Troubleshooting JPI RPM disagreeing with analogue tach

[TigerMooney]

On takeoff and throughout cruise my JPI RPM indication disagrees with my analogue tach (JPI 50-80RPM less). Are there any simple initial thoughts on which one to trust and which one may be off? I just submitted my engine data to Savvy for analyzation and of course they came back saying my engine wasn’t making full power on takeoff…however I believe it is based on the analogue tach. 

[Fly Boomer]

2 minutes ago, TigerMooney said:

On takeoff and throughout cruise my JPI RPM indication disagrees with my analogue tach (JPI 50-80RPM less). Are there any simple initial thoughts on which one to trust and which one may be off? I just submitted my engine data to Savvy for analyzation and of course they came back saying my engine wasn’t making full power on takeoff…however I believe it is based on the analogue tach. 

This could add some clarity:

https://www.aircraftspruce.com/catalog/topages/trutach.php

[Ragsf15e]

There are some free apps on your phone that work pretty well to check rpm.  I’d take a look with something like that to see which one is accurate.  

https://apps.apple.com/us/app/video-tachometer/id1492583587

 

[oregon87]

2 hours ago, Fly Boomer said:

This could add some clarity:

https://www.aircraftspruce.com/catalog/topages/trutach.php

This!  It's like having two watches and never really knowing what time it really is. 

Edited June 16, 2023 by oregon87

[N201MKTurbo]

Or if you are a CB like me:

https://www.amazon.com/AGPtek®-Professional-Digital-Tachometer-Contact/dp/B004Q8L894/ref=sr_1_1_sspa?keywords=optical+tachometer&qid=1686945059&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

it is about $200 cheaper.

[Fly Boomer]

3 minutes ago, N201MKTurbo said:

Or if you are a CB like me:

https://www.amazon.com/AGPtek®-Professional-Digital-Tachometer-Contact/dp/B004Q8L894/ref=sr_1_1_sspa?keywords=optical+tachometer&qid=1686945059&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

it is about $200 cheaper.

I especially like the “anti-jamming technology” — you never know when foreign nationals are trying to prevent you from getting an accurate reading.  For that price, I’ll buy one.

[N201MKTurbo]

Anti jamming probably means it has a lock in amplifier. Easy to realize and isn't affected by ambient light. this tach requires you put a reflective sticker on your prop.

[TigerMooney]

Thanks for all the tips! I actually have a third digital tach on my panel too…so three watches and I don’t really know what time it is  

Is it reasonable to ask my IA to calibrate my analogue tach an annual? Or is the original tach not able to be adjusted?

 

if it’s indicating RPMs faster than actual, then it’s also running hours off my engine faster….

Edited June 18, 2023 by TigerMooney

[Fly Boomer]

1 hour ago, TigerMooney said:

if it’s indicating RPMs faster than actual, then it’s also running hours off my engine faster…

Your engine doesn't know what the tach says.

[PeteMc]

My rule of thumb would be to have your mechanic check it next time you're there, but no need to RUSH in.

Until then, I'd trust the JPI.  There are way too many potential issues with the analog tach.  And it's not like they're showing a 300-500 RPM difference.  I'd say the number is probably within the margin of error of the analog tach.

[Pinecone]

Since the JPI is counting pulses from the sensor, I suspect it is more accurate.  Especially compared to a legacy mechanical tach that works from a spinning cable generating a current that moves the needle.

[FlyingDude]

My r-1 rpm tach was off so I had to send it in for repair. So it's not like all electronic tachs are always correct. Just verify both tachs with a laser tach. If the inaccuracy is consistent, you might leave them as is and make a mental note. 

 

[0TreeLemur]

On 6/19/2023 at 9:12 AM, Pinecone said:

Since the JPI is counting pulses from the sensor, I suspect it is more accurate.  Especially compared to a legacy mechanical tach that works from a spinning cable generating a current that moves the needle.

Agree, digital tachometers are generally very robust.  Usually they either work or they don't (low/noisy signal).  Lots can go wrong with analog tachs.

[TigerMooney]

The main concern is after uploading engine data to savvy analysis pro, I’m making 2610 rpm on takeoff (according to JPI), and they recommend a prop governor adjustment. But I don’t know if 90 RPM is enough to necessitate a R2 of my analogue tach.

Aircraft Quality Instruments in KS said they will adjust it for $185, plus an hour labor on each side of that.
 

Once my IA shoots it with a laser during annual, I guess I’ll make a decision. 
 

Would you guys r2 the tach and adjust the prop governor  if it’s just 90 off at takeoff power?

Can anyone recommend Aircraft Quality Instruments, or have a recommendation for another shop?

Edited August 16, 2023 by TigerMooney

[TigerMooney]

On 6/17/2023 at 9:12 PM, Fly Boomer said:

Your engine doesn't know what the tach says.

That is 100% correct, sir…however if it’s reading 100 RPM fast….its reducing maintenance intervals and valuable SMOH time at a faster rate. Of course the consumables like oil and piston rings/cylinders know no different. 

[Fly Boomer]

1 minute ago, TigerMooney said:

That is 100% correct, sir…however if it’s reading 100 RPM fast….its reducing maintenance intervals and valuable SMOH time at a faster rate. Of course the consumables like oil and piston rings/cylinders know no different. 

Actually, if your instrument is reading fast, you will pull back the prop and accrue time at a slower rate.

[Pinecone]

The optical tachs are easy to check.  Just point it at a florescent light and it should read 3600 RPM.

In the late 60s/early 70s I did a HeathKit optical tach for use with model airplanes.  To calibrate it, you pointed it a a florescent and adjusted it to read 3600.

[PT20J]

On 8/16/2023 at 6:48 AM, TigerMooney said:

Would you guys r2 the tach and adjust the prop governor  if it’s just 90 off at takeoff power?

According to the Lycoming power curves for an IO-360A, 90 rpm low (2610 instead of 2700) at sea level/ 59 deg F is 192 hp (loss of 8 hp). 1 hp = 33000 ft-lb/min. So, at 2700 lb gross weight, 8 hp would reduce the rate of climb of 8*33,000/2700 = 97.8 fpm.

[jetdriven]

The JPI is going to be the accurate tachometer because it uses an hall-effect sensor that counts pulses of the spinning magnet inside a magneto which is geared to the crankshaft.

[TigerMooney]

On 8/18/2023 at 7:39 PM, PT20J said:

According to the Lycoming power curves for an IO-360A, 90 rpm low (2610 instead of 2700) at sea level/ 59 deg F is 192 hp (loss of 8 hp). 1 hp = 33000 ft-lb/min. So, at 2700 lb gross weight, 8 hp would reduce the rate of climb of 8*33,000/2700 = 97.8 fpm.

Thanks, That is some great info…and some math that would certainly made my brain hurt!

While my plane is in for annual I think I’ll have the IA pull the tach and I can sent it to AQI in Missouri who said they can calibrate it for less than $200…but I know its a gamble with a 40 year old Stewart instrument…but I think a new one would be about $500 all together…hopefully after that, a prop governor adjustment I’m not too deep into this to restore full takeoff power.

[Stanton R]

On 8/16/2023 at 1:05 PM, TigerMooney said:

That is 100% correct, sir…however if it’s reading 100 RPM fast….its reducing maintenance intervals and valuable SMOH time at a faster rate. Of course the consumables like oil and piston rings/cylinders know no different. 

The Tach time on a mechanical tach has nothing to do with the needle. It is actually just counting the number of times the engine is turning. So unless the cable is slipping or the gears are bad the tach time is correct. The needle is moved by a spinning magnet that pulls the needle over farther the faster in spins.