My heads (milled 4.8 truck heads) were advertised at like 11.1:1. On hot days in the summer, it'll knock. Of course the CA summer blend *might* have something to do with it. I've since gone to a 160 Tstat, which should help too, but I've traditionally run roughly a cup of Torco per 15 gal tank, and that gave the slight margin I needed. I worked out the calcs once and it was something like 92-3 octane when I was done. HOWEVER, I did and still do have some fairly gnarly back pressure, which raises combustion temps even further, so some good headers and that problem might be gone altogether on 91.
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