Nitrous Oxide
It's been a very long while since I've done one of these posts, so I apologize in advance for the lapse. If any others want to take up the mantle and help pick up the slack, please do.
This is going to be another one of those posts that's largely academic, since few of us will ever go this route. It's fun to discuss and the knowledge is useful, though, so I'll proceed.
The chemical formula for Nitrous Oxide is N2O, but it also often goes by the name, "NOS", which is the brand name of Nitrous Oxide Systems, one of the more popular kit offerings and parts suppliers. N2O means two atoms of Nitrogen and one atom of Oxygen. This is significant for several reasons. Primarily, the limiting factor of producing power in most engines comes down to how much Oxygen you can stuff into the cylinder(s). You need between 12 and 14 times as much Oxygen (by weight) as you do gasoline for combustion, so adding fuel is rarely a problem. Air generally found in our atmosphere is constituted of a lot of different gases, of which Oxygen represents about 1/5. In N2O, the Oxygen represents a little over a third. What's interesting is how this Oxygen is freed up. On its own, N2O is a non-flammable, relatively inert, gas. When heated to around 570°F, is begins to break down. The Nitrogen and Oxygen separate and we now have accessible Oxygen which can be put to use in our combustion. Finally, the way the N2O is stored becomes quite a bonus as well. Compressed N2O is stored in liquid form in gas tanks. When the N2O is delivered from the tanks, it decreases in pressure drastically. And because temperature and pressure are interrelated, a drop in pressure also means a drop in temperature. Releasing N2O into your intake tract drops temperatures by a significant portion. This, of course, leads to a denser intake charge, resulting in even more air making it into the cylinders.
Unfortunately, to which many a poor soul can attest, tweaking with the amount of Oxygen entering the cylinder without adjusting the fueling can lead to a bit of heart ache. Burned pistons and valves are not uncommon as well as even larger failures that stem from detonation and pre-ignition. With all of the extra Oxygen available, we need to add the fuel to match or we quickly develop a very lean running condition.
How that fuel is added is how each Nitrous Oxide system is generally described. N2O systems come, usually, in one of two flavors: Dry Shot or Wet Shot.
In all cases, the actual installation involves locating your N2O bottle somewhere on the bike and routing the lines to an electrically activated pneumatic solenoid. The solenoid then controls the flow of N2O gas into a set of injectors mounted into the intake tract. The start of the flow of the N2O usually triggered by a button near the hand controls. Many systems also include a safety that prevents the system from being used unless the bike is at wide open throttle. Use of the N2O at partial throttle positions usually ends up being fatal for the engine. To further combat any issues arising from the addition of more fuel and Oxygen, ignition timing is usually retarded a few degrees when the N2O is in use.
Metering of the N2O is usually handled by a pressure regulator as well as jets. Like the jets you find in carbs, these jets control the amount of N2O that is provided. And like carbs, the size of the jets are usually represented by a number (which may or may not be arbitrary). The combination of the pressure of the N2O system as well as the jet size is usually summarized with a rough estimate of the horsepower added to the engine when the system is in effect. 15 shot, 20 shot, 40 shot, and even 75 shot all refer to the estimated horsepower added. As with most things, more power usually means more cost, more planning, and more difficulty in engineering.
Dry Shot
A Dry Shot Nitrous Oxide System is so named because there is no auxillary addition of fueling to compensate for the addition of N2O. Dry shot relies solely on the vehicle's existing fuel delivery system to administer additional fueling. For carbbed engines, the N2O is added upstream of the carbs. The idea is that the denser charge passing through the carbs will pick up more fuel. While this is true, it's usually not enough. The same process is used in fuel injected engines, though usually with a greater effect.
A Dry Shot system is usually the easiest to install and maintain, but also the most prone to damaging the engine and its components. Also, Dry Shot usually results in less overall power than the other types. This is purely due to the failure to add enough fuel to fully utilize the extra Oxygen. Because of the lack of additional fueling, the amount of N2O that can be added is usually minimal. Things must remain matched.
Another common tactic is to intentionally richen the mixture by installing oversized main jets into the carbs. Since the main jets have the most effect at WOT and the use of N2O requires WOT, this throttle position is reserved for when the user wished to take advantage of the Nitrous system.
Because of the balance of risk involved in using a Dry Shot system versus the low gains, Dry Shot systems are not usually favored by those looking for real power or longevity, though they are much more common in modern fuel-injected engines where electronics can account for the gas addition without many other changes.
Wet Shot
A Wet Shot system is quite a bit different than a Dry Shot system in that all of the fuel necessary to utilize the additional Oxygen provided by the N2O is added at the same time, along with the N2O, usually with fuel injectors located in each intake port, after the fuel metering (so after the carbs or MAF/MAP of an EFI system).
To add this fuel, it's usually necessary to engineer a fairly complex system. If your bike didn't have a fuel pump before, it definitely needs one now. A fuel pressure regulator also becomes a necessity. Fuel injectors must be added to each port, after the carbs, so now the button press to release the N2O into the system now also actuates fuel injectors.
It doesn't sound like a lot extra, but all of these added parts on an already small machine usually requires quite a bit of forethought to pull off successfully. Furthermore, the electricity needed to run the fuel pump, solenoid(s), and injector just may not be available on some bikes.
Conclusion
Nitrous Oxide can be a game changer. Compared to most other performance options, it is generally very inexpensive to install, but it also costs money over the long term. Refilling N2O bottles can get spendy, especially for those of us addicted to torque and horsepower. A poorly executed system can also result in a lot of broken parts and it's not difficult to find videos on YouTube of installations that turn their operators into human fireballs. Well executed systems, however, can result in a lot of extra power for very little investment of time and money and usually do little to long term reliability of an engine.