John and Ron--
I do not believe the Genturi generates any back pressure as it has an opening at the base of the vertical stack, which creates a chimney effect pulling the fumes up.
I have seen it tested, where you take a very small piece of paper (1/4" square), with the generator running, and let go of it next to the opening at the base of the Genturi. It will get sucked up the stack and come back down floating to earth as if it were shot out of a cannon.
I have one, but have yet to use it.
'07 Winnebago Journey 34H - CAT C7, Koni's, MCU's, SS Bell Crank, Safe-T-Plus
'07 HHR Toad, SMI AFO, Blue OX