New to me..

[Bruce Fenstermacher]

Darn it thought I might have found some paperwork.

I'd put one 8" on and if I needed it then I'd get a second or I'd even try it with a 4". The 8" and 4" will be a slightly different length and shape only noticable by most when they are next to each other. To bad these 4" venturi aren't worth anything, seems everybody has a few in their hanger.

[docfoley]

Of note, my venturis were mounted on opposite sides of the aircraft and were never mounted together.

[GAHorn]

Darn it thought I might have found some paperwork.

I'd put one 8" on and if I needed it then I'd get a second or I'd even try it with a 4". The 8" and 4" will be a slightly different length and shape only noticable by most when they are next to each other. To bad these 4" venturi aren't worth anything, seems everybody has a few in their hanger.

Did I miss something? How did a mis-matched set of venturiis get into the discussion?
I believe a mis-matched set will only diminish the performance of the pair, neither achieving their potential.
I'm concerned the more powerful venturi will likely "backwash" the smaller, and will see the smaller as a "leak" in the system.

Cessna had dual, identical venturiis connected in "parallel" for their instrument airplane kit, to drive an AN DG, and and AN Horizon.
Here's how they did it:

To see a larger version, and other details, click on the #2 link below.

If a search is made in these forums, in two seperate threads, I talked about "upgrading" my panel to the "super" venturiis, because I wanted to assure plenty of vacuum existed for an additional (modern) 3-1/8" pictorial gyro.
1. http://www.cessna170.org/forums/viewtop ... on+project
2. http://www.cessna170.org/forums/viewtop ... +venturiis
Prior to the mod, my dual, standard venturiis produced about 4" of vacuum at 7500' MSL cruise. Lower altitudes produced a bit more, up to 5".
After swapping them out to super-venturiis, adding a third gyro (the new horizon) and an interconnecting manifold, and regulator, I see 4.5" at all atlitudes from S.L. to 7500', presumeably due to the added regulator. (I've not tried it above that yet.)
I'm certain that dual super-venturiis will prove satisfactory with whatever two gyros you have installed, and that's what I"d recommend.

Charley, I suspect Cessna put them both on the starboard side for a reason. There have been operators who noted reduced efficiency with those mounted on the port side. (Prop-swirl effects perhaps ??) Anyway, I've had excellent service from my right-side venturiis, and absolutely refuse to consider downgrading to a vacuum pump. (That should re-liven this discusson.)

[docfoley]

George,
I'm glad you joined in! I had read al the other threads actually. You are correct that you. A not mi. The two venturis in the same circuit. You could use them as two separate circuits if you really wanted to separate the instruments.

Can one super Venturi run a modern DI and HSI? I wouldn't have thought so. I am seriously thinking about changing to the RC Allen gauges instead of keeping the AN's. they're 2 lbs lighter too

I have a 337 for the installation of the vac system. I didn't check the date but I think it was later than the original date of sale. If the factory didn't do the install that would explain why I had right and left venturis. Personally I like the way they look when mounted together.

[Bruce Fenstermacher]

Charley,

I feel one super venturi can certainly run two modern gyros and here is why. My first 170 had one 4" venturi running two modern gyros. There was no regulator and it was not required as the most I ever saw the vacuum gage pull was 4.25". It shouldn't have been enough vacuum but it was for my gyros which I felt must have been in decent shape to work under these conditions. it was not perfect and there where times the gyros were slow and not up to speed but it was fine for 90% of the flying 90% of us do where we look outside and follow a GPS anyway.

So based on this and the fact your aircraft has one hole on each side anyway, I'd get one super venturi, mount it on the right side and see how it works. Bet it would be fine. Worse case is you buy another and either mount it on the left side or make a new hole on the right and tie it in. Of course most would just get two right of the bat and save on the shipping and time spent under the panel but what fun is that.

I've never tried a mixed venturi system case I've never had a super venturi to test with. But I believe what would happen is the system would pull an average of the two or about 6". You have to remember the weaker venturi is not a hole in the system. It to is puling vacuum.

If you had two 4" venturi the vacuum in inches of mercury does not double to 8" but the volume of air moved does. Yes it's been reported two 4" venturi work better that one and I wouldn't be surprised to find together the set might pull nearly 5" of mercury but I think it more the volume the set of 4" pull that makes them work better. A mismatched pair of venturi would still be doubling (at least) the volume but as I stated I'd expect the force or inches of mercury to be an average of the two.

Any fluid engineers out there care to comment?

(I'm not a fluid engineer but I do have the second highest post count here which has to count for something )

[docfoley]

I am going to try with just one venturi at 8" to start with. I was thinking of just mounting both 4" but the transition phase where you may slow down and lose adequate vacuum bothers me. I don't know that the mismatched venturis would be an even split. If I had the holes already in the plane I would mount it to try it

[Ryan Smith]

...Charley, I suspect Cessna put them both on the starboard side for a reason. There have been operators who noted reduced efficiency with those mounted on the port side. (Prop-swirl effects perhaps ??) Anyway, I've had excellent service from my right-side venturiis, and absolutely refuse to consider downgrading to a vacuum pump. (That should re-liven this discusson.)

I don't see how spiral slipstream would have any bearing on the effectiveness of venturis being placed on the starboard side versus port. A simple change in the pitch of the propeller would have an effect on venturis if spiral slipstream were a factor.

Attached Is a picture of my family's 170 that my grandfather bought right as I was born; the sole object of my affection since I was old enough to know what airplanes were. Someone at the airport we kept it at bought it last week and brought it back down from NY where it had been for about four years after my father sold it in 2008. You can see the venturis are on the port side. I seem to recall some issues with them interfering with the static port, more so than being ineffective due to their placement. We didn't have any AN gyros in the airplane, we had a modern AI and DG in the airplane and had no issues with vacuum, but then again it didn't have a suction gauge.

[docfoley]

Nobody seemed to have a solid answer for the static problems....
I see no need for a failure prone vacuum pump either, at least not for my kind of flying.