A few years ago when I had my heads off with the old cam I was getting just a wee bit of detonation on the tops of a few of the Pistons. I brought a spark plug into the machine shop and it was way too hot and I'm pretty sure the AR 3934 s are what he sold me or that was the heat range and I changed brand but I don't think so. But I did now LOL... 220 lb of cranking compression, I may be dealing with a combustion chamber heat issue?
