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Unfortunately my ca18 shat itself - compression in cylinders was 160, 160, 155, 120. drove worse than the a12!
I hope (for your sake) that it is just a cracked ring, or a head-gasket as lagwagon suggested, but I'd be willing to bet $$$ that you've spun the 3rd & or 4th big end bearing..... This is a common problem for CA18's.
I'm pretty sure the stock CA18DET has cast pistons, but unless you are looking for a 10 second car you should be o.k to stick with the cast ones.