I'm thinking about NOS when my rebuild is finished - but I can only use it at the drags down here.
You aren't even allowed to have it set up in your car on the street ( even if it's disconnected).
Many people have watched The Fast & The Furious too many times and have some misconceptions about nitrous
Facts are: When it's injected into the inlet manifold it -
1. Lowers the engine intake temperature providing a dense inlet charge.
2. Increases the oxygen content of the inlet charge.
3. Increases the rate at which combustion occurs.
Main things to watch when fitting NOS are to pay particular attention to : Fuel pressure delivery and Ignition timing. Usually the timing should be retarded 4 - 8 degrees - also watch the temperature of the NOS bottle. Also higher octane fuel would be beneficial and spark plugs may have to be a couple of ranges colder than normal.
For small NOS applications engine mods would be limited however if you wanted a huge increase in power using NOS : Ditch the cast pistons, use high quality head studs, upgrade the ignition system.
As for power gains it depends on your budget and application but anywhere from 10 - 80 HP increase or there abouts.
Hope some of this helps.......................