Dynojet Q421 Jet Kit for YFZ450 04-05 Fits 04-05 YFZ450 Stage 1 and 2 Stage 1 kit are intended for motorcycles with a stock engine using the stock airbox, air filter and stock pipe Stage 2 kit are Intended for motorcycles with a stock or mildly tuned engine using a well designed aftermarket pipe with a modified airbox and a stock replacement air filter Stage 2 kit are designed for applications where individual filters cannot be installed and for applications where airbox modification improves the engine's performance, in most cases power increases of approximately 8 percent can be achieved