What is a 'hot V' configuration? PH Explains
How relocating the turbochargers of a V-type engine can deliver a plethora of benefits
An engine with a 'hot V' is a V-type powerplant - such as a V6 or V8 - that has its turbocharger, or turbochargers, mounted between the cylinder heads. The flow of fresh charge and exhaust gas through the cylinder heads is also reversed in hot V engines, so that waste gases are dumped into manifolds sited between the heads.
Opting for this configuration offers a range of benefits, including an engine that's far more compact. Consequently, hot vee designs are often employed by those developing high-performance sports cars which benefit from low bonnet height and a low centre of gravity. Some manufacturers, such as Mercedes-Benz, refer to this particular engine configuration as a 'hot inside V'.
How is a hot vee configured?
In a conventional twin-turbo vee engine, the turbochargers would usually be positioned outside of the cylinder heads. Fresh charge would flow through the intake manifold at the top of the engine, through the intake ports and into the cylinders. Waste gases would then flow out of the exhaust ports on the outside faces of the cylinder heads - then on into the exhaust manifolds and turbochargers.
A hot V set-up is the other way around; fresh charge is fed into the cylinder heads through separate intake manifolds bolted to what would conventionally be the exhaust ports, then exhaust gases are delivered into manifolds in the middle of the vee. These gases are then fed into the engine's turbo - or turbochargers - before being routed down the back of the engine and into the rest of the exhaust system.
What are the benefits of a 'hot vee' configuration?
There are several key advantages to utilising a hot V, the most prominent of which is packaging. In a conventional turbocharged vee engine, the placement of the turbochargers outside of the cylinder heads results in wide powerplant. Consequently, the increased width of the overall engine assembly makes it harder for manufacturers to position the engine low in the chassis. This has a negative impact on the car's centre of gravity and, depending on other design elements, may result in the manufacturer having to move the engine forwards or backwards to avoid clashing with other assemblies. This, again, may compromise the car's weight distribution.
Mounting the turbochargers between the banks of the engine results in a far narrower, more compact arrangement. Although it raises the centre of gravity of the engine itself, it gives the manufacturer the ability to site the engine in an overall far more advantageous position. This is of key consideration for those developing high-performance sports cars or supercars.
The plumbing required for a hot V can also be far simpler and shorter than a conventional set-up, particularly if water-to-air intercoolers are used. This can improve the engine's efficiency and quicken its responses; the exhaust manifolds for a hot V are typically very compact, for one thing, resulting in a shorter path from the exhaust port to the turbocharger's turbine. This means less exhaust energy is lost before the gases hit the turbine, helping the turbocharger spool more quickly and reducing lag.
Using a hot V is also thermally advantageous. In a conventional turbocharged vee engine, the temperature of the intake manifold is increased by heat rising from the engine below. This raises the temperature the incoming charge, which can reduce performance and - in the worst case - lead to damage through detonation and pre-ignition. In a hot V, the intake manifolds are positioned in far cooler areas. This helps keep reduce any unwanted temperature increase, aiding engine performance, while the 'hot' side of the engine is concentrated on the top.
A brief history of the hot V in production applications
BMW was the first manufacturer to deliver a production petrol engine that featured a hot vee. Its twin-turbocharged N63 V8, which arrived in 2008, packed a pair of Garrett MGT2256S turbochargers between its heads, which aided it in producing 408hp and 443lb ft. BMW cited several benefits, including improved performance and a more compact package, at the time of launch.
Many diesels have also benefitted from the hot V configuration, such as Ford's 6.7-litre 'Scorpion' Power Stroke diesel V8 - which arrived in production form in 2010.
Some older engines, such as Pontiac's turbocharged 301 V8 from 1980, feature turbochargers mounted atop the engine. These are not 'reverse flow' configurations, however, as fresh charge still flows into the cylinder head from the top and the exhaust gases out of the bottom. Instead, exhaust gases are collected from conventional manifolds then simply piped up to the turbocharger at the top.
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