I am a fan of venturiis and operate 90% VFR and occasional “light” IFR. (I use clearances to ascend to VFR and descend to VFR/non-prec. appchs.)
There have been plenty of discussions about the pros/cons of various vacuum systems. In my experience the venturi system SUCKS…. and since that’s what it’s supposed to do….that’s GOOD!
1- It is reliable. If you’re flying…it’s sucking. There are those who fear icing conditions. That’s also good. This airplane is prohibited from being flown in icing conditions. If you get into icing conditions…LEAVE it….and use your back-up-system like FAA and Cessna intended (the needle/ball/airspeed) to stay right-side-up until you get out of that condition you’ll swear will never happen to you again.
2-It has ALL THREE of my gyros up-and-running before I get 200-feet above the runway on take-off. ( I don’t do “zero-zero” takeoffs in single-engine airplanes. In fact, I quit doing zero-zero takeoffs in ANY airplane after the one I did from a private ranch-strip with no runway markings in a Hawker-jet and discovered that after 60 kts all the little pebbles in the runway-asphalt appeared as streaks coming at you and was of zero-use in determining if you are headed for the side of the runway or not. After I heard “V-One Rotate!” …. that was the last time I’d ever perform a zero-zero takeoff in the last 30 years of my professional career.)
Therefore, I don’t need my gyros to be running until after takeoff….and even tho’ I’ve departed with “clearance void” times on an IFR flight-plan with an 800’ overcast sky…I’ve got plenty of attitude information before entering the clouds. (And why anyone would fly single-engined airplane in serious, wide-spread, low-IFR conditions is a wonder to me. I’d like to be able to descend to VFR and see which pasture to select if more important things than venturiis fail.)
3-Venturiis require almost No Maintenance…other than a pre-flgiht inspection for condition, security, cleanliness …. and confirming in-flight that adequate vacuum exists. Some folks have experienced low-vacuum with venturiis….. I believe most are due to improper installation and/or improper INSTRUMENT-system maintenance. Vacuum system integrity, plumbing-condition, and filter changes are still necessary…. But you’ll never lose vacuum due to a failed vacuum-pump or suffer expensive gyro-instrument repair-bills when the carbon brushes in “ modern” pumps disintegrate between 300-500 hrs as designed. (The old Pesco “wet” pumps never did that…. so if you have a vacuum pump… I advise trying to find/keep the old-style “wet” pump system.)
Another reason some folks have had poor experiences with venturi system is odd installations. Cessna placed venturiis on the right side of the fuselage. I believe they did testing and found that the ideal location. (See the IPC) Some folks have them “split” with one on the left and one on the right….and it’s been reported that the one on the left does little-to-no-good. I even know of one FAA Inspector who owned a 170-B and his venturiis were mounted on the belly…aft of the cooling-air exit, behind the lower “lip”…. in some mistaken belief that the warm air exiting the engine comp’t was a good thing…. (apparently ignoring the fact that warm air is less-dense than cool air and would provide less vacuum…. but also placed where considerable contamination and degradation from oil-blow-by kept them filthy.
4- My original venturiis are available cheap, if you want them. The only reason I replaced them was because of my addition of a 3rd gyro…a modern “pictorial” A.H. which requires more vacuum than the old A.N. types and the fact that I’d be burdening the system with more gyros than intended with the standard system. I’m pleased with the result. (A.N. gyros require approx 3” of vacuum while 3-1/8” modern “pictorial” gyros need 4.5” - 5”. If you have that kind of vacuum and your instruments don’t work properly with a half-inch less than spec…get them repaired or overhauled!)
If you are the least bit worried about it….install the so-called “super” venturiis in the original IPC-described manner and be done with it. Keep in mind that venturiis are described by their capability. A 4” venturi is not 4” long…. it is designed to produce 4” of vacuum., etc. The small 2” venturi was intended to run an air-driven T&B (and sometimes used to evacuate a “relief-tube”). The so-called “super venturi”….is a 9” venturi and designed to produce 9” of vacuum. That description is a measurement taken from a closed system and does not allow for actual air-flow losses. My dual 9” “super” venturiis produce 5” at cruise…and 4.5” in the pattern…because I had to install a vacuum-regulator to bring the system down from 7” which would over-work my gyros. (see the article I posted in a link below)
As already stated, I have TWO art. horiz’s and a WW2 AN-D.G. Cessna installed back in the day…
The vac pump you presently have is an obsolete and no-longer supported mod/STC. It also req’d a change of cowling nose-bowl from original, but that’s not likely to cause any grief for you….leave that alone…Just wanted you to know.
The following posts/threads might prove useful for your project:
viewtopic.php?f=39&t=6800&p=147650&hili ... is#p147650
viewtopic.php?f=5&t=15377&p=143059&hili ... on#p143059
viewtopic.php?f=24&t=7955&hilit=2nd+horizon
Installing dual venturiis might find this post/thread helpful:
viewtopic.php?f=24&t=9901&p=88023&hilit ... zon#p88023
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