A lot of useful topics brought up here, but let's discuss vacuum performance of the dual 6.6" vac motors used in our new machine.
Lift of these two vacs in series is 215" water or OVER 15.5" hg. CFM is 140 CFM max. This is the most powerful we have seen in a portable of this size. Portable machine design requires more LIFT for longer hose runs. Each section of hose reduces the lift at the wand. Whichever portable can generate the most LIFT at the wand, will also generate the highest CFM at the carpet- wand interface.
This graph shows our testing results with dual 8.4" Ametek vacs we did a year ago. The 6.6" Ametek vacs are very close in performance to the 8.4" results.
The Dual 3-st vacs on this graph were in parallel.
Quad vac was 4 2-st. in series/parallel.
Triple vac was 2 3-st. in parallel with high CFM vac in series.
Dual 8.4" series was 155% of Dual 3-stage in parallel.
Measured w/wand moving on the carpet. Dual 6.6" vacs are ~150% of Dual 3-stage vacs.