With my turbo 3.8 (Grand National) it made a little more power at 12 but timing was being taken out. At 11.5, hp was off only a few (3-4) ponies but no timing was being taken out. We left it at 11.5. If you run 12.5 on one of those with over 20 psi boost, you will be replacing headgaskets. This was 93 pump gas on a cool fall day.