"HT" stands for "Happy Time" - originally a derogatory slur comparing the engine to cheap, unreliable Chinese fireworks... It stuck as like those fireworks, they're far more reliable than you'd think and they're an absolute blast to play with lol
TBH, I don't think there
is an "exact designation for the generic 66cc Chinese 2 stroke" beyond the "in-frame 66cc Chinese 2 stroke" ...which is a bit cumbersome - thus the adoption of "HT" and "China Girl"... I could be wrong, if so I'd love ta hear what it's supposed to be
As for the "80cc" thing, well other than it's obvious, if ethically dubious marketing value - the story is that the "Chinese use different math/technique to measure displacement" ...which sounds good, even viable - except what bothers me about it is the 49cc doesn't seem subject to this same math... which leads me to believe in my original thought - that it's just advertising hype *shrug*