Re: Increase HP of 289 / Retain Stock Appearance 289
I think that "back in the day" the engine were more typically underrated, not overrated, for insurance purposes. However, the automotive industry did change the way engines were rated sometimes around the early seventies, I believe, to use gross ratings instead of net ratings, or something like that. Gross HP was higher than net, or brake HP, as I understand it, but people more knowledgeable can correct me and properly explain that.
My engine builder told me he used heads like my 351W has on a 289 he built for himself many years ago; the 69, 70 351W heads had bigger valves. Download the Cam Quest software from Comp Cams and try out different combinations and see how impacts the virtual dyno results.