Can a MP gauge overread

[DrQuinn]

I started to keep some more detailed stats to see how not even close to the book numbers I am getting.  Depending on altitude I typically setup for closest settings under 75%bhp and lean.  Usually I am 15 or more mph short of the known unrealistic POH figures.  I just started keeping fuel statistics though and I am running at 7-8 gph, certainly way better than the books range of 9 - 9.4.  

I have a portable tach that gets the actual RPM, leaving either I'm leaning waaaaay too much (which I don't think I am) or the MP gauge is off by an 1 - 1 1/2 ".  Does anyone know if this is likely?  Would my mechanic be able to verify the meter similar to the tach?

[Marauder]

I started to keep some more detailed stats to see how not even close to the book numbers I am getting.  Depending on altitude I typically setup for closest settings under 75%bhp and lean.  Usually I am 15 or more mph short of the known unrealistic POH figures.  I just started keeping fuel statistics though and I am running at 7-8 gph, certainly way better than the books range of 9 - 9.4.  
I have a portable tach that gets the actual RPM, leaving either I'm leaning waaaaay too much (which I don't think I am) or the MP gauge is off by an 1 - 1 1/2 ".  Does anyone know if this is likely?  Would my mechanic be able to verify the meter similar to the tach?

There are a number of factors that come into play. The POH I’m sure was at part a marketing document. Number of antenna, rigging and even condition of the paint will affect some bearing on the true airspeed you can achieve.

You certainly can verify your electronic RPM with an external RPM gauge. Your MP should be close to local barometric adjusted for altitude with the engine off. It may not be linear, but I think it is a low failure item.

Based on the fuel flows you are talking about, it sounds like you are way off. Do you have an engine analyzer?


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[DrQuinn]

I use an external gauge to set RPM and I have no reason to believe that it is inaccurate; it gets the same readings as my mechanics.  I will have to check the manifold pressure reading with engine off next time I'm at the plane.

I have a EI ubg-16 but it's not hooked up for fuel flow or MP unfortunately.

 

Edited March 24, 2018 by DrQuinn

[Igor_U]

If I understood you correctly, you are running your engine at roughly 2gph less then "per book" meaning you are developing that much less power and result should be lower TAS.

Without looking at POH or doing the math, seems to me you are at the ballpark...

So, yes, MP gage could be reading low. What does it read with the engine of, on the ground? It should read close to Altimeter setting value at See level and (negative) difference would transfer to the reading in flight as well.

 

 

[DrQuinn]

4 minutes ago, Igor_U said:

If I understood you correctly, you are running your engine at roughly 2gph less then "per book" meaning you are developing that much less power and result should be lower TAS.

Without looking at POH or doing the math, seems to me you are at the ballpark...

So, yes, MP gage could be reading low. What does it read with the engine of, on the ground? It should read close to Altimeter setting value at See level and (negative) difference would transfer to the reading in flight as well.

 

 

Thanks for the hint, I will definitely check on that at startup next time.

[Browncbr1]

Your MP gauge will read high at low power settings if there is a leak in the 1/8” aluminum line that goes to the gauge, or an actual induction leak.  I had a flare break on the bunt reducer fitting where the 1/8” pipe was attached at the baffle and both of my MP gauges showed 19-20” with throttle closed.   

You can use the “Baro” iPhone app to check gauge accuracy on the ground with engine off.  

[Guest]

You can use a pitot static tester on the MAP gauge, one inch per thousand feet should be close enough to check the gauge.

Clarence

[bradp]

And yes they can fail in high or low readings and in non lob at fashions.  Just be cognizant that the entire system is involved in the reading (line, calibration hole, gauge, etc). 

[ShuRugal]

Usually I am 15 or more mph short of the known unrealistic POH figures.

POH isn't that unrealistic: My '64C is frequently within 5mph of the POH numbers.

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[carusoam]

My M20C’s MP gauge started to read higher than normal....

i grabbed the 1/8” vac line while in flight... suddenly, my MP was reading like an altimeter...

The soft aluminum line has a calibrated drilled hole in it.  A crack developed from vibration, when I grabbed it, the crack turned into a break...

Expect a larger number than usual, when the line leaks, or it’s calibrated hole gets torn open...

At full throttle, the error is hard to notice...

At idle, the error is huge, indicating lots of power that isn’t actually being produced...

PP thoughts only, not a mechanic...

Best regards,

-a-

[DrQuinn]

Thanks for all the responses.

To confuse matters I checked it with engine off and it actually under-reads what the pressure was by 1 1/2".

My mechanic suggested that he could have it calibrated or that I just play around with different settings over the summer (while keeping an eye glued to the cht's) and keep records so I can find settings my airplane like.

[carusoam]

Consider getting it calibrated....

It is impossible to set the power very precisely without a proper MP.

If you fly IFR, you will have your go to settings for various regimes of the flight.  All based on MP and rpm...

When things get difficult, like High DA, while loaded to MGToW, you want to know the power you have right now, not a close estimate....

The one day you get carb icing, same thing... you want to know how much power you have, now.

calibration or gauge replacement isn’t very expensive.  Drilling the calibrated leak hole is a pretty simple task.  Requires the right diameter drill bit....

Missing 1.5” of MP is the equivalent of approximately 5% of total available hp... (1.5/29.92 x 100)

Where this shows up is on T/O performance and climb performance....  it is better to have real numbers in your hands for those hot and or heavy days.... and damp and cold days....

PP ideas only, not a mechanic.

Best regards,

-a-