I was thinking of selling my warrior and getting a 400ex what mods would be required in order to beat my friends stock z-400

 

A better rider.

The z is not that much faster. The only noticable difference in my experience is in the top end.

 

yea thats pretty much it, people say the 400ex is slow but dirt wheels did a test between a prdator, yfz450, z400 and 400ex and lap times on a 400ex were only a second or or so slower than a z400, dont believe everything you hear about a 400ex, the d@mn good quads

 

416 kit and you could easily run with or out run a stock z 400.. If you have enough power to pull the 16t sprocket. Its all over...

 

I can beat a stock z easily in 1st gear, I had a bad start and still beat him. If I started out in second I would have beat him by even more, and the night after I beat him, I adjusted the discs on my exhaust so I could have beat it by more. The rider was fair skill level.

 

just a better rider

 

Its more than a rider difference. They are even right off the bat, but the z pulls alot harder on top and it revs alot higher.
I would say you need to get into the motor to be faster, not much , but no pipe is going to do it.
I assume you are talking a drag race.

 

A piped and jetted (open air box) 400ex will beat a stock z400.

 

This rider I raced ,for straight line racing, was pretty much same skill as me. I was full out and he was full out, and he wasn't catching me. I am not lying to you. Only mods I have are what you see below, no ITP wheels yet.