An Overview of wet. Dry And Direct Port Systems

These are three basic types of nitrous systems: dry, wet, and direct port. The most misunderstood is the "dry" type of system. A "dry" nitrous system simply means that the fuel required to make additional power with nitrous will be introduced through the fuel injectors (remember, fuel makes power, nitrous simply lets you burn more of it). This keeps the upper intake dry of fuel. We accomplish this by two methods. First, is to increase the pressure to the injectors by applying nitrous pressure from the solenoid assembly when the system is activated. This causes an increase in fuel flow just like turning up the pressure on your garden hose from 1/2 to full. The second way we can add the required fuel is to increase the time the fuel injector stays on. This is accomplished by changing what the computer sees, basically tricking the computer into adding the required fuel. In either case, once the fuel has been added the nitrous can be introduced to burn the supplemental fuel and generate additional power.
The second type of nitrous kit is the wet style of kit. These kits include carburetor plate systems and add nitrous and fuel at the same time and place (normally 3-4" ahead of the throttle body for fuel injected applications or just under the carb as with plate systems). 
This type of system will make the upper intake wet with fuel. These systems are best used with intakes designed for wet flow and turbo/supercharged applications.The reason for this is the fact that fuel flows differently than air or nitrous. This difference in flow characteristics can lead to distribution problems and, in some cases, intake backfires. Intakes designed for wet flow (such as with carburetors) cause much less separation of the nitrous/air, and fuel. Because modern fuel injection intakes are designed to flow air only, they have tighter turns and a more compact design as a result. Thus, they generally do not make good candidates for wet flow nitrous systems.
The last type of system is the direct port system. Just as it's name implies, it introduces the nitrous and fuel directly into each intake port on an engine. These systems will normally add the nitrous and fuel together through a nozzle known as a Fogger nozzle. The fogger nozzle mixes and meters the nitrous and fuel delivered to each cylinder. This is the most powerful and one of the most accurate type of systems. This is due to the placement of the nozzle in each runner, as well as the ability to use more and higher capacity solenoid valves.
A direct port system will have a distribution block and solenoid assembly which delivers the nitrous and fuel to the nozzles by way of connecting tubes. Because each cylinder has a specific nozzle and jetting (both nitrous and fuel), it is possible to control the nitrous/fuel ratio for one cylinder without changing that of the other cylinders. These systems are also one of the more complicated systems when installation is considered, as the intake must be drilled, tapped, and the "plumbing" made to clear any existing obstructions. Because of this and the high output of these systems, they are most often used on racing vehicles built for the strain of such high horsepower levels.