I really can't imagine preignition with aluminum heads and 9.2:1 compression. Check your cranking compression. It should be between 165 and 180 to insure no preignition. Food for thought, My engine is 11.2:1, Eddy heads. Cranking compression is 175 on all cylinders... No preignition... timing is all out by 1800 rpm (35*) with 28* of mechanical built into the distributor and no vacuum advance. Again, no preignition and I run pump gas 93 octane.