the issue you have is the fact that the 2.0 head motor had dished pistons vs the 88 2.6 with flat top pistons. the 2 together push the compression ratio over 10:1 without the shim. If you want to lower the compression ratio back down close to stock, there are companies that make thicker head gaskets to any thickness you want. Or you can run alcohol injection to fix the detonation issue. where are you setting the timing to, and is the advance working properly?