Venturi performance

[SAFARIAIR]

Hi gang I i am having a little trouble getting the required inches of vacuum out of the venturi system on the old 170B. Running at 3.5 in level flight at 90 kts.

[mit]

Just to stir up George! Put on a belt driven Vac Pump!

[Bruce Fenstermacher]

If you have a stock venturi system your not having any trouble at all. That is about all they will do. The stock venturi were designed to pull 4" of vacuum. Your is perhaps a bit low which could be lots of things from a bad vacuum gage to restrictive plumbing.

[SAFARIAIR]

Thanks for info Bruce.I just over hauled the complete system, only running DG,but it requirers 5in.Is there any way that I can just a little more out of the stock system? If not what would you suggest with out going to a vucuum pump. thanks again.

[lowNslow]

You could have a bad regulator or, Bruce mentioned, some restrictive plumbing. If you do have the original 4" hg venturi you should replace it with the 9" available from Spruce.
http://www.aircraftspruce.com/catalog/i ... nturi3.php
The mounting is the same as the 4" so requires no mods to the mounting holes.

[Bruce Fenstermacher]

When you say overhauled you mean you overhauled the gyro and replaced all the vacuum lines with high flow fittings?

You are correct that a modern gyro is rated at about 5" of vacuum, Old AN gyros only needed 3.5". New gyros have been adequately run on 4". Now each of us has our own definition of adequate.

What can you do to "fix" your problem. You could add a vacuum pump. To do so would mean you have to buy a STC product or design your own and get an approval for it. I'm not sure the add on STC'd pump we occasionally see is still available or supported. I've heard not. This system also required a modified cowl. And after spending $$$$$$ if you can even still buy it, you will be installing a system that fails about every 500 hours.

You could also change out your engine to a 0-300-D with an angle starter AND a vacuum pump pad. Bet you won't go through that expense just for a vacuum pump.

What you can do fairly easy is add a second 4" venturi if you presently only have one. This won't "fix" the problem but doubling the volume seems to increase the vacuum to a bit better than 4" and many people think their gyros run adequately (there is that word again) with this system.

BUT if your going to ad a venturi why not buy a "Super Venturi" or 8" or 9" venturi and install that either with your current single venturi or a replacement for it. These are available from Spruce and Wag Aero. You will also most likely need a regulator for the larger venturi because it may produce to much vacuum.

And surprise surprise we've recently had another thread dealing with the same topic including George Horn's Super Venturi installation. Here is the thread for that discussion: http://www.cessna170.org/forums/viewtop ... f=2&t=7955

Here is another thread: http://www.cessna170.org/forums/viewtop ... 49&p=66082

And on last thread but there may be more: http://www.cessna170.org/forums/viewtop ... 46&p=66071

[SAFARIAIR]

Thanks for all the info/help looks like the best alt is with the super venturi Colin

[n2715d]

Beware venturi internal corrosion. Our 170-b with dual AN-5807-1 4-inch vacuum venturis (without a vacuum regulator) produced between 5 and 6 inches of vacuum running DG and AH for 6 years and then dropped off to 4, 3, and finally 2 inches max. Replaced hoses and removed and blew out venturis with compressed air. No change. Replacing venturis with 2 new venturis from Spruce restored 5.5 inches of vacuum. We can only assume that corrosion in the narrow vacuum channels of the steel venturis degraded their performance.

[GAHorn]

Insects are a more likely explanation. (And not necessarily mud-daubers.) Insects and spiders build nests internal to the circular vent near the mouth. A soft aluminum or brass wire worked into that area can bring out "cotton" spider-nest material.