I've not heard exactly what makes the k20a2 and the k20z (2005) different. the k20z is middle ground between the k20a and k20a2. I've been told its just a slightly more aggressive cam, but I don't see why that would call for a different engine code. if you can find a k20z, it'll be at a premium to the k20a2, so I'd stick with the a2 head.