Piston Choice on 318

8.5:1 is stock CR for most 318s. I dont see a flat top piston with 2 valve reliefs being that CR unless those are for a magnum application, which if I am not mistaken is what you need for magnum heads is 2 reliefs. LA style heads should be one relief with a flat top piston if you are approaching 9:1 or just above that mark.