Actually, Jim Laroy and his son Cody found there are some differences. In fact, I've seen Jim say the ports on the 452s are a smidge better than the 906s. "I guess" if you're gettin down to the nitty gritty you would want the 452s, but for the vast majority, they only real difference is the "hardened" seats. I use quotation marks, because induction hardening is good for the original valve job.....sorta. lol