My friend and I both have '99 YZ400's. Both bikes are in about the same condition. His bike is all stock except for a White brothers E-series pipe on it, just the end section, not the header. My bike is all stock except for a Promoto end cap (with the spark arrestor screen installed) and a WR flywheel and stator. When comparing the two bikes power characteristics, his bike is alot snappier off the bottom, and feels like it has more power. Could that WB pipe be giving him that power advantage? As a side note, that WR flywheel is about 10-12 ounces heavier than stock, although I didn't notice really any power loss after installation, just a smoother powerband.