Vacuum Gauge

[Bruce Tuncks]

Does anybody have a vacuum gauge among their engine instruments? They don't seem to be in use much, and I wonder why not.

 

A change in vacuum readings, especially a sudden drop, could be the first indication of something wrong, such as an inlet manifold leak.

 

The gauge would be easy to install if the tapping-point was just downstream of the carby. So there must be another reason why they are not widely used.

 

 

[pmccarthy]

Every hoon had one on his car once.

 

 

[GraemeK]

Isn't that what your MAP gauge reads? But instead of vacuum, it reads out in absolute pressure?

 

 

[Bruce Tuncks]

Thanks guys. There is a whole lot of stuff out there on manifold pressures which is not relevant to my Jabiru.

 

If I ever get a turbocharged constant-speed prop plane then there will be a lot for me to learn.

 

In the meantime, the only use it would have in my Jabiru would be as a diagnostic tool.

 

And you are right Graeme, it should not be called a vacuum gauge at all.

 

 

[jetboy]

I put one on my 2200 to measure the prop load vs rpm temporarily. You just remove a small screw from the middle of a port on the carb underside and place the hose there.

 

It doesnt matter what type of gauge you use if you convert the units of scale. I just borrowed a gauge from a set of 4 carb balancing ones from a m/cycle supplier, these come with a damping valve fitted so you can remove the chatter, depending on what you are trying to use it for. they are compound gauges, not sure why for m/cycle tuning because bikes dont usually have turbos. Bought the gauges because the cars i was working on needed 4 carbs balanced - Honda S600 & H1300 Coupe

 

 

[Thruster88]

Does anybody have a vacuum gauge among their engine instruments? They don't seem to be in use much, and I wonder why not.A change in vacuum readings, especially a sudden drop, could be the first indication of something wrong, such as an inlet manifold leak.

The gauge would be easy to install if the tapping-point was just downstream of the carby. So there must be another reason why they are not widely used.

Your ear should alert you to a sudden drop in map, absolute power in a fixed pitch prop airplane that's the tachometer.

 

 

[Yenn]

Most of the time when flying the vacuum gauge would read very little vacuum, which is the same as a high manifold pressure.

 

When I was a motor mechanic many years ago I always had a vacuum gauge in my toolkit as it was a very good diagnostic tool. I still have one, buthave never bothered to connect it to an aero engine. I find now that the EGT on every cylinder is a good diagnostic tool. On my first or second flight using a lycoming o-320, I had misfire while running up prior to take off. The EGTs let me pinpoint which plug had been at fault.

 

 

[facthunter]

If you have a manifold leak it will be most obvious at idle when the vacuum is higher and gas (airmass) flow least. If you have a supercharged engine set the MP to the pressure height and no leaks happen. You need a damper (small orifice restriction) sometimes or the needle is a blurr, from pulses, especially on the 2 carb layout on the 912 Rotax.. MP is only low (High vacuum though it's nowhere near a vacuum) at idle with no load. It's an indication of a good engine to a point academically. It's no real value though in the big scheme of things. It can show your valve timing is out. I recall. Nev

 

 

[skippydiesel]

Very easy to fit to the Rotax 912/914 range, as a factory fitted vacuum spigot is already mounted on the carbi balance tube.