CCA per CI is what I was told in the past. The past being before all the added demand on electrical system with AC, stereos, pw/pl and so on. My 71 with a 318 got a 640 CCA one, forgot what brand. Was a 5 year battery, bought in late 70's, never needed another one as car was totaled in 89. I would say I got my money out of it. Then I had an 86 Olds Ciera with 4 cyl and extra electrical. Part store tried to sell me an $80 one, high CCA cause of electric demand. Told them what I wanted, and the place wouldn't warranty it. I told them big deal, just give me the battery I asked for. Saved $40. Thing lasted for years.