# Thread: Vacuum Gauge vs Manifold Pressure Gauge

1. ## Vacuum Gauge vs Manifold Pressure Gauge

No, not that itty-bitty little gauge that tells you if the vacuum pump has failed...

Does anyone know of a link that describes the difference between a manifold pressure gauge as used in an aircraft and a vacuum gauge as used by every engine mechanic in the world, other than the scale and the direction the needle spins to indicate greater vacuum? (And, I suppose, the \$\$\$ needed to certify the thing for aircraft use.)

What I'm wondering is if the manifold pressure gauge is as useful for diagnosing engine problems as a vacuum gauge, ie, does the pointer indicate less vacuum if the rings are bad, or jump around 4"Hg or so if a valve is sticking, and so on. I've never flown a constant-speed prop plane so I have no experience with the instrument.

Thanks.

Bob

2. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

The two instruments are the same in that they sense the atmospheric conditions within the manifold. For normally aspirated engines the gauge measures a vacuum in that the pressure will always be lower than atmospheric. For turbo or super charged engines the pressure will be higher than atmospheric (with waste gate closed). The only difference then is in the nomenclature, which means that the indicator can be used in the same manner as with an automotive vacuum gauge.

3. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

The vacuum gauge mesaures pressure below ambient. The manifold pressure gauge measures absolute pressure without reference to ambient.

Dan

4. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

Originally Posted by rdj

What I'm wondering is if the manifold pressure gauge is as useful for diagnosing engine problems as a vacuum gauge, ie, does the pointer indicate less vacuum if the rings are bad, or jump around 4"Hg or so if a valve is sticking, and so on.
Bob
Mechanics normally use a compression gauge to check rings and valves.
BB

5. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

Originally Posted by BBerson
Mechanics normally use a compression gauge to check rings and valves.
BB
I agree, but a vacuum gauge can also be an added diagnois instrutment.

6. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

Originally Posted by Dan Thomas
The vacuum gauge mesaures pressure below ambient. The manifold pressure gauge measures absolute pressure without reference to ambient.

Dan
I did some more research and, yup, you're right. The vacuum gauge has ambient pressure on the non-manifold side of a diaphragm, while the manifold pressure gauge has a vacuum tank on the non-manifold side.

So if that's the case, does the 'vacuum tank' of the manifold pressure gauge have a 'restriction in the plumbing' like the vertical speed indicator has to dampen its response? I'm familiar with the difference between a vertical speed indicator and a variometer, for example, where the VSI is less useful for soaring because of its limited sensitivity due to that inbuilt damping. I'm wondering if the manifold pressure gauge has the same issue as the VSI?

In my earlier motorcycle days, it was common to use a vacuum gauge to troubleshoot intermittently sticky valves or lifter float, stuff you couldn't see with a static compression test. The vacuum needle would jump around fairly rapidly during those events. I'm wondering if the manifold pressure gauge does the same, or is it too damped like a VSI?

7. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

A manifold pressure gauge will give you an accurate indication of the power setting of your engine regardless of altitude. The engine's power output is directly related to the absolute pressure of the air in the intake manifold. To get an indication of the engine power setting from a vacuum gauge you would need to know the ambient pressure at the altitude at which you're flying, then calculate the manifold pressure from that and the vacuum gauge reading. Vacuum gauges really are of little value other than helping a driver to "keep his foot out of the throttle" to get better gas mileage. Most have only an automotive level of accuracy and reliability. Many of the aftermarket ones found in hot rod shops and catalogs are pure junk, they're just glitzy toys.

If you're going to put a gauge in an aircraft, use a proper manifold pressure gauge.

And no, there is no simple, universal equation for the relationship between manifold pressure and precent of rated power. It needs to be calibrated for each engine and is a function of RPM as well as manifold pressure. If you have a certified engine the manufacturer has provided that information in the engine operating manual. And again, for a certified aircraft that information will be included in the aircraft operating manual for the engine(s) it is certified with.

8. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

Originally Posted by rdj
I did some more research and, yup, you're right. The vacuum gauge has ambient pressure on the non-manifold side of a diaphragm, while the manifold pressure gauge has a vacuum tank on the non-manifold side.

So if that's the case, does the 'vacuum tank' of the manifold pressure gauge have a 'restriction in the plumbing' like the vertical speed indicator has to dampen its response? I'm familiar with the difference between a vertical speed indicator and a variometer, for example, where the VSI is less useful for soaring because of its limited sensitivity due to that inbuilt damping. I'm wondering if the manifold pressure gauge has the same issue as the VSI?

In my earlier motorcycle days, it was common to use a vacuum gauge to troubleshoot intermittently sticky valves or lifter float, stuff you couldn't see with a static compression test. The vacuum needle would jump around fairly rapidly during those events. I'm wondering if the manifold pressure gauge does the same, or is it too damped like a VSI?
I don't think it's damped, but the length of line between the manifold and gauge, and its small diameter, will damp it anyway. You might get a twitch if a valve is sticking or leaking, but in aircraft there are other ways to detect that: vibration at idle or runup, a failure to get to max static RPM, and so on.

Dan

9. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

Thanks all,

Yes, I agree most available vacuum gauges are cheap junk. I wasn't intending to put one in the plane, I was just wondering if the manifold pressure gauge would produce the same occasionally useful results, or if it was damped for display purposes like the VSI.

And the 'engine' I'm looking at designing the plane (motor glider) around is the 2276 Great Plains redrive VW conversion, so certified isn't even close to describing it. (I hesitate to even bring it up, because I don't want to start another one of those threads.) As for determining rated power, well, that's what fuel flow is for. And I'm keeping on eye on those new solid-state MEMS torque transducers, hoping the automotive industry starts using them in mass. Then it will be easy--torque*RPM. But now we're drifting off-thread...

10. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

Originally Posted by rdj
Thanks all,

Yes, I agree most available vacuum gauges are cheap junk. I wasn't intending to put one in the plane, I was just wondering if the manifold pressure gauge would produce the same occasionally useful results, or if it was damped for display purposes like the VSI.

And the 'engine' I'm looking at designing the plane (motor glider) around is the 2276 Great Plains redrive VW conversion, so certified isn't even close to describing it. (I hesitate to even bring it up, because I don't want to start another one of those threads.) As for determining rated power, well, that's what fuel flow is for. And I'm keeping on eye on those new solid-state MEMS torque transducers, hoping the automotive industry starts using them in mass. Then it will be easy--torque*RPM. But now we're drifting off-thread...
Let's drift a little more off thread. The VSI isn't damped for display purposes; its lag is an undesireable product of its design. It has a calibrated leak of its internal bellows, and so the altitude must change somewhat before the bellows expands or contracts to produce an indication. the glider guys use a variometer, sometimes called an instantaneous vertical speed indicator (IVSI) that has tiny pistons in cylinders, suspended by tiny springs, that sense any change in vertical position (such as when the glider encounters a light thermal) and the pistons pump air in or out of the bellows to force a quick indication on the face of the instrument.

Dan

11. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

The VSI isn't damped for display purposes; its lag is an undesireable product of its design.
Correct. Ignore my earlier 'damped for display' statement. That's what happens when my fingers go faster than my brain (which is most of the time)

12. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

Originally Posted by Dan Thomas
Let's drift a little more off thread. The VSI isn't damped for display purposes; its lag is an undesireable product of its design. It has a calibrated leak of its internal bellows, and so the altitude must change somewhat before the bellows expands or contracts to produce an indication. the glider guys use a variometer, sometimes called an instantaneous vertical speed indicator (IVSI) that has tiny pistons in cylinders, suspended by tiny springs, that sense any change in vertical position (such as when the glider encounters a light thermal) and the pistons pump air in or out of the bellows to force a quick indication on the face of the instrument.
I've never seen such a system in my life. It would be extremely undesirable too, since it would go against the limit in normal flights (turbulence) and provide nill useful information. Most gliders fly on a total energy system system, measuring the net movement in the air instead of the net rise/sink, so ballistic speed changes are "filtered out". Great for quick centering.

Top of the bill is the Sage, that still has a 1 second lag for small (20 fpm) changes:
Models and Product Info

13. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

I've never seen such a system in my life. It would be extremely undesirable too, since it would go against the limit in normal flights (turbulence) and provide nill useful information. Most gliders fly on a total energy system system, measuring the net movement in the air instead of the net rise/sink, so ballistic speed changes are "filtered out". Great for quick centering.
Here. Scroll down to Figure 5:

http://www.free-online-private-pilot...struments.html

14. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

Because the two chambers have crossovers I would venture to assume they dampen each other and provide a sensitive but less twitchy output to the diaphragm. Curious on just how small those pistons are and what the spring tension is to be able to be reliable over time.

15. ## Re: Vacuum Gauge vs Manifold Pressure Gauge

When building this, I wanted to know the Vacuum and Boost. I hooked up two Vacuum gauges to this engine. One was before the Blower and gave me the Vacuum reading. The other was after the blower and gave me boost pressure.

I did not take enough pics of this setup. To late now, I sold this. I won the Nationals with this truck. This truck also placed First in a few other events. Long story on this truck, but I built her by myself in about 3 months. I was trying to make a point. I built this truck to make that point.

The two gauges you can see in the pic have nothing to do with boost or vacuum.....

1983 S10....

Tony