The ability of a battery to supply current (Amps) is different from the current requirements of things like a starter or glow plugs. The starter and glow plugs will draw approximately the same current whether supplied by a 390 cold cranking amps battery, or an 800 one. The 800 version will probably supply it for a longer time before discharging, although CCA isn't a measure of battery capacity, Amp-hours is, which is somewhat related to the advertising term "reserve capacity".