The International Cessna 170 Association

n2582d wrote:If approval wasn’t an issue, I’d try routing one side to the engine manifold, and the other side to your venturis. That would permit gyros to be spun up as you taxi out. A problem with this idea might be that tapping into the intake manifold would cause those three cylinders to run leaner, similar to having a vacuum leak there. A moot point as there is no approval for such a modification.

Actually I have an STCd system to do just that, the SVS system from SafeFlight. It has a manifold with check valves functionally similar to the one you picture except there is only one inlet from the instruments. One outlet goes to my dual venturis, and the other to the base of the 1-3-5 intake manifold through a manual shutoff valve. I had the manifold modified with a 3/8” NPT pipe boss, as the existing 1/8 primer boss is way too small for the required flow. The system is intended as a backup vacuum source, so there is also a required vacuum switch to drive a light warning of loss of the primary source.

The boss I had installed on the right intake manifold replaces the primer boss near where the manifold attaches to the sump, so it doesn’t seem to inordinately affect the cylinders on one side over the other. I do get about a 100 rpm rise at idle due to the additional airflow with the system on, but the engine maintains its normal smoothness. I leave the system on most of the time so I have he gyros powered on the ground. When the throttle opens on takeoff, the manifold vacuum goes away, but the venturis quickly take over, so im never really without vacuum. About the only time I shut it off is sometimes when I’m a little high on final to get the engine down to a true idle.

rnealon1 wrote: ↑Wed Oct 18, 2023 1:56 am Reviving a thread, I am pondering the venturi/vacuum system in my aircraft. I have dual venturis, one on each side of the fuselage. They run the AH and DG; turn coordinator is electric. There are 2 suction gauges mounted one above the other. I have not found any documentation of the installation, but did locate the installation instructions on the forum.

Both instruments are functional by the time I look at them after takeoff; the AH is erect by the time I lift off. In flight both gauges indicate about 5.2" hg.

I was curious if the 2 gauges correspond to the 2 venturis, and if so which is which. I experimented by reaching out the window to block the back of the left venturi while the engine was run up, but the suction decreased on both gauges.

Also curious if losing suction from one venturi would cause the other to take up the slack, but have not conducted an in flight experiment on this yet.

Any thoughts on this are welcome.

Bob

The thought I have on this is that the IPC shows both venturiis to be mounted on the right side of the fuselage. This configuration is also reported to provide more vaccum than one mounted on each side.

Dual vacuum gauges are a curious item. I wonder if they are tied to separate gyros on a whim someone had at some time. Completely unnecessary in the standard installation since both gyros are plumbed to the same vacuum source.

Gary,

I feel certain your manifold /check valve scenario is the explanation; I haven't been able to squeeze in far enough to find it yet. If it is there I am also certain it has not been inspected in a long time.

Miles, I am intrigued by your Safe Flight SVS, but have not been able to find any info on it.

My venturis are 10" long and provide 5.2" hg in cruise.

Thanks,

Bob

rnealon1 wrote: ↑Thu Nov 02, 2023 7:08 pm Gary,

I feel certain your manifold /check valve scenario is the explanation; I haven't been able to squeeze in far enough to find it yet. If it is there I am also certain it has not been inspected in a long time.

Miles, I am intrigued by your Safe Flight SVS, but have not been able to find any info on it.

My venturis are 10" long and provide 5.2" hg in cruise.

Thanks,

Bob

A big part of the problem is that I mis-remembered the manufacturer’s name. It’s Precise Flight, not SafeFlight. I found an installation manual on ACS’s website, but they don’t seem to offer them for sale any longer. I’m guessing the SVS is no longer made (not much demand for vacuum systems these days…), but here is one advertised as new surplus. There is an AD on them, but as I recall it’s just a test for a sticking shuttle valve. I use mine every time I fly, so valve sticking has never been an issue.

Also, venturis are rated at how much deadheaded vacuum (in inches of mercury) they will generate at a specified speed, not on any dimensional measurement. It sounds like you may also have a vacuum regulator in your system. I’m guessing you have 8” “super” venturis, so a regulator would be required to prevent overdriving your instruments. That’s the setup on my airplane.

Here’s a photo of the logbook entry when I had mine installed, showing the STC#s for both the airframe and engine.

Hi Miles,

Thank you, I did see information on the Precise Flight system. You are correct that there is an Airborne 2H3-23 Vacuum Regulating Valve in the system. Now just need locate and probably replace the manifold.

Bob

Well, research complete and there is no manifold. The 2 venturis are both feeding a single vacuum system and driving dual suction gauges for some reason.

Everyone can relax now that the mystery is solved

Huh, weird. Which gauge is correct?

DaveF wrote: ↑Tue Nov 07, 2023 3:07 am Huh, weird. Which gauge is correct?

Yep. Need a tie-breaker. Add a THIRD vacuum gauge!

It’s like the old adage that says the man with two watches never knows what time it is…

Thanks, I have a line on a 3rd gauge!

Bob

rnealon1 wrote: ↑Tue Nov 07, 2023 4:42 pm Thanks, I have a line on a 3rd gauge!

Bob

Bob, I KNOW that YOU know ..why that jet you fly has a stby (3rd tie-breaker) horizon….

(It’s sure easy to forget the basics when we’ve become accustomed to them for so many years, heh?)

I only recently noticed that the altimeter in my 170 has no third indicator-hand. I’ve owned/flown this thing for decades and simply haven’t spent any time thinking about it since it’s only rarely been above 10K’. But I seem to recall a nasty accident (Gulf Shores perhaps..?) where a DC9 hit the water on approach because the crew mis-read their 3-pointer altimeter. This resulted in a regulatory change requiring a single-pointer-type altimeter in large aircraft,… aviation history, like other history, has a way of being forgotten.

Well, I just realized mine is the same...

Well Bob, at least your altimeter DOES have a 10K’ indicator. My altimeter has neither the 3rd hand NOR the “Triangle-pointer” …NOR does it have the Barber-Pole (less than 10K’ indicator). It has only the hundreds and thousands needles…. Makes me wonder if it’s even legal….yet it passed 91.411 and 91.413 checks recently.

Here is a quick review of various types This TOP altimeter is what was mandated for at least ONE in Heavy and Turbine aircraft after the Gulf Shores accident in which a (Delta? National?) flight impacted the water on approach when they thought they were passing down thru 10,000 and they were actually passing down thru 1,000.


And Here is a Chart which gives a Brief explanation of the FAR requirements: