The International Cessna^® 170 Association

Just curious about a 337 I found in the library here concerning the addition of a second Venturi. Why add a second Venturi?

I suspect it is in order to provide enough vacuum for more than one gyro. Seems like a DG and Needle and Ball would take a lot of vacuum to operate properly. Add in an artificial horizon . . .

An artificial horizon...makes sense. Thank you.

The original airplane (back in the late 40's/early 50's) had a minimal equipment list requirement for VFR flight (the most common configuration.) In effect, all that was req'd was a T&B and a sensitive altimeter for IFR flight, as the "needle/ball/airspeed" was the primary IFR instrumentation.
Shortly, the requirement for Art Horizon and D.G. was made.
This meant that the previous low-power (either electric T&B...or vacuum unit) could easily be met until the requirement was changed.
A vacuum T&B only requires 2" of vacuum, and that's a small venture (as seen recently on our newest Member's upper, left-side venturi.) Subsequent, standard installations included an Electric T&B and one vacuum D.G., which required one 4" venturi.
The addition of the AN gyro horizon, coupled with the AN D.G. required dual 4" venturiis, which typically was backed up with an electric T&B. This was the standard Cessna setup for the mid-50's IFR package.
Anyone with a VFR airplane (one venturi) who wished to upgrade to full IFR had to install a second 4" venturi. Those who did not make the modification via Cessna Service Letter may have chosen to do so by field modification. Either way, a Form 337 may have been considered prudent.

Thorough explanation, George, thank you.... I have an electric turn and bank and a gyro. I have the static port on the left, a single 4" venturi on the right and I would like to install my artificial horizon somewhere, someday. So, this is just a 337-operation...nice.

Just to add to George's post, the next step in the evolution of DG and AHs is the modern pictorial type gyro. These gyros run on 4.5 to 5.5" of vacuum to operate properly. A single 4" venturi will spin then but not fast enough and operation is marginal. 2 - 4" venturi spins them better because the volume is doubled (not the pressure) and most VFR pilots are satisfied but the modern gyros still are not being spin to their rated speed. This is why you might want to consider replacing either your single 4" venturi or dual 4" venturi setup with a single or dual 8" venturi (super venturi) setup.

There are other threads on this of course including my recent report on my single 8" venturi installation.

Bruce Fenstermacher wrote:This is why you might want to consider replacing either your single 4" venturi or dual 4" venturi setup with a single or dual 8" venturi (super venturi) setup.

Figures....

Edit: Zoinks! According to Aircraft Spruce the Super Venturi requires (maybe they all do) a suction regulator. $800? Is there a less expensive way to go about this?

First look in your aircraft to see if you already have a regulator. You will need one with the super venturi but you don't have to buy it new. I bought one for $10 from a flee market table.

I got one of these, it works.
http://www.chiefaircraft.com/aircraft/v ... h3-12.html

Thanks. I saw one on the Wag Aero site and eventually called their techy. He said one would want to fly it and try it, that a regulator may not be necessary, as long as the suction checks out at 4.5-5". They have a similar one, maybe the same, for almost the same price.

Wag has the same or similar "super venturi". If you fly over 105 mph you will need a regulator.

Ok, got it. Thank you! I'll take a look to see if I already have a regulator, or not, then keep my eyes peeled for a used one.

I don't know much about venturies and am wondering what the different sizes refer to? I see reference to 4", 8", etc, what does that mean, physical size (measured where?, dia?, length?), amount of suction (in-Hg), or what?

hilltop170 wrote:I don't know much about venturies and am wondering what the different sizes refer to? I see reference to 4", 6", etc, what does that mean, physical size (measured where?, dia?, length?), amount of suction, or what?

It is referring to the amount of suction. The 8" venturi is not much bigger then the 4" and fits the same hole pattern as the 4".

More specifically it is referring to the amount in inches a column of mercury would be raised by the vacuum over the level at atmospheric pressure.