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Hi Eric. I ran a set of #12's for about 4 years after I took the #670's off of my stock '67 400. The 12's (RAIII) are excellent heads, make a lot of power, and flow very well. They also want about 100 octane fuel. On my engine, I had severe detonation at light load conditions. Nothing helped: (Timing retarded/water injection/104 octane boost, etc) I ended up putting lower compression heads on the engine. The hot ticket these days is to install Ross dished pistons. That way, yu can run the "good" Pontiac heads on 91 pump gas. Next time I overhaul one of mine, that's what I'm gonna do! Weird: the 670's seemed to do a little better on my stock 400 than the 12's, but detonated much worse under heavy load. The 12's only seemed to detonate at LIGHT load. These days, with 91-93 octane, yu need about 85-95 cc chambers if you are running iron heads. The rule of thumb is 150-175 psi cranking compression. Good luck!
Jeff
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