Ac or Dc motor

http://www.google.com/search?q=site:diyelectriccar.com+ac+dc+motor

Lots of answers to your question there. Probably more answers than you want...

AC seems to be the best choice from what everyone says, but it comes at a price premium.

AC motors inevitably involve losses in the electrical provision of their static field. Permanent magnets have no such downside. Therefore efficiencies can be higher - up to 93% Check out the Lynch motor - Agnimotors.com
http://www.agnimotors.com/home/index.php?option=com_frontpage&Itemid=47

Penman said:

AC motors inevitably involve losses in the electrical provision of their static field. Permanent magnets have no such downside. Therefore efficiencies can be higher - up to 93% Check out the Lynch motor - Agnimotors.com
http://www.agnimotors.com/home/index.php?option=com_frontpage&Itemid=47

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But you can also get AC motors with permanent magnets. The Oxford YASA motors, for example.
This allows for regen, which would be pretty important in a track car.

ragnar said:

Whitch is better if im building a track car.. ac or dc motor?

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The question is too open-ended. People always ask these types of questions, and invariably get a million (obvious exaggeration) different answers.

To even consider the question, you have to provide (at least) some basic information. What type of vehicle will you be racing, what type of venues, etc? If you're racing compact sedans with small four cylinder engines, maybe an AC motor would be great. If you're competing against large V8s, you might need the torque of a series DC to not get obliterated on the straights. I'm over-simplifying to make a point.