What Makes a V Engine Hot or Cold?

The hot in "hot vee" refers to the exhaust system and where it's located. The cold side is the intake. Internal-combustion engine design has long experimented with the exact locations of the intake and exhaust ports, but today you'll find them on opposite sides of the engine. Air and fuel are drawn into the cold side, are combusted, and exhaust is pushed out the hot side. Think of a single cylinder, and it's easy to picture the two sides.

A V configuration engine joins two banks of cylinders at their bases, and each bank has its own hot and cold side. Traditionally, the cold sides are paired so they're on the inside of the V and the hot sides are on the outside. This has many packaging and cost benefits, allowing engineers to place a single intake manifold in the center of the V that can feed both banks of cylinders while exhaust manifolds on the outsides of the V can easily route down and around various other components.

Why Would You Want a Hot V?

While the traditional layout works great for naturally aspirated engines, it's not as optimal for turbocharged engines. This is true for several reasons, including the fact that turbochargers take up more space in an engine compartment than exhaust manifolds, making them harder to fit in the same area. Turbos are also more efficient the shorter the intake and exhaust paths are, getting more work out of the exhaust and providing more boost to the intake with less lag.

A hot vee engine addresses both of these issues by locating the turbochargers inside the V and reversing the intake and exhaust sides of the engine. Doing so places the bulky turbochargers out of the way and a pair of slim intake manifolds on the outsides of the V where they take up less space than traditional exhaust manifolds. This allows a twin-turbocharged engine to fit in a space designed for a more compact naturally aspirated engine.

Because there isn't much space inside the V, this configuration naturally pushes the turbochargers as close to the exhaust ports as possible. This is good for power, response, and efficiency. Turbochargers work by harnessing the heat and velocity of the exhaust to drive a turbine that compresses the intake air. Compressed air can then be combined with more fuel to create more power than is possible without compression. To get the most out of the exhaust, you want to minimize any loss of exhaust heat and velocity before it reaches the turbocharger, so closer is better. This means the turbocharger will be activated more quickly, reducing turbo lag and turning as much exhaust energy into compressed air as possible.