RE: OS 25 FX how to get more power
A quick look at the Novarossi page seems to show that the lower number plugs are hotter plugs. If you are running the engine stock, with low nitro and a muffler, it makes sense that a hotter plug will lead to more RPM. However, on a mousse can or mini-pipe, or if you've run up the nitro, you might need a colder plug to avoid pre-ignition.