My 6v starter spins as nicely as a 12v starter (same engine and correct 6v battery cables). No problems getting similar spins on a 6v. You can get good spins on a six volt if all parts and connections are in good shape. And, as Cletis stated, an 8v battery is not the best way to get a 6v system to spin like a 12v system (you'd probably need to deal with the voltage regulator and might wear out light bulbs more quickly).

The 8v battery "fix" was (is?) popular on old 6v systems, especially farm tractors. My bet is that a majority of those set-ups had (have) replacement battery cables for 12v systems.

HAVE A NICE DAY