The ‘Roos cannot hear the venturii..... It’s been widely broadcast to them that YOU David, ...are flying the airplane and they KNOW to get the hell out of the way!
(Seriously... are you CERTAIN they are actually “bronze” and not simply anodized...(or alodined)... in that color?
On further note.... the 4-inch venturi and the 9-inch venturi are referring to their capability to develop 4” and 9” of Hg/vacuum.... not their actual length. (The 4” venturi is 10-1/2” in length for example.)
It was Cessna’s intent to install the 4” venturi in a VFR airplane. The IFR kit (which utilized AN horizon and AN gyro-compass) used dual 4” venturiis. Keep in mind that until the 1950s an IFR equipped airplane only required a Turn and Bank as the “primary” gyro. The AH and DG were considered “high tech”... previously only needed to drive an autopilot such as in the B-17 and B-24 aircraft.
The early AN gyros require less vacuum (about 3.5”) than later 3-1/8” (so-called “pictorial” gyros) which require 4.5” vacuum.