Vacum Gauge

Hope this is the right place for this question. My 74 Aristocraft with 165hp Inline 6 has a vacum guage that was installed in the dash somewhere along the line, not OEM. Thing is I have no idea what I should be looking at. How does a vacum gauge work, what does it tell me and what would be good numbers to see. Never had this gauge on any of my boats before, thanks.

Re: Vacum Gauge

lamphega said:

Hope this is the right place for this question. My 74 Aristocraft with 165hp Inline 6 has a vacum guage that was installed in the dash somewhere along the line, not OEM. Thing is I have no idea what I should be looking at. How does a vacum gauge work, what does it tell me and what would be good numbers to see. Never had this gauge on any of my boats before, thanks.

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Howdy,


Well, depending on where it's connected, the vacuum will measure manifold vacuum. It's really only useful at idle for adjusting the carburetor. It's not very useful at any other power setting since boats usually operate at fairly high throttle positions. You won't get much vacuum indicated.

On a boat, it just gives you a place to develop a vacuum leak and cause a lean running condition. Running lean can cause detonation. You don't want that.

I would remove it and plug the hole.


Regards,


Rick

Re: Vacum Gauge

vacum gauges use to be use in autos in the early 80's as a guide to throttle/gas use the higher the vacum the more gas you was using. as a boat has no gears and saves fuel when on plain i dont think slowly throttling up is a gas saver.

Re: Vacum Gauge

Yep, pretty much useless on a boat, plus a headache if the hose develops a leak.

Re: Vacum Gauge

glenn property of pam said:

vacum gauges use to be use in autos in the early 80's as a guide to throttle/gas use the higher the vacum the more gas you was using. as a boat has no gears and saves fuel when on plain i dont think slowly throttling up is a gas saver.

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Nope -- it works the other way around. High vacuum means the throttle plates are nearly closed and hence the engine is burning less fuel. You WANT high vacuum. Blip the throttle and you will see the needle drop. Wide open throttle means you will see very little vacuum. It was useful on road vehicles because you typically strove to maintain something over about 10 inches of vacuum for economy. If vacuum got below that point you were lugging the engine. An engine at idle will typically show in the 15 - 16 inches area. A pulsating vacuum gauge signals issues as does low vacuum at idle.

Re: Vacum Gauge

Unless you have a way to manipulate the mixture at the helm also...it's useless.

Aircraft use them, however the pilot has the ability to lean/enrich the engine depending on operating enviroment/performance requirements and when parked is pulled back full lean to lessen the chance of accidental startup.

As stated previously...I'd plug the hole. Buy a handheld gauge for tune-ups.

Re: Vacum Gauge

Oshkosh1 said:

Unless you have a way to manipulate the mixture at the helm also...it's useless.

Aircraft use them, however the pilot has the ability to lean/enrich the engine depending on operating enviroment/performance requirements and when parked is pulled back full lean to lessen the chance of accidental startup.

As stated previously...I'd plug the hole. Buy a handheld gauge for tune-ups.

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Um, a vacuum gage is not used to adjust mixture in an aircraft engine.

(and there are no vacuum gages used in aircraft at all........That would be a manifold pressure gage that reads in inches of mercury.......yes it is technically still measuring a vacuum but it's referenced to absolute pressure ........... more like a barometer........ in fact......Exactly like a barometer. When the engine is not running, the manifold pressure gage indicates barometric pressure)

You're thinking of an exhaust temperature gage which is absolutely used to adjust mixture.........

Re: Vacum Gauge

HT32BSX115 said:

That would be a manifold pressure gage that reads in inches of mercury.......yes it is technically still measuring a vacuum but it's referenced to absolute pressure ...........

You're thinking of an exhaust temperature gage which is absolutely used to adjust mixture.........

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Yes...just thought he'd get the gist...You actually CAN minipulate mixture to reach the highest MP if needed. MP also gives a better indication of how much torque you're delivering in a max performance demand situation. Granted...you may also be overheating...but if you find yourself in a boxed canyon or at the mercy of a mountain rotor...on the way down...it may be your last option.

CHT in conjuction with EGT will allow the most precise mixture to be found under normal circumstances.

Taking off from Cheyenne...heavy...on a hot day will do it too!

Re: Vacum Gauge

You actually CAN minipulate mixture to reach the highest MP if needed.

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apologies to the OP. This is sort of off subject........... but, I must respectfully disagree. No pilots operating handbook, and Neither Lycoming, Continental or Franklin ever suggested adjusting mixture to control manifold pressure. Manifold pressure is only regulated by carburetor butterfly valve position, (MSL) altitude and temp (AKA density altitude) ....... manifold pressure might change with RPM a little but it would be an inverse relationship.

I.E. If you were running rich at a non-full throttle (open butterfly) power setting, leaning the engine to best power would actually increase RPM slightly (with a fixed pitch prop) . The increased RPM would then result in a slightly lower manifold pressure because the engine would be moving more air through it at that previously fixed (or even FULL open) throttle setting. In a turbocharged engine the MP might increase slightly if you lean the engine to best power because the increased power will produce hotter higher pressure exhaust gases to the exhaust turbine.....but we're talking about N/A engines.....

I have only flown carbureted, fuel injected, turbocharged and non-turbocharged aircraft since the 1970's. The airplane I have owned since 1981 has a 180 hp engine with a Hartzell constant speed prop. It also (of course) has a manifold pressure gage. Mixture adjustment from full-rich to either max power or the best economy setting, doesn't change the manifold pressure at all.. Maybe there's an engine out there that operates like you're saying but I haven't seen it.......

Re: Vacum Gauge

My original point was that without the ability to manipulate it...ANY vacuum gauge of any type at the dash/helm/IP is just another leak waiting to happen, and serves no useful purpose. Obviously aircraft operate in a much more dynamic enviroment where so many variables can influence virtually every other aspect of flight.

I'll end my part by just saying that in some vintage military/racing aircraft...you really need watch the MP when using water injection.

Re: Vacum Gauge

I'll end my part by just saying that in some vintage military/racing aircraft...you really need watch the MP when using water injection.

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Hmm...Well, I only flew a couple of those many years ago and just a tiny little bit! (and actually got a ride in a P-51 once but I cannot speak much about it....... it didn't have dual controls....Dmmmit!)

Water injection will change MP somewhat, but mostly just cools the air charge and increases the density..... so with turbo & Engine driven super-charging, you can really jack up the boost..... AND you're absolutely right! I was told the same thing! All I have now to watch very closely is the N1 and the EGT and there's "idiot" gages for them too!


Cheers,


Rick

Re: Vacum Gauge

This is my iboats forum right??lol!

If you're ever up for the fly-in let me know! I think EVERY GA pilot should expirence the well known "rock your wings" & "fly it down to the green dot" callsl at least once!

Regards...

Re: Vacum Gauge

Oshkosh1 said:

This is my iboats forum right??lol!

If you're ever up for the fly-in let me know! I think EVERY GA pilot should expirence the well known "rock your wings" & "fly it down to the green dot" callsl at least once!

Regards...

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I have wanted to go to Oshkosh ever since I started flying! Now with avgas at close to $6/gallon I donno!
My Stinson Stationwagon burns 10 gallons per hour crawling along at about 125mph..... Back when I got it 80 octane was under $1/gallon.

Time sure flies.......

Re: Vacum Gauge

A vacuum gauge is a diagnostic tool just like a tach, dwell meter, or compression gauge. Obviously it would be unreasonable to install 4, 6 or 8 compression gauges for constant monitoring. But a vacuum gauge is as simple as a pitot speedometer and does have a useful purpose. Since the gauge is already there, why not use it. It is particularly useful at idle. A nice steady reading of about 15/16 inches is an indication that the carb is set right and that there actually are NO vacuum leaks. There are generally a number of ports in the intake manifold to insert the hose barb fitting for the vacuum line going to the gauge. Just don't use a ported fitting on the carb. You want manifold vacuum, not ported vacuum.

Re: Vacum Gauge

Silvertip said:

A vacuum gauge is a diagnostic tool just like a tach, dwell meter, or compression gauge. Obviously it would be unreasonable to install 4, 6 or 8 compression gauges for constant monitoring. But a vacuum gauge is as simple as a pitot speedometer and does have a useful purpose. Since the gauge is already there, why not use it. It is particularly useful at idle. A nice steady reading of about 15/16 inches is an indication that the carb is set right and that there actually are NO vacuum leaks. There are generally a number of ports in the intake manifold to insert the hose barb fitting for the vacuum line going to the gauge. Just don't use a ported fitting on the carb. You want manifold vacuum, not ported vacuum.

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I agree with silvertip, it is already installed and does provide useful info. There are a number of online sources that show you how to use it as a diagnostic tool. If the hose is cracking/leaking then maybe, but now you have a hole in the dash to fill.

I routinely hook one up during my spring tune-up. I have considered mounting and leaving it installed.

Re: Vacum Gauge

i got one of them too lamphega. mines oem though.
15-16 inches is about what mine reads. it's steady at idle. any fluctuation would be an indicator something is amiss. there are vacuum gauge interpretations (what various fluctuations mean) on the www.
when at wot there isn't much vacuum. the more throttle ya give it, the lower the gauge goes.
i don't use it much, but is agree it's a diagnostic tool that can lead you towards a repair if needed.


vac. reading at idle on my boat.


vac. reading at near wot.

Re: Vacum Gauge

The subject of the purpose, usefulness or whatever of a manifold vacuum gauge in a boat comes up from time to time on every boating forum I visit. I am shocked amazed by the number of people who do not realize what an important little instrument it can be.

It is a diagnostic tool. It is a tuning tool. It is also probably about one of the best indicators of how much your engine is being loaded, which is absolutely essential to know if you are fine tuning the engine tune or picking the best prop for your application. They are simple, cheap and easy to install. Vacuum leaks rarely occur if the installation is done correctly.

Years ago when much of the smaller commercial fishing fleet on Canada's West Coast had gas engines, many fisherman ran their engines according to manifold vacuum. The conservative ones went by "never run less than 10 in vacuum at continuous operation". Those a little braver might be about 8" in or so. Convention for pleasure boats is a minimum of 5 in.

Sure you can run less, but not if you expect to get best engine life.

Vacuum gauge useless? Not in my opinion.

Re: Vacum Gauge

Years ago when much of the smaller commercial fishing fleet on Canada's West Coast had gas engines, many fisherman ran their engines according to manifold vacuum. The conservative ones went by "never run less than 10 in vacuum at continuous operation". Those a little braver might be about 8" in or so. Convention for pleasure boats is a minimum of 5 in.

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But using like the above description is using it as a throttle "position" gage.

You could put an aircraft manifold pressure gage on a boat and do the same thing and as long as you didn't change the load or prop pitch, the manifold pressure would still approximate the throttle position (and carb butterfly setting etc)

You could use it as a diagnostic tool if you were diagnosing a problem.

It's like a timing light, dwell meter, fuel pressure, etc....

I would hook it up if I needed to 'fix' something but I wouldn't permanently install one for regular operation.