Tachometer and Manifold Pressure gauge overhaul

[NotarPilot]

My MP gauge is indicating about 3" too high (As compared to my Insight G3 engine monitor) and likely needs a calibration or overhaul. My tach is only registering 2500 RPM at WOT and might also need the similar work. A quick google search came up with about 7 different shops that overhaul these gauges. However, not knowing anything about these shops' reputation I wanted to see if anyone here could recommend a good instrument overhaul shop from their own experience.

Before overhauling the tach I intend to verify the RPM with an optical tachometer my A&P has.

[BigTex]

I'd give John Dors a call with Instrument Overhaul Service. They overhauled my engine cluster when no one else would touch it. The unit came back looking brand new. They are out of El Cajon, CA. 619-449-5565. www.instrumentoverhaul.com

[1964-M20E]

I'd do an independent third instrument verification of each and see if the G3 is correct.  I assume it is but this will let you know for sure.  Then I'd consider using the G3 for fine adjustments and use the mechanical gauges as is for coarse adjustments and not worry about fixing them.  The mechanical instruments are still working just may not be as accurate as the G3 and if you did not have the G3 you would be happy with what the mechanical gauges were telling you.

[Marauder]

My MP gauge is indicating about 3" too high (As compared to my Insight G3 engine monitor) and likely needs a calibration or overhaul. My tach is only registering 2500 RPM at WOT and might also need the similar work. A quick google search came up with about 7 different shops that overhaul these gauges. However, not knowing anything about these shops' reputation I wanted to see if anyone here could recommend a good instrument overhaul shop from their own experience. Before overhauling the tach I intend to verify the RPM with an optical tachometer my A&P has.

Another thing you may want to consider is moving over to approved primary electronic versions. I replaced my combination fuel pressure/manifold pressure gauge and tach with versions from EI. I didn't know that fuel pressure is actually determined by the analog ones by bringing fuel into the cockpit!

[N601RX]

The Iphone Engine Rpm app is good for verifying the tach. It works by performing a FFT of the engine sound and always matches my JPI exactly.

[NotarPilot]

What's an FFT? I have an iPad but no iPhone and can't seem to find that app in the App Store.

Marauder, thanks for the suggestion, I like the EI gauges but not sure I'm ready to spend that amount on such an upgrade yet.

BigTex, thanks for the suggestion, that's just what I was looking for. I'll give them a call.

[1964-M20E]

(FFT) Fast Fourier Transform is a mathematical analysis of an input signal and then it extracts out the underlying characteristics of the signal.  In this case I suspect it uses the sound of the engine and extracts the RPM.  I used FFT in college and it was interesting but I haven’t used it since.

[MARZ]

First I'd ask if its really 3" high? At sea level not started your manifold pressure should match the barometer  - info from this link - http://www.advancedpilot.com/downloads/prep.pdf

 

 

 

The ambient air has equalized in all parts of the engine portrayed here, represented by dark blue.

I've shown the throttle (yellow) fully open here, but with the engine at rest it doesn't matter --

even with the throttle fully closed, there's enough of an opening for air to get by and equalize.

(The throttle never really closes all the way -- a "fully-closed" throttle must still pass enough air

for the engine to idle.) In this picture, the air pressure is at ambient pressure in the intake, in the

induction plumbing, and in the combustion chamber. This will show on the MP gauge as 29.92

inches at sea level on a standard day. I know, it's hard to read it that accurately on the usual

instruments, but you should see it very close to 29.9, and that's "close enough." If the sea-level

airport has a big high-pressure area located over it with a local station pressure of 31.10, for

example, then your gauge should show 31.1 inches of manifold pressure. If the airport is located

at some higher elevation, the MP gauge will show an inch less for each thousand feet above sea

level. (This rule-of-thumb is close enough at normal airport elevations, though it breaks down at

altitudes above 10,000 feet.)

It is a good habit to note the MP gauge reading before engine start, and do a quick calculation to

see how close it is. Set your altimeter to the field elevation, note the altimeter setting in the

Kollsman window, subtract one inch per thousand feet above sea level, and your MP gauge

should show very close to that value with the engine not running. At a 6,000-foot elevation

airport, for example, set 6,000 on the altimeter, read (say) 29.5 in the Kollsman window, subtract

six, and check that your MP gauge shows approximately 23.5 before start.

Anything else is an error in the instrument.

[N601RX]

(FFT) Fast Fourier Transform is a mathematical analysis of an input signal and then it extracts out the underlying characteristics of the signal. In this case I suspect it uses the sound of the engine and extracts the RPM. I used FFT in college and it was interesting but I haven’t used it since.

I used it in college and thought it was one of those things i would never use again. At my last 2 jobs I have used it on a weekly basis.

Here is a link to the Ipad rpm app. http://tunelab-world.com/tach

[NotarPilot]

Mike, on the ground it'll display about 31" with the engine off. With the engine running it'll show 3" above what the engine monitor shows. The engine monitor is less than 1 year old.

[MARZ]

Mike, on the ground it'll display about 31" with the engine off. With the engine running it'll show 3" above what the engine monitor shows. The engine monitor is less than 1 year old.

60 foot elevation at your home field is close enough to sea level and if the barometer was close to 31" at the same time you checked the gauge it would tell me that it is OK 

 

I'd check the solid line to your gauge or connections - thin line, susceptible to cracks if nothing is apparent there then id consider pulling it and having it checked. 

[BigTex]

If with MP reads 31" with the engine off, the gauge is likely fine.  As mike said, it might be something with the line.  If you run the engine up to 2000+ RPM, does it read correctly?  I had the same thing and it turned out to be a clogged line.  There's a very small pin hole in the line that can get clogged.  I had my line blown out and it work fine after that.

[mike_elliott]

If with MP reads 31" with the engine off, the gauge is likely fine.  As mike said, it might be something with the line.  If you run the engine up to 2000+ RPM, does it read correctly?  I had the same thing and it turned out to be a clogged line.  There's a very small pin hole in the line that can get clogged.  I had my line blown out and it work fine after that.

What Gary says...Look at the MP line coming from I believe #1 cyl. On my IO360, there is a fitting hiding under an adel clamp that secures the line to #1 Rocker cover. That fitting has a small hole in it that could be clogged on yours.

[NotarPilot]

I actually swapped out the gauge with another MP gauge for a Mooney and it reads a hair over 29".  I'm pretty sure this means it's the gauge.

[BigTex]

I actually swapped out the gauge with another MP gauge for a Mooney and it reads a hair over 29". I'm pretty sure this means it's the gauge.

When does it read 29"?

[Dale Logsdon]

Adjust your altimeter to 0 feet( no matter what your current elevation is). Next read your barometric pressure in the kollsman window. This is what your M.P. gauge should indicate without the engine running.

[NotarPilot]

Just to let everyone know I went with Berkshire Instruments in Montrose, CO and was very happy with their work. Both gauges came back working properly and calibrated. $220 both both gauges to be repaired and calibrated.