Turbos, Nitrous, And Superchargers

Over the past 30 years, V-twin engines have become larger and larger. Where 80 cubic inches (actually 82) was once the norm for a Big Twin, stock engine displacement first increased to 88ci and eventually to 96ci straight out of the dealer showroom. With the plethora of performance parts available, owners have found it easy to increase displacement to 95ci or 103ci and even larger. In fact, new Big Twins with the CVO option ship with a 110ci engine. In addition, 120ci and larger crate and hand-built engines are readily available. The theory behind increasing displacement is that in order to make more power with the internal combustion engine, you must stuff more air into the cylinders during each intake stroke. More air allows more fuel to be added. Increasing the engine's air and fuel supply produces greater combustion heat and pressure on the pistons, resulting in higher power production at the crankshaft.

Nevertheless, regardless of how large your engine's displacement is, there are diminishing returns on filling a cylinder because a normally aspirated engine relies on atmospheric pressure-14.7 pounds per square inch (psi) at sea level-along with vacuum created by piston suction to draw air and fuel into the cylinder. And the stock V-twin engine is not very efficient because it only fills each cylinder between 60 to 70 percent. With proper induction and exhaust modifications, volumetric efficiency in a normally aspirated engine can exceed 100 percent. Yet, with a power adder, volumetric efficiency can easily surpass 100 percent. And that means more power. Best of all, a power adder can be used to stuff more air and fuel into the cylinders without increasing engine displacement.

There are three types of power adders commonly used on street engines: superchargers, turbochargers and nitrous oxide. All three methods have plusses and minuses. We'll take a brief look at each so you can determine if one is the right option to take your engine to the next level of performance without increasing displacement.

Superchargers
A "supercharger" or "blower" is a compressor driven by gears or belts. In the case of the V-twin, the compressor is mechanically driven off the crankshaft. A supercharger creates pressures and is used as a forced-induction device to increase cylinder fill. By design, a supercharger pressurizes the entire induction tract above normal atmospheric pressure of 14.7 psi. By increasing the pressure differential between the manifold and cylinder, a greater mass of air (often called charged air) is allowed to flow into the cylinder than would by atmospheric pressure alone. Superchargers develop pressure (also called boost) relative to engine rpm because they are connected to the engine's crankshaft. Since a supercharger is crankshaft driven, boost starts instantly right off idle and builds as rpm increases. This is a key trait and benefit of a supercharger.

Supercharger boost is defined as pressure above 14.7 psi or one atmosphere, which is equivalent to normal atmospheric pressure at sea level. For example, 8 pounds of boost pressurizes the engine's induction system 8 psi above the normal atmospheric pressure. Therefore, the engine would essentially see 14.7 psi plus 8 psi for a total of 22.7 psi, or the equivalent of approximately 1-1/2 atmospheres of pressure. Increasing boost to approximately 15 psi, the equivalent of two atmospheres, is sufficient to effectively double an engine's displacement along with a corresponding horsepower increase. One thing to remember, however, is that it takes horsepower to rotate a supercharger, so there is a loss of some horsepower due to the mechanical and pumping processes for instantaneous response and boost.

Types Of SuperchargersCurrently, there are two supercharger designs for the V-twin engine: positive-displacement (roots) and centrifugal. A positive-displacement or roots-style supercharger (also called a rotor blower) draws air into cavities formed between spinning rotors. With each complete rotation of the rotor elements, a fixed volume of air is pumped from the inlet side to the exhaust side of the blower housing. The continuously shifting voids displaced air and fuel trapped in the recesses between the rotors and supercharger housing, forcing the air/fuel mixture into the engine's intake manifold. Air/fuel is compressed when the shifting voids are subjected to the air already occupying space in the intake manifold. With this design, air compression takes place in the manifold, not the supercharger housing. Roots-style superchargers build instantaneous boost pressure at low rpm and maintain it as rpm increases, but efficiency reduces at high rpm due to increased heat inside the blower housing and leakage past the rotor seals.

Centrifugal superchargers were originally designed for high-altitude piston-driven aircraft engines and are similar in design to a turbocharger in that they use a fan-like impeller rotating at high speed to develop boost. But unlike an exhaust-driven turbocharger, a centrifugal supercharger is mechanically driven by belts or gears. A centrifugal supercharger uses its rotating impeller to apply radial force to the air in-between the impeller blades and housing. As the air flows away from the impeller, it compresses in the housing and flows into the intake tract. Compared to roots-style blowers, centrifugal superchargers can be turned at higher rpm for high boost and are more efficient, reducing the temperature rise of the intake charge and parasitic power losses. A centrifugal supercharger also builds boost differently than a roots-style blower. Where a roots-style supercharger builds boost early and sustains it as rpm increases, a centrifugal supercharger builds boost exponentially. This means that doubling the supercharger's rpm results in the boost quadrupling.

TurbochargersThe internal-combustion engine is a very inefficient device because only about one third of the energy released during combustion actually powers the crankshaft. The remaining two-thirds energy either is absorbed by the cooling system or exits out the exhaust in the form of heat. A turbocharger harnesses some of the lost energy by using the engine's hot combustion gases as they exit the exhaust system to spin a compressor that forces a greater amount of air into the cylinders. As such, a turbocharger is the equivalent of an exhaust-driven supercharger. For these reasons, a turbocharger is sometimes referred to as a method for obtaining almost "free horsepower." We say "almost" free horsepower because a turbo suffers discrete pumping losses due to increased exhaust backpressure, but the loss is typically less than the mechanical loss incurred by a supercharger. A turbocharger system includes several components. The major component is the turbocharger, while air ducting, exhaust system, fuel pump, control regulator and waste gate are supporting components.

For proper performance, a turbocharger unit must be matched to an engine's displacement and application. A turbo unit includes three major subassemblies: exhaust-turbine housing, bearing housing and compressor housing. Both the exhaust and compressor housing contain an impeller with integral blades. The impellers are connected by a shaft supported by bearings. The turbo system directs hot exhaust gases from the engine's exhaust system directly through the turbo's exhaust turbine housing. The high-velocity gases cause the exhaust impeller to spin, which turns the compressor-side impeller. As the compressor wheel spins, air is drawn into the compressor housing. Then, centrifugal force guides the air out of the turbo housing and into ducting leading to the carburetor (or EFI throttle body) and intake manifold under pressure. This results in the engine's intake tract, from the exit side of the turbo unit to the cylinder, being pressurized. As engine rpm increases, turbo boost also increases. And more exhaust gas is created as boost increases because a greater amount of air and fuel are forced into the cylinders. This cycle feeds on itself because the increased exhaust gases spin the turbine wheel faster, spinning the compressor wheel faster, which, in turn, stuffs even more air/fuel mixture into the cylinders.