i can tell you on a 400ex your most likely going to want to, going from the stock exaust to a slip on will COMPLETELY change the way your bike breathes and the amount of backpressure your getting.
You pretty much just spent a ton of money on a preformance part to make your bike not preform by skipping this easy process.
* Spode Freestyler *
Team Spode Racing
Thank you to my sponsors: DWT, Motoworks, Lone Star Racing, Dirt Fiend Racing, Go Pro Cameras, Amsoil, Spy Optics, Motoconcepts, We All Ride Clothing, Leatt Braces, ODI Grips
2004 YFZ450