and the same casting.
Putting the turbos as close to the exhaust valve as possible means they see all the engine’s heat. They can also use the pulses of exhaust as each piston forces out the spent fuel and air to help drive them. The turbos spool up much faster. That means more boost more quickly. Exactly what your right foot wants.
There are packaging benefits, of course. No turbo stuck on the outside, and no exhaust manifolds to deal with means a narrower total engine package. You can now put it in more vehicles with less work. And fit it into smaller vehicles, like the AMG GT.
That’s not to say that there’s nothing of the engine outside of the V. That’s where you put the intake. On most V engines, the intake sits inside the V. that means that the intake manifold is warmed by the heat of the engine. That’s not good for power. On a hot V engine, the intake manifold is on the outside of the V. One intake for each bank of cylinders.
On a conventional turbo engine, the compressed air usually travels from the back of the engine, where the turbos are, all the way back to the front. Then it passes through the intercooler, before finally meeting the intake manifold. That leads to more issues with turbo lag, as the pressurized air has to go a long way before reaching the cylinders. With the Hot V, the pressurized air can come out the front of the engine, make a 180° turn, and immediately enter the intakes. That in front of the engine path is shorter, which means less lag. It also makes intercooler routing easier. Most Hot V engines use a pair of intercoolers for each bank. An air to water intercooler to cool the intake charge mounted next to the engine, and a second cooler mounted in the bumper to cool that water back down to repeat the cycle. Without losing boost pressure or increasing lag.
So moving the turbos to the V reduces lag and makes the engine more efficient. Both in power output and packaging. So it’s no wonder that the idea is spreading to more and more manufacturers. And now you know how it works.