Turbos, Nitrous, And Superchargers
Power Adders For the Street
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.
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.
A phenomena associated with turbochargers is called “turbo lag.” Turbo’s are load-sensitive and require inertia to work, which means it is important that the turbine impellers are spinning fast enough to generate boost. When the engine’s throttle is abruptly opened, airflow in the intake tract is moving slowly and the turbo’s impellers will be spinning slowly and have little inertia. This condition results in a short delay before sufficient boost develops. The delay is known as turbo lag. Turbo lag can be minimized by optimizing the design of the turbo impellers and matching the size of the turbo unit relative to engine displacement.
“Spooling up” is another term often associated with turbos. When the engine’s throttle is quickly opened, exhaust gases require only a short time to gain momentum and accelerate the turbine wheel to a fast rpm. This is called spooling up. Smaller turbochargers generally spin up more quickly than larger ones and result in less turbo lag but flow less air for less top-end power. Minimizing turbo lag, especially at low rpm, is critical for enjoyable street riding. This is why a turbo unit must be matched to the engine’s displacement and its application (read: its most important rpm band) for optimum performance.
**IntercoolersAs turbochargers and superchargers compress air to increase boost pressure, the induction air charge is heated. Heated air is less dense than cool air and requires a higher level of fuel octane to avoid detonation. Detonation not only is power limiting but also can destroy vital engine components. One method engine builders use for reducing heat and increasing efficiency with superchargers and turbochargers is to install an intercooler.
An intercooler is a small radiator-type device placed somewhere in the intake tract between the blower/turbo and the engine. The intercooler is designed to cool the heated and compressed air exiting the blower or turbo before the air reaches the engine. Since colder air is denser and contains more oxygen, a richer air/fuel mixture can be used to make more power. Cooler air also reduces the potential for detonation, which is the equivalent to raising the octane level of the fuel.
**Nitrous OxideA third power adder option is nitrous oxide. “Juice” and “squeeze” are typically associated with nitrous. A nitrous oxide system (NOS) is not mechanically driven and does not pressurize the engine’s induction system (intake port, manifold, carb or EFI throttle body), as do superchargers and turbochargers. Essentially, nitrous oxide is power in a bottle. In other words, it is a chemical-based supercharger that increases pressure in the engine’s combustion chamber.
From a chemical standpoint, nitrous oxide is a non-toxic non-flammable clear-gas oxidizer, which is stored in a liquid state under pressure in a bottle. Releasing nitrous from a bottle instantly changes it into an oxygen-bearing gas. The oxygen-bearing gas can increase the percentage of oxygen in the cylinder to roughly 50 percent, which is more than double that of a naturally aspirated engine. More oxygen in the cylinder allows more fuel to be added. As mentioned earlier, increasing the air (oxygen) and fuel in the correct proportion produces greater combustion heat and pressure on the pistons, resulting in higher power production.
With the push of a button, a nitrous system injects nitrous oxide into the engine’s intake tract along with air and the appropriate amount of fuel to ensure proper combustion. As combustion takes place, the chemical bond between nitrogen and oxygen is broken and the oxygen becomes usable for combustion. The increased amount of oxygen in the cylinder allows fuel to be added, resulting in higher combustion heat and greater power production. In simple terms, whenever the rider decides to push the nitrous button, he can have increased cylinder pressure and greater power.
One major difference between nitrous oxide and blowers/turbos is that blowers and turbos pressurize the intake tract 100-percent of the time. In contrast, a nitrous system is active only when the rider pushes the button; otherwise, the engine operates as normally aspirated. Although a NOS can be activated at any rpm, activation is typically only after the engine has reached sufficient rpm to minimize potential engine-damaging detonation. For a street engine, that means about 4,000 rpm and up. On the other hand, a nitrous system is normally activated at a lower rpm on a modified race engine run on high-octane fuel.
**Wet Or DryThere are two basic categories of nitrous systems: wet and dry. Wet systems deliver both nitrous and fuel into the induction tract. Dry systems differ in that they only supply nitrous, which is sprayed into the induction tract. When extra fuel is needed by a dry system, it is added through the engine’s existing carburetor or fuel injectors. With EFI, this is usually done by modifying the ECM’s fuel curve. Dry systems are easy to install, but a return-style fuel system is required. For EFI, a wet system may take more work to install, but it also allows control of both the fuel and nitrous levels.
**Final ThoughtsAll three power adders we have discussed will provide a significant power increase over a normally aspirated engine. In mild form, any of these power adders will give your engine an added 30 to 60 ponies. Compression ratio, spark advance, cam timing and the exhaust system are critical for making a power adder system perform optimally. And, depending on the amount of boost or nitrous being used, more durable parts such as forged pistons and beefier cases along with other stronger engine components, may be required. When installing a power adder system, it is best to ask your supplier what engine design guidelines they recommend.
The bottom line is that everything is a balancing act. The simpler you keep things the safer you will be. Pay attention to details. Detonation is the leading danger to any boosted engine, so the amount of boost, static compression ratio, cam timing and ignition timing must be matched to the application. To be safe, start with a rich air/fuel mixture and somewhat retarded ignition timing. For the most part, power adder systems work well with cams having moderate lift, low duration and reduced overlap. Since a power adder engine stuffs the cylinders with increased amounts of air/fuel mixture, it also generates a higher volume of exhaust gas compared to a naturally aspirated brethren. Therefore, be sure to install a low-restriction exhaust system capable of flowing a sufficient volume for the engine’s displacement and rpm. Additionally, power adder systems typically require a low-pressure fuel pump for adequate fuel supply. Make sure your engine’s fuel system is up to the manufacturer’s recommendations. And remember, EFI systems will need ECM recalibration.
If you have already increased the displacement of your V-twin engine and are wondering how to take your engine to the next performance level, a power adder system may be your ticket to increased performance without cracking the cases.