Found most recently in the brand new Porsche Panamera, a hot V engine has exhaust ports pointing inwards; the opposite direction to a conventional setup. The ports point towards the centre line of the engine block with the turbochargers placed neatly between the two banks of cylinders. This means that the turbos are much closer to where the combustion in the engine is occurring, rather than being bolted to the outer sides of the engine block. So why use this setup over a convention layout? Let us take you through it:
Turbochargers are fed by exhaust gases from the engine and therefore they rely on the velocity of the gasses to spool up properly. As exhaust gas temperature reduces, the velocity reduces due to a lack of pressure, which reduces the rate of the turbocharger spooling up. So, as a rule of thumb, you want to keep your turbocharger in a warm area of the engine bay. Most components on a car are normally aided by cooling, so a turbocharger should be placed in a position where other nearby components are not affected by heat soak. Additional piping is normally used to achieve this, but the hot V setup eradicates that need.
The shorter distance to the turbos means that there is less tubing for heat to be lost from the exhaust gases. The catalytic converters (which are normally found underneath the car) are positioned also within the V as they also work best when hot. The high temperature between the banks of pistons allows the turbochargers and cats to increase in efficiency in comparison to a more conventional outboard system.
In general, the smaller an engine can be in sheer size while keeping power output high, the better. With manufacturers trying to cut costs, companies like BMW tend to design one basic engine block that is used in many of its models with slight changes to differentiate the model variations. The tight package created by a hot V setup allows the powertrain to be easily transplanted from one chassis type to another, with this versatility making an engine convenient for spreading across a company’s model range.
A more compact engine setup also makes the control of the engine within the engine bay much easier. Instead of turbochargers and other ancillary components hanging out the side of the engine block, the hot V locks the majority of the engine’s weight together in a tight package making its oscillation much more predictable.
With the turbos sitting much closer to the exhaust ports, the control of the turbocharger can be much more acute. Engines orientated in a V-formation have a firing order that can make controlling turbocharging difficult as the impeller spins in an irregular fashion.
To eradicate this fault in a standard turbocharged system, a large amount of excess piping has to be put in place to keep the change in rotational speed of the impeller more predictable. With the balance between the engine and the turbochargers able to be better controlled due to their close proximity in a hot V engine, throttle response is much sharper making the car itself much easier to control.
The hot V setup has grown in popularity with manufacturers recently, with AMG producing the its 4.0-litre V8 for the AMG GT, C63 and soon E63 with hot twin-turbos, as well as Porsche introducing the concept to its new Panamera. Turbocharging has changed from the days of lumpy power deliveries like the Ferrari F40, with engines designed to barely feel turbocharged at all with almost perfect linearity. The implementation of hot V engines has furthered this journey down the path of ‘invisible’ turbocharging and it won’t be long until this automotive witchcraft spreads itself to the majority of forced-induction V engines.