text: Nitrous is an often used abbreviation for the chemical compound nitrous oxide (N2O) also referred to as NOS. The term NOS is derived from the abbreviation of the company name Nitrous Oxide Systems (NOS), one of the pioneering companies in the development of nitrous oxide injection systems for automotive performance use. More recently, the term was used prominently in the film "The Fast and the Furious", and not surprisingly, shortly after the film's release, various automotive newsgroups and forums featured unprecedented numbers of inquiries about "NOS" and how to install and use it. To a biochemist, however, NOS refers to nitric oxide synthase.
  There are two main categories of nitrous systems: "dry" & "wet". A nitrous system is primarily concerned with introducing fuel and nitrous into the engine's cylinders, and combining them for more efficient combustion. There are 4 main sub types of "wet" system: "single point", "direct port", "plate", and "plenum bar" all of which are just slightly different methods of discharging nitrous into the plenums of the intake manifold.
  In a "dry" nitrous system, extra fuel required is introduced through the fuel injectors, keeping the manifold dry of fuel. This property is what gives the dry system its name. Fuel flow can be increased either by increasing the pressure in the fuel injection system, or by modifying the vehicle's computer to increase the time the fuel injectors remain open during the engine cycle. This is typically done by spraying nitrous past the mass airflow sensor (MAF), which then sends a signal to the vehicle's computer telling it that it sees colder denser air, and that more fuel is needed. This is typically not an exact method of adding fuel. Once additional fuel has been introduced, it can burn with the extra oxygen provided by the nitrous, providing additional power.
  A "wet single-point" nitrous system introduces the fuel and nitrous together, causing the upper intake to become wet with fuel. In carbureted applications, this is typically accomplished with a spraybar plate mounted between the carburetor base and the intake manifold, while cars fitted with electronic fuel injection often use a plate mounted between the manifold and the base of the throttle body, or a single nozzle mounted in the intake tract. However, most makes of nitrous systems combined with unsuitable intake designs, often result in distribution problems and/or intake backfires. Dry-flow intakes are designed to contain only air, which will travel through smaller pipes and tighter turns with less pressure, whereas wet-flow intakes are designed to contain a mixture of fuel and air. Wet nitrous systems tend to produce more power than "dry" systems, but in some cases can be more expensive and difficult to install.
  A "wet direct port" nitrous system introduces nitrous and fuel directly into each intake port on the engine. These systems are also known as direct port nitrous systems. Normally, these systems combine nitrous and fuel through several nozzles similar in design to a "wet single-point" nozzle, which mixes and meters the nitrous and fuel delivered to each cylinder individually, allowing each cylinder's nitrous/fuel ratio to be adjusted without affecting the other cylinders. Note that there are still several ways to introduce nitrous through a direct port system. There are several different types of nozzles and placements ranging from fogger nozzles that requires one to drill and tap the manifold, to specialty direct port E.F.I. nozzles that fit into the fuel injector ports along with the fuel injectors.
  A multi-point system is the most powerful type of nitrous system, due to the placement of the nozzle in each runner, as well as the ability to use more and higher capacity solenoid valves. Wet multi-point kits can go as high as 3,000 horsepower (2,400 kW) with only one stage, but most produce less than half that amount with two, three or even four stages.Fact|date=October 2007 These systems are also the most complex and expensive systems, requiring significant modification to the engine, including adding distribution blocks and solenoid assemblies, as well as drilling, tapping and constructing plumbing for each cylinder runner. These systems are most often used on racing vehicles specially built to take the strain of such high power levels. Many high-horsepower race applications will use more than one nozzle per cylinder, plumbed in "stages" to allow greater control of how much power is delivered with each stage. A two-stage system will actually allow three different levels of additional horsepower; for example, a small first stage can be used in first gear to prevent excessive wheelspin, then turned off in favor of a larger second stage once the car is moving. In top gear, both stages can be activated at the same time for maximum horsepower. A more recent improvement on the "staged" concept from WON is the "progressive" delivery system, which allows a simpler single stage system to act even better than multiple stages, delivering a smoothly progressive increase in power which is adjustable to suit the user requirements.
  Another type of system is called a "plenum bar" system. These are spraybars that are installed inside of the plenums of the intake manifold. Plenum bar systems are usually used in conjunction with direct port systems in multi-stage nitrous systems.
  It is possible to combine the use of nitrous with a gaseous fuel such as propane or compressed natural gas. This has the advantage of being a "dry" system and yet still maintaining proper air/fuel mixture. Such a system requires exact choice of jet sizes and gas pressure regulation to provide a consistent pressure to the jets. Other advantages include better air/fuel mixing and distribution and less risk of knocking due to the increased octane of propane and CNG.
  Some mechanism to disable the nitrous system when knock is detected by a knock sensor would be beneficial.
  Nitrous oxide injection systems for automobiles are usually not emissions legal. For example, in New South Wales, the Roads and Traffic Authority Code of Practice for Light Vehicle Modifications (in use since 1994) states in clause 3.1.5.7.3 that "The use or fitment of nitrous oxide injection systems is not permitted." [cite book
  title = Code of Practice for Light Vehicle Modifications
  publisher = Roads and Traffic Authority of New South Wales
  isbn = 0731029232 ]
  There have been reported instances of police officers arresting drivers of vehicles equipped with N2O injection systems on the grounds that he or she intends to inhale it,Fact|date=October 2007 although such auto-grade N2O is often mixed with about 100 ppm sulfur dioxide, making inhalation noxious or even fatal.
  * [http://auto.howstuffworks.com/question259.htm Howstuffworks "How does nitrous oxide help an engine perform better?"]