# Vacuum venturi

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#### Dana

##### Super Moderator
Staff member
Anybody using a vacuum venturi driven gyro on a slow plane?

My plane has (came with) a gyro turn and bank, driven by a venturi mounted under the fuselage, between the landing gear. It never worked, at speeds up to 80 kts. The instrument itself works (verified on the bench with a vacuum pump) but the venturi draws no vacuum. It wasn't plugged, but I took it off and cleaned it anyway, driving at 80mph in the car it produces some vacuum, but not the 2" the instrument requires (I haven't put it back on the plane yet).

From what I understand, they're rated at 100 mph, so a venturi that draws 2" of vacuum would only draw 1/2" at 50 mph, but being in the prop blast... I don't know. I shouldn't really need it in a plane like mine, but I had one in my T-Craft years ago and it saved my butt once when I waited a little too long to turn around in bad weather, so it's comforting to have.

Dana

#### pictsidhe

##### Well-Known Member
I have a feeling 50mph isn't going to give you 2"
Does it need much airflow? Intake vacuum is used for car brakes, you might have enough velocity in the intake runners for that to work at full throttle. Easy enough to try, just make sure you have a one way valve to keep nasties out of your gyro and the cockpit. Diesel cars have a small vacuum pump for their brakes, maybe 1lb in weight.

#### pictsidhe

##### Well-Known Member
Another thought. Those sort of things work on a pressure difference. If you somehow feed the high pressure side from a pitot tube and use the venturi for the vacuum side. You'll have several times the pressure drop of a venturi. It might make the difference.

#### gtae07

##### Well-Known Member
I almost hate to say it, but sell the vacuum gyro and buy a digital instrument (like a Dynon D1 if you don't have an electrical system, or maybe a GRT mini if you do). It'll weigh less and work better.

Staff member

#### StarJar

##### Well-Known Member
Intake vacuum is used for car brakes
Old post but....Brakes pull a limited amount of air to fill a chamber. If you pulled a continuous stream from the intake, for hours, your engine would be running lean the whole time.
Where's my gold star?