What are some inexpensive mods that will add horsepower and speed to a ltz400. My friend is getting a yfz and I want to be able to keep up.

 

lid off and a 400ex boot, uncap the muffler, rejet to 145 could get ya 3 hp

 

Have you thought about changing sprockets for more more top speed? or aftermarket exhaust and changing the jets. A good air filter, I run a UNI filter witch is foam. I would not run a K&N filter because they let in alot of dirt. If you do exhaust and filter you may be able to run with out a air box lid just depends, you may not.