Turbos, Nitrous, And Superchargers

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.