I use a dry kit and spray before the throttle body. Also I was told when you take out the timing, just to clear up, take it from whatever you have it presently set at. If you run 16 deg. int. timing, then you would subtract the 2 deg. per 50 hp shot from there. I find this works for me, but I also only use the n2o at the track. It is definitely a good idea to get the car on the dyno to play with the timing. Also make sure your car is in good working order. Fix any engine problems before you spray.

Also what kit do you use? I have a Zex kit and it has a control box that you run vaccum lines between fuel pressure regulator and the intake. This is what boosts the fuel pressure when the nitrous flows.

Hope this helps.