Some of the things stated in this thread are confusing to me...Nitrous makes power in three different ways. Air box fogging utilizes 2 of these methods, and spray bars utilize all three methods.
Oxygen Concentration: Simply stated - Air is 20.9% O2 and Nitrous is 33%. The more nitrous you spray, the closer your O2 concentration gets to 33%. You can't go higher than 33% concentration, and this is one of the reasons why air box fogging benefits start to max out at shot sizes over 75 - 90 HP or so. You can keep adding N2O, but you stop making any noticible power at this point because of the 33% concentration limit.
Liquid Phase Induction: This is how spray bars make power not limited to 33% concentration...If the nitrous is injected close enough to the back of the valve, a good portion of it is injected into the cylinder as liquid phase. It gets into the cylinder, changes phase into gas expanding wildly, and actually chemically breaks down into nitrogen and oxygen well before combustion. The point is that you can get an enourmous amount of liquid nitrous into the cylinder before it changes phase. This is the principle difference and advantage that spray bars have over box foggers. The more liquid phase N2O you can stuff under the valve, the more power you can make. In theory a good spray bar is only limited by hydro-lock (not likely!).
Phase-Change Cooling is the third method: Since natural gas law states that density has to go down linearly with absolute temperature, the colder you get your intake, the more moles of O2 you can suck in. Although Gixx points out that the temp at the tip of a spray bar is much colder, the physics are such that the same amount of phase change cooling occurs in spray bars and box foggers (for the same jet size). The only difference is that the farther you get away from the injector, the more heat is pulled out of the surrounding air.
Another effect that is kind of a combination of item 2 and 3 is that the gasoline injected from a fuel injector is less likely to vaporize and expand prior to passing into the cylinder due to the low temperatures it sees from N2O. The trick with fuel induction is to get good atomization without gas phase vaporization. You want small drops that are still liquid phase and not taking up volume.
Maybe this confused things. Maybe not. My $.02.