M6 (act drive) should turn at 8000 -8100. An M8 7700-7900 rpm.
As a rule... the smaller the displacement is, higher rpm's are needed to make power... smaller motors are peaky with less tourque. Engines with a bigger bore make more twist at lower rpm... wider power band, easier to clutch etc....
So... yes your M6 should spin at a considerably higher rpm than the M8.