An 8V battery is a solution looking for a problem, mostly used by people who are too lazy or unskilled to maintain a 6V system properly. EVERY connection must be kept pristine clean and tight, and the battery must be fully charged at all times. Cables must be #0 or larger everywhere, hot and ground. With a lot of guidance (and some ridicule) by my father, I was able to keep a flathead V8 in a 47 Mercury running and starting well on 6V, and it had Offenhauser aluminum cylinder heads with 11.25:1 compression.
Jerry