RE: y8 or z6 head
It depends. If you plan on going turbo later then the z6 would be a better choice, but for now and N/A the y8 is better. The y8's combustion chamber is smaller and the result is about a half a point of compression and that half of a point will give you a little more performance right now. Also, the ports are a little different. The z6's are what's called "tumble" ports and give a little better top end. The y8 ports are "swirl" ports and are better down low; but that's getting picky.
Basically, go with what's easier or cheaper.
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