JPI 830 vs Original 1986 Engine Gauges: which is more accurate?

[Heybluez]

I have a JPI engine monitor and notice that reading from the JPI and the engine gauges do not match -- or are even close (see images).  Is this due to the JPI is more accurate or does the JPI need to be configured?  

 

IMG_1868.HEIC

[Jerry 5TJ]

What do the two MAP gauges read with the engine off?  Likely the JPI will match local atmospheric pressure to 0.1” Hg.  

The old tachometer gauge uses a fundamentally different mechanism to measure RPM compared to the electrical JPI. I’d tend to trust the JPI.  

Trust but verify.  

 

Edited May 26, 2019 by Jerry 5TJ

[Mooney in Oz]

30 minutes ago, Heybluez said:

I have a JPI engine monitor and notice that reading from the JPI and the engine gauges do not match

Have you checked the programming of the MAP and RPM on the 830?

Horse Power Constant (HPC) for your IO360, 200 HP is from memory 120.  Someone else feel free to correct if I am wrong.

Page 47 in the PG explains the process for both.

If you don't have a PG, you'll find the latest here:  https://www.jpinstruments.com/wp-content/uploads/2012/06/PG-EDM-730_830-Rev-A-02-JULY-2009.pdf

[alextstone]

My new JPI 830 reads about one inch higher MP than the factory guage so I'm curious too...

[gsxrpilot]

"A man with a watch knows what time it is. A man with two watches is never sure."

One more reason to install gauges that are certified Primary and remove the old stuff.

[carusoam]

There are so many things that can cause a difference...

So the original question is a tad misleading....

In the mix could be...

• mechanical gauges
• electronic gauges
• digital electronic gauges
• digital electronic gauges with mechanical displays
• old gauges
• new gauges
• Out of calibration gauges
• gauges calibrated yesterday

We have also witnessed old and dying gauges as well as mis-installed gauges.....

When it comes to some points of data... it is important to know what sensor type is being used... and what location is it reading from..?

My favorite quirk in early Mooney MP gauges... it has a calibrated leak involved in its reading... the word calibrated in this case is left open to who drilled the hole with which drill, and what has happened to the hole since it was born...  it is there to keep the blue goo (100LL) out of the gauge... my M20C had a crack form and propagate from the calibrated hole... the soft 1/8” aluminum MP line cracked and broke in half after 5k+hours...

So to answer the question briefly...

• It all depends...
• The JPI can be more accurate
• The original gauge can be calibrated to be accurate
• verify first, then trust... these are not soviet built gauges...
• 

Things we can have easily verified...

• All temps can be tested for reading the same after sitting in the hangar for a day or so...
• Any suspect temp gauge can be further tested over a range using... ice water and boiling water...
• MP can be compared to local atmospheric pressure when the engine isn’t running... (doesn’t account for broken/clogged weep holes)
• visually inspect the health and well being of the weep hole.
• RPM can be read from a smart phone app that is using sound as a sensor...
• FF gauges take additional effort to set their K factor.  Based on total fuel used....
• %bhp LOP requires proper compression ratio and accurate FF data for the calculation...
• %bhp ROP depends on proprietary fuzzy calculations that may not match your MAPA key numbers or POH/STC...

 

So if we are only talking about the MP gauge and RPM gauge that haven’t been calibrated since the day they  left the factory 50years ago...  compared to a JPI that has never been calibrated after installation...    hmmm that’s a tough choice... 

 

PP thoughts only, not a mechanic...

Best regards,

-a-

[Gone]

Just to throw a wrench into this discussion, I would avoid the use of the term "calibrated" in this discussion because its use is fundamentally misleading.  It assigns value/property/worth to a measurement which does not exist here.  To quote the international weights and measures council (either CIPM or their technical arm BIPM - supported both by NIST (US) and NRC (Cdn) signatures), calibration is a term used to signify the connection of a measurement (these are all gauges reading a measurement) back to some base or intrinsic property or value which are held and maintained by our national measurement systems.  Such is not the case here.

The best we can ask of these instruments, without a calibration certificate provided by someone that includes a demonstration of the competence of the person making such connection, (or organisation - within our very good international system of accreditation of such competence) is to "verify" their operation within some tolerance.  Very little real traceability of measurement is required for this - just some comparison with a more "trusted" source without chasing after the components of the measurement that relate it to NIST or NRC.  Much cheaper and good enough for our purposes.  

But - and this is reason I am bringing this up, the control of the measurement process determines how good the measurement will be.  In the case of the mechanical tach cable - it may have history, but the number of cycles it goes through have a real impact on its performance. In materials science, fatigue is the weakening of a material caused by repeatedly applied loads.  Compare the repeated loading and unloading of the tach cable to some electronic reader counting the number of times a specific identification mark passes in front of it - such as this one.  Light and/or electrical connection only.  Still subject to fatigue loading but with a lot less impact on the validity of the measurement. 

 

The fact that EI still probably thinks this instrument is calibrated (when it is not really) is moot.  It far outperforms (in terms of stability, precision, and resolution) the tach that came with my '65 Mooney E (which was replaced once before and still exhibited unacceptable performance before that one was then replaced by this little puppy).

The design of our engines dates back close to 80+ years ago (1930's and 1940's). Many of our factory installed instruments (from the 50s and 60s) still use the technologies of that era.  They still work - but when we are forced to ask ourselves why there is difference between them and some newer technology we have installed - albeit not certified as primary - by law we are forced to accept the certified technology that demonstrates less control of its measurement process. 

And to reiterate, measurements from both technological eras are not really traceable anyway (would add significant cost to the instruments) although both have been adequately verified as functioning within some stated tolerance - and that is OK.  As Carusoam has pointed out, the source of the measurement for the electronic tach is different than the source of the measurement of the mechanical gauge - and for me (engineer) that makes a big difference in how much I can trust it.

My 20 cents worth (you can get paid that much too if you can provide the definition of traceability of measurement contained in the VIM).

[Aerodon]

Set your altimeter to airfield elevation and read off the ambient pressure.  Then compare with your JPI and your OEM MP.  

Now if they are all the same (within 0.5"), then I would say the difference you are seeing is a result of where the JPI and OEM are measuring the MP.  I've seen OEM MP measured off one of the 'primer ports' in a Cessna and then also off an inlet 'riser' in a carbureted engine.  

If at all possible, try T the JPI transducer off the OEM MP line somewhere.

Aerodon

 

 

[Jeev]

On 5/26/2019 at 4:52 AM, Mooney in Oz said:

Have you checked the programming of the MAP and RPM on the 830?

Horse Power Constant (HPC) for your IO360, 200 HP is from memory 120.  Someone else feel free to correct if I am wrong.

Page 47 in the PG explains the process for both.

If you don't have a PG, you'll find the latest here:  https://www.jpinstruments.com/wp-content/uploads/2012/06/PG-EDM-730_830-Rev-A-02-JULY-2009.pdf

Definitely do this, it may take a few adjustments to get it right and Ill bet this is your fix..  If your are still way off check where your JPI MP sensor is located, I suggest T'ing it right behind the factory MP gauge. 

[Yetti]

An optical rpm gauge or an app for your phone can give a third reading

[StevenL757]

On 5/26/2019 at 8:51 AM, gsxrpilot said:

"A man with a watch knows what time it is. A man with two watches is never sure."

One more reason to install gauges that are certified Primary and remove the old stuff.

Completely agree.

It baffles me why people opt for an 830 when you can spend only slightly more, invest in a EDM-900 or the EDM-930, use it as your primary engine monitor, get rid of the factory indicators, and free up panel space.

To @Heybluez, whilst I can understand an argument for redundancy, the two different instrument sets are - in some cases - pulling data from different places in the engine, so you're never getting an accurate reading.

Save yourself the confusion and don't be penny-wise/pound-foolish.  Get an EDM900 or 930 and lose the 830.

[gsxrpilot]

I actually don't agree with the redundancy argument. If the factory gauge goes out, you're not airworthy or legal to fly. So you don't really have redundancy for dispatch purposes. 

If the factory gauge goes out in flight, it's no issue and I'm confident I can still get to my destination or back home. It's not an emergency and therefore redundancy is really just situational awareness, and is of very limited benefit.

Like @StevenL757 said, I just can't understand people who install the 830. I understand being on a budget. In my first Mooney, I just couldn't spend the total amount to get a 900. But I still needed/wanted accurate temp data. So I got a G2, because it's PRIMARY for CHT/EGT, and it was cheap. Later with a better Mooney, I decided to spend more and get the 900. The 830 just seems like a very poor ROI.

[StevenL757]

42 minutes ago, gsxrpilot said:

I actually don't agree with the redundancy argument. If the factory gauge goes out, you're not airworthy or legal to fly. So you don't really have redundancy for dispatch purposes. 

If the factory gauge goes out in flight, it's no issue and I'm confident I can still get to my destination or back home. It's not an emergency and therefore redundancy is really just situational awareness, and is of very limited benefit.

Yea, good points.

[chrixxer]

Horse Power Constant (HPC) for your IO360, 200 HP is from memory 120.  Someone else feel free to correct if I am wrong.

That's where I ended up with mine, and it now tracks the book numbers pretty closely.
https://r.tapatalk.com/shareLink?share_fid=55491&share_tid=30175&share_pid=505797&url=https://www.mooneyspace.com/index.php?/topic/30175-Mixture-too-lean%3B-misfires%3B-0%2E7-GAMI-spread/page__view__findpost__p__505797&share_type=t


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[chrixxer]

On 6/6/2019 at 9:31 AM, gsxrpilot said:

Like [mention=12500]StevenL757[/mention] said, I just can't understand people who install the 830. I understand being on a budget. In my first Mooney, I just couldn't spend the total amount to get a 900. But I still needed/wanted accurate temp data. So I got a G2, because it's PRIMARY for CHT/EGT, and it was cheap. Later with a better Mooney, I decided to spend more and get the 900. The 830 just seems like a very poor ROI.

 

In my case, the 700 to 730 swap was a ~$1,100 no brainer; same hole, same probes/wiring harness, 1 hour install. I was quoted almost $9,000 for a 900. Then I decided I wanted All The Savvy Things, and installed additional probes (RPM, MAP, OAT) to get the horsepower display, etc. (already had a FS-450 connected to the GNS430, so moving those connections over was a no-brainer, too.) All told I'm about $3800 into the -830, ignoring the troubleshooting (not grounded properly; factory CHT probe failing and corrupting the inputs for the JPI), less than half the cost of the certified replacement.

And, it's only to tide me over until I can go full glass, with the SkyView or G3X and the associated EMI module.

So for me, it made sense. I get a modern UI, USB data offloading, and an accurate set of digital instruments and saves thousands over an EI CGR or JPI 900-series; thousands the plane needed spent elsewhere (sigh).

I agree, unlimited budget, redo the right side panel around a certified replacement. But for those of us in the real world, especially we who already have an EDM-700 or -800 ...


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Edited June 19, 2019 by chrixxer

[rbridges]

2 hours ago, Yetti said:

An optical rpm gauge or an app for your phone can give a third reading

can't you just stick your finger out and see how many whacks you feel with the prop in 60 seconds?

[Yetti]

10 minutes ago, rbridges said:

can't you just stick your finger out and see how many whacks you feel with the prop in 60 seconds?

Most people lose count while focusing on the pain in their finger.    https://hobbyking.com/en_us/turnigy-fidget-spinner-tachometer.html    The apps for your phone work off exhaust sound,  Pretty accurate also.   Now you can have 4 readings to choose from.

[Yetti]

But that reminds me of sticking fingers into grandma's fan