Maxxis Razr front and rear (for best traction), on .190 wheels. (for strength) and a widend/strengthend axle.

 

???

Some strange advice for an XC Racer.

I assume you're riding a Z400 from your icon.

You will want to be stock width - depending on the XC you'll be racing, maybe a +1 setup on the front at max - no wider, unless the trails you'll be racing in are very wide. Stock width in the rear will work well.

Not sure about the strength of the LTZ axle, but the 400EX's stock axle works great.

Leave your stock wheels on the quad unless you replace them with stronger units like race rings or beadlocks. .190 wheels are not stronger than stock. If you change the front wheels, make sure not to run any wider offset than stock - wider than stock offsets will make you much more likely to bend tie rods in the event you hit a tree. They will also produce more feedback to the handlebars which will fatigue you over the course of a race.

Depending on the level you'll be racing at, the stock engine will likely be fine. In XC fuel economy is important. If you can get by without refueling in a 2hr race you will be at an advantage over the guys that have to refuel.

Focus on handling before you mess with the engine. Shocks, steering damper, etc... RAZRs are good tires - I personally like the RAZR2's better. Holeshots provide a lot more sidebite, but will not slide well. Which is better? Depends on your riding style.

Also focus on making the machine reliable. You can't win a race you don't finish. On a Z400, that means frame gussetting. There are numerous areas that need to be strengthened on both the frame and subframe.

Make sure the machine is comfortable to ride. Handlebar bend, grips, etc. If you plan on running "bark busters" look into the Cycra Pro-Bends - these will allow your wrists more freedom to move around over other designs.

 

tbr being from Calif., might be thinking of desert XC. Not eastern trail riding where you are squezing between trees.