I understand the change in a fan curve relating to rpm but many impellers used in standard collection systems top out after a certain rpm and speeding it up doesn't gain much additional. In addition, a vfd can't really increase much over 60 hz efficiently due to the limit on voltage. I can see an increase in cfm with my radial fan that is rated up to 4400 rpm but there is a limit to how much the motor can speed up within the amp limits. I doubt the Smart is designed to run much over 60 hz, but rather is oversized so when at low pressure runs slower than 60 hz and under high pressure speeds back up. A curved blade impeller by nature won't pull cfm at 23"sp unless sized over 18" in diameter. I have seen pictures of the 5 hp regular smart that operates up to 15" and it is curved so the extra couple of inches must come from the housing design or perhaps a few extra hz. I wonder if the blade design for the high vac models differ and how as most high pressure impellers I've seen are belt driven and radial so as to operate well over 3600 rpm. Dave