I read on the net (can't remember where) where some dude is claiming his all stock 400EX with a Big Gun exhaust, re-jet and K&N is making 39hp! I can't find anyone else that says they make this much hp with so few mods. Even these pipe companies that dyno these bikes don't claim that much hp. I found a web page that displays all sorts of dynographs for various bikes and they had a stroked 440EX with a Big Gun at 41 crank horsepower. This guy with 39 on a stock bike plus the above mentioned mods has got to be padding his numbers or making an uneducated guess.

 

There's no way you'll get anywhere close to 39 rear wheel Hp on a 400EX by simply adding a pipe - to get that kind of HP, some real mods will need to be made.