Venturi Recommendations

[Bruce Fenstermacher]

I've reviewed past posts regarding venturi installations and would like recommendations as to what to install to improve vaccuum.

I currently have one stock venturi and no regulator running a new style DG and AH. This current system pulls 4inchs or less and barely runns the gyros.

Which is better. Installing one 8 inch venturi and regulator or add another 4 inch venturi to my existing one.?

I understand that the 8 inch will pull more vacuum than needed and should be regulated down to 4.5 which the gyros need. Cause there is more vaccuum available with this system at least 4.5 would be available at slower speeds.

If I add another 4 inch venturi I've only doubled the volume of vaccuum not increased it and although the gyros may operate better the system would not have the required 4.5 inchs ever and even less at slower speeds.

If this thinking is right then it would seem to me the best upgrade would be one 8 inch venturi and a regulator.

I plan on using the setup for light IFR work.

What is the opinion of the forum?

[wa4jr]

I agree with your logic here. I currently have two 4" venturies running a modern horizon and an old AN DG that is coming out soon to make way for a modern DG. I see no problems now with the perfomance of my gyros, but it does seem that one 8 or 9 inch venturi with a regulator would be the best way to go...but then those regulators are something else to mount in an already crowded area and they are quite expensive are they not? If I see that the modern DG will not run well off of my present 4 inch venturi, I'll probably bite the bullet and go with a single larger venturi since I too do light IFR work.

[GAHorn]

I recommend a dual 9" venturis set-up as illustrated in the 170B Parts Catalog, pgs 132-135. (even without a regulator, this is a better set up , especially with modern 3 1/8" gyros.)

[Bruce Fenstermacher]

Wow George, dual 9" venturis. I'd have never guessed. You seem to imply that is what the B model had when you say ''as illiustrated in the IPC". Did they? I thought they where dual 4" venturis.

George, could you elaborate why you would go with 2 venturis. Unregulated is interesting. Sure don't want to spend money on that regulator if I don't have to. Have you seen this done and do you know how exceeding 4.5"(gyro rating) effects the gyros.

How important is volume in this equation. If Pulling 8"of vacuum from a single venturi going to 2 gyros is that the equivilant of each gyro seeing 4"of vacuum cause the volume theoretically is split?

Does anyone know at what airspeed a venturi such as the 8"or 9" pulls its rated vacuum?

Does this line of guestioning suck?
(sorry just had to throw that in)

Thanks

[GAHorn]

Wow George, dual 9" venturis. I'd have never guessed. You seem to imply that is what the B model had when you say ''as illiustrated in the IPC". Did they? I thought they where dual 4" venturis.

George, could you elaborate why you would go with 2 venturis. Unregulated is interesting. Sure don't want to spend money on that regulator if I don't have to. Have you seen this done and do you know how exceeding 4.5"(gyro rating) effects the gyros.

How important is volume in this equation. If Pulling 8"of vacuum from a single venturi going to 2 gyros is that the equivilant of each gyro seeing 4"of vacuum cause the volume theoretically is split?

Does anyone know at what airspeed a venturi such as the 8"or 9" pulls its rated vacuum?

Does this line of guestioning suck?
(sorry just had to throw that in)

Thanks

Well, I should apologize for relying on memory and slipping into colloquialisms.
They are indeed what some refer to as "4-inch" venturi's. (Venturi nomenclature refers to their capacity by most useage, or to the physical length according to some others (incorrectly I believe. I never should have done that.) The 4-inch capacity venturi is about 10" long, and like a lot of guys, I'm not especially accurate when it comes to describing length. I should have been more careful.)
Let me start over.

The correct venturi is the AN 5807-1. This is an Air Force-Navy Aeronautical Standard that became inactive around 1950 (and in fact was actually cancelled about 1970.) At 100 knots it's design calls for 4.5" in a standard atmosphere. It is probably what you meant when you said a 4-inch venturi.
Anyway, why re-invent the wheel? Cessna did this homework for us already. The 170 was designed/certified for instruments with the installation illustrated in the Parts Catalog. This was for the older AN type of gyros which required less vacuum (3.5-4" Hg) than "modern/pictorial" 3 1/8" gyros which require 4.5-5.5" Hg. (The AN gyros had massive rotors which, once spun up, tended to maintain their speed with less air due to momentum. The newer gyros have lightweight rotors that need lots of air to keep them going at rated rpm.)
The dual venturis will provide approx 6" Hg when installed as depicted. While this increased vacuum will not be harmful in the short term, it will in theory add to the wear rate of gyros, and may contribute to more precession than ordinary with regard to D.G.'s, so a regulator will be helpful in allowing the excess vacuum to be relieved. Regulators can be found in salvage yards and surplus fairly reasonably. Disassembled, cleaned, etc., they rarely cannot be made to function like new again.
A venturi's suction is affected by forward speed, altitude, venturi size, and location on the airframe. Any additional gyros added to the system will also reduce the capacity, hence the need for the dual setup. (You'll likely need more than one venturi to guarantee sufficient vacuum at high altitudes and/or low airspeeds.)
In any case, unless the IPC is followed, you'll be making an installation that deviates from that approved originally. Your inspector is who will have to determine whether any deviation is airworthy.

[zero.one.victor]

Other than ease-of-approval considerations, I would be inclined to go with the single,9"-rated venturi just for the cleaner look and not having to drill more mounting holes in the fuselage. My Spruce catalog sez the 9" venturi is 11-1/4" long,compared to 10" long for their 4" model.
Spruce lists several suction regulators with different hose connection configurations,all for around $235. My regulator is mounted inline & tie-wrapped into place--it doesn't take up much more room than just the hose itself so I don't think space restraints are much of a consideration.
The vacuum system on my ragwing,added by a previous owner & field-approved,uses two venturi's on the RH side of the airplane. The paperwork doesn't indicate model number or capacity,so I assume they're 4". I have a modern horizon and DG,suction gage indicates around 5" at cruise speed,I believe 4" to 6" is in the green.
I believe I'd add a regulator no matter if I changed to the 9" venturi,or just added another 4"er. Maybe if you add another 4"er and plumb the AH & DG each to their own venturi,a regulator wouldn't be needed?

Eric

[N3243A]

If you need a new regulator the Airborne brand of FAA PMA vacuum regulators used to be around $235 in Aircraft Spruce. In the the latest catalog they jumped up to over $600 each!! I imagine that poor Airborne has been sued to death over vacuum pumps and had to raise prices to cover litigation, but almost a 3-fold price jump? Sorry but no way would I pay $600 for a new Airborne regulator. You can also get them used from salvage outfits like Wentworth for about $200. Rapco, a long time maker of PMA brake and vacuum system parts has (for some unknown reason) a NON-PMA vacuum regulator for about $140 in Spruce. Save $460 by "forgetting" the PMA sticker? You make the call

George has talked about using the old Garwin vacuum regulators that came as OEM with the C-170 but I haven't found one yet. The guy I talked to at Wentworth didn't have one at the time.

Bruce

[frainiea]

Back in January I went through this routine. I installed a modern DG to go with a modern AH. With the two four inch venturi's the vacuum was to low. I purchased two 8 inch venturi’s from Wag-Aero ( Cat No G-611-000) @ $62.00 each. The mounting pattern was different so I had to rivet on a new patch about 8”x 8” and used new hardware. I mounted them in the original spot on the right hand side. It was much easier on this side because all I had to remove was the side panel and the glove box. By the way mine did not come with the ¼” plugs in the downstream end so I had to wait a few days to complete the project. I added a vacuum reg. I picked up from Cleo Bickford (he has several more I think) for a very reasonable price. You might contact him (he is the 170 Association Historian and a very 170 knowledgeable guy) and see if you can work out a good deal for a regulator.

Any way after completion I have no problem pulling 5 inches at 10,000 ft indicated altitude and the gyro’s work fine. I usually get spin up of the DG on taxi.

My user friendly A&I signed off on the whole job.

[N3243A]

Frainiea,

When you mentioned that your 2-4" venturis would not drive your modern gyros, was it a marginal situation or worse and not at all acceptable? I have found that a single 4" venturi will drive my modern AI just fine. Does a modern DG require more vacuum than the AI, resulting in the need for more than 8" of total vacuum? Apparently, Wag-aero sells 8" venturis and Aircraft Spruce sells 9" (super venturis). Would the additional inch of vacuum with the super venturi make the installation viable do you think? Like Eric, I would much rather have one venturi for a cleaner (and lighter) look.

Thanks for the tip on regulators from Cleo. I wil give him a call and see if he has any more to sell.

Bruce

[funseventy]

Those are for the instruments?!!!

And all this time I've been using them as fueling steps. They are a lot more reliable as fueling steps than they are at providing vacuum. HA, HA!!

Kelly

[N3243A]

Those are for the instruments?!!!

And all this time I've been using them as fueling steps. They are a lot more reliable as fueling steps than they are at providing vacuum. HA, HA!!

Kelly

Even as fueling steps, they leave something to be desired.

[zero.one.victor]

I thought they were air horns--I've been trying to figure out where the horn button is so I can use them to scare deer off the runway!

Eric

[GAHorn]

I'm always surprised that so many disparage venturis. I find them a very reliable and cheap source of vacuum. The few times I've discovered an airplane that had mal-functioning venturis had maintenance issues such as:
1. Improper fittings in the vacuum system. Using hydraulic or ordinary brass plumbing hardware creates restrictions due to internal design. (Most ordinary fittings are designed for pressure, not vacuum, and most make sharp turns internally adding restriction.) Fix: Use correct Aeroquip 816 (not the same as AN816) Aeroquip 8891, Statoflex, or Airborne (c) fittings, etc. in a vacuum system. These are identifiable by having their turns made with a long radius rather than sharp corners, and are usually mfd out of steel or spun aluminum, rather than cast, forged, or machined metal.
2. Old and/or incorrect hoses. Correct vacuum hoses designed for air/vacuum/pneumatic systems (such as Mil-H-5593, Aeroquip 306, or Stratoflex 111 or 193 hose) rather than pressure-type hoses. Pneumatic-type hoses have inner liners that are designed/vulcanized to their outer materials so they won't delaminate and/or collapse under vacuum and aren't dried-out by air. Other types are designed to carry fluids, oils, etc. under pressure and the inner liners may collapse, wrinkle, etc. diminishing the effective vacuum and may dry-out and send rubber particulates thru your gyros.
3. Venturis are covered in multiple layers of paint, dented, eroded, etc. and otherwise damaged. While they are pretty fool-proof devices, buying a used venturi is asking for trouble. If the inner throat is worn from exposure to sand, dust, accumulated debris, or has lost it's sharp edge at the exit,...or if the inner venturi is no longer concentrically located within the outer tube (the tolerance is only +or- .010") then it will never produce the design vacuum. It it's auxillary source plug is leaking, (or if in use, if the base gasket is leaking, dried out, missing, never-installed, etc.) then it cannot produce design vacuum.
The valid shortcomings of a venturi system include:
1. No vacuum produced before takeoff (unless a standby, or alternate source is used.)
2. Icing can reduce/prevent vacuum. (But it can do the same for flight in a 170! An iced up 170 with a vacuum pump is also in trouble, and even a vacuum pump equipped airplane is in short-term trouble without a heated pitot and anti-iced propeller.)
3. Installation location is important. (Some installations attempt to overcome icing paranoia's by locating the venturi near the lower cowling exit, reasoning that warm air there will prevent the ice from occuring. Unfortunately, this also locates the venturi where it will become contaminated with dirt blown-up/thrown by the prop and any oil misting/leaks from the engine compartment. Both will reduce the efficiency of a venturi.)
A venturi-driven vacuum system is subject to most of the same problems of a pump-driven system such as aged hoses, misrouted lines, incorrect fittings etc. But it has none of the sudden-failure habits of dry vacuum pumps and none of the oily-belly problems of a wet pump system. (And, by the way, all the problems mentioned above in #1 and #2 are also known to add to the short life of dry vacuum pumps. When a dry pump system doesn't provide enough vacuum due to the masking effects of those faults, the mechanic usually cranks up the vacuum regulator/relief. This seriously overworks the pump, and then the owner is surprised/disappointed when he goes through pumps regularly. (If I did have a pump-driven system, I'd opt for a wet-pump and live with the belly.)
In my own day-dreams, after I get a heated pitot, electric windshield, prop anti-ice and TKS system approved for the 170, I think I'd consider one of the engine-intake vacuum system backups for the all-weather interceptor 170. That way I could spin up the gyro's before takeoff and still have a backup vacuum source if the venturi ices up and the struts begin to look like popsickles. I just hope when it finally falls out of the sky, I hit the ground and wake up from the bad dream. (just kidding, folks.)

[zero.one.victor]

I also think the venturi's are a valid source of vacuum for the type of (light) IFR flying that a 170 is suitable for. I've heard a lot of comments to the effect that venturi's are no good,they ice up,etc--well,the pitot and the airframe are gonna ice up too. The real drawback is the spin-up time required after takeoff--you don't want to plunge into IMC until the gauges are working & you're on them.
Of course,as a VFR-only pilot,my opinion doesn't have a lot of personal IFR experience behind it.

Eric

[funseventy]

The "HA!" placed at the end of the comment meant it was a joke. I too believe a venturi is way more reliable than a dry pump.

If I wanted to start a fight I would've mentioned that I can't even use them as fuel steps because I pulled them off as well as the gyros to save weight.

Now that it'll get all of you in an uproar!!

Have fun with that, Kelly