The International Cessna^® 170 Association

If installing a second 4" venturi in series to my existing 4" venturi give me 8" of vaccuum or just a more consistent 4". Thanks Tom

An added 4 inch venturi will NOT, in my opinion, provide vacuum greater than 4 inches under normal circumstances.

Imagine this.....if one air hose supplying 4psi to a fitting, is supplanted by another air hose
supplying 4psi, wil the pressure be increased?.....or will there simply be more volume of air
available at 4psi?

In other words, adding a second 4 hg venturi simply allows additional gyros to be operated at 4hg...
(they don't give more vacuum....they give more volume) or it will help existing gyros
operate more reliably and at higher altitudes.

If you want higher vacuum you need greater HG venturiis.

The normal venturi installation would not be considered in "SERIES". That would mean that one is mounted to the back of the other and airflow would flow through one into the other. I have a feeling that is not what you meant.

One must know this to help make sense of a dual venturi installation over a single. First and most important the AN gyros of the time ran on 3.5 of inches of mercury. A 4" venturi pulled more vacuum than needed for these instruments. On IFR installations Cessna doubled the 4" venturi to ensure there was enough volume for all the vacuum instruments. They did not need to increase the vacuum.

Now we have modern pictorial gyros and they run on 4 to 5.5 inches of mercury. This is more than a the common AN venturi can deliver. The 4" of vacuum still makes the gyros work but they are running at the ragged minimum and as they age they don't tend to work as well. Doubling the volume helps the gyros run and many are satisfied but the gyros are still not being supplied with the vacuum they are rated for.

The only way to increase the vacuum with a venturi is to get one designed to pull more vacuum. These are available and advertised as 8" or 9" venturi. With one of these installed you will now need a vacuum regulator to regulate the vacuum down to 4.5".

If you have a mixed panel of lets say a pictorial artificial horizon and a AN directional gyro you might want to consider plumbing a larger 8 or 9" regulated venturi to the artificial horizon and the unregulated original 4" venturi to the AN gyro. Running an AN gyro at higher than rated vacuum can make it work as well as the pictorial types on low volume, not so good.

Thanks to all for your advice! I'll keep you posted on the results!

When you are low and slow the AN vacuum instruments may be unreliable on single venturi. DG starts to slightly roam right when you want precision.

Two heads are better than one

regards

I have two ORIGINAL 4" Hg venturiis for sale... unpainted/natural-aluminum, as removed/working perfectly from my airplane 1/2 price ea. (just like Cessna ordered 'em)
(Spruce gets $66 for each. I'll sell both for that...or one for $33, plus postage..about $6.)

(I installed dual 9" venturiis (measure 11" long) when I added a SECOND (pictorial, 3-1/8" horizon) to my panel, in addition to the original AN horizon and AN DG.) Here's the previous panel out-lay: But if you have ONE of the 4-inch venturiis...and don't care to drill more holes, and more plumbing...you might wish to consider replacing it with ONE 9-inch venturi. (Not sure it will increase your VOLUME of air however...it will only force you to buy an expensive regulator ($180+) and associated plumbing also!)

Here's how I plumbed the regulator and second filter into a manifold after adding the second horizon to those existing instruments: And here's how it's all connected to the dual venturiis, per Cessna dwgs:

Now George, you are remembering this thread is not in the TradeMart (yet) aren't you.

Bruce Fenstermacher wrote:Now George, you are remembering this thread is not in the TradeMart (yet) aren't you.

If a TIC170A Member mentions something for sale in a discussion thread (in which the item is germane to the topic) I don't usually relocate it to TradeMart. Only if it is unrelated to the thread, or if the item(s) exceed the topic under discussion do I usually seperate it out. (After all, they DO have the privilege of posting the item for sale regardless.) If it's a NON-Member, I place it in Members Intercom.

Aryana wrote:Per the Cessna drawing that George posted, can someone clarify where the air intake would be located? Does the air get sucked in on the instrument farthest from the venturi?

I see all the instruments are plumbed in series on the drawing, whereas George's system looks to have them plumbed in parallel off a distribution block. Is there any advantage/disadvantage to either method?

The advantage of using the manifold with the regulator attached directly to the manifold, is that ALL instruments receive the same amount of vacuum regardless of individual gyro-demand.
If the gyros are plumbed in series, each gyro may successively receive reduced vacuum (theoretically however, they all flow the same volume of air.) In the Cessna dwg, the horizon receives greater vacuum than the DG. An additional disadvantage of the Cessna system is that if an individual filter is dirty, the ALL the downstream gyros are affected....but if a manifold is used, each instrument can be individually filtered. (I did the latter partly in order to demonstrate to the new gyro mfr'r that for warranty purposes, I'd installed a new filter for their unit. The existing AN gyros already had a filter installed at the previous annual inspection which was about 4 mos previous.)
I have the regulator adjusted to 4.5" and all gyros are working satisfactorily,....in fact the modern (Sigma-TEK)* 3-1/8" erects first, before lift-off in every case. Additionally, if I had it to do over, I'd probably not pay extra for the "fast erect/pull-to-cage" function, as it has been completely unnecessary.

Sigma-TEK has a current offerign of a NEW gyro at rebuilt prices....about $500....much cheaper and more dependable than an elect. design. Since I'm using venturiis... I feel no seperate power source is necessary for a 2nd gyro. I enjoy the addition very much.

gahorn wrote:

Bruce Fenstermacher wrote:Now George, you are remembering this thread is not in the TradeMart (yet) aren't you.

If a TIC170A Member mentions something for sale in a discussion thread (in which the item is germane to the topic) I don't usually relocate it to TradeMart. Only if it is unrelated to the thread, or if the item(s) exceed the topic under discussion do I usually seperate it out. (After all, they DO have the privilege of posting the item for sale regardless.) If it's a NON-Member, I place it in Members Intercom.

George, that is why your in charge of that department.