Factory Gauge vs Engine Monitor RPM Differences

[schule]

Any recommendations on the best method to accurately resolve the differences between a factory RPM guage and engine monitor?  Is the factory guage designed to be viewed from a particular angle to be accurate?

My factory RPM guage seems to read around 50-75 RPM less than a new enigne monitor.  I believe the engine monitor reads off the left side Magneto, but I am not at all certain where the factory guage reads from.  

I would like to tune the RPM back to the 2700.  The monitor reads 2670 and the factory guage is something slightly less.  I don't want to be on the wrong side of legal RPM range, but a little more power will be nice to jump over the upcoming mountains.

Any ideas would be appreciated.

 

-Mark

[N601RX]

Use a strobe light.

[Piloto]

Next time when taxing at night try to view a hangar fluorescent lamp through your propeller. When line up adjust engine RPM to 2400 RPM. Adjust the RPM so the blade will look still. At this point your RPMs are exactly 2400RPM +/- .00001%. This is due to power companies in the US generating power at 60Hz. If in Europe (50Hz) set the RPM to 2000RPM.

 

José

[xftrplt]

Brilliant, José!

But, I believe, 1200 or 1800 rpm will also work, with less stress, or give more data points. 

[Piloto]

Quote: xftrplt

Brilliant, Jos

[Piloto]

Quote: xftrplt

Brilliant, José!

But, I believe, 1200 or 1800 rpm will also work, with less stress, or give more data points.

[1964-M20E]

If I’m not mistaken the factory tachometer is mechanically driven and there is some error when you view an analog gauge form an angle.  High accuracy analog gauges have a mirror on the face of the dial behind the needle so you can ensure your are correctly lined up to the face of the gauge.   Most likely the digital readout is more accurate than the mechanical gauge.

I hope this does not erupt into a glass vs. analog gauge discussion here.

 

[Hank]

Quote: 1964-M20E

If I’m not mistaken the factory tachometer is mechanically driven and there is some error when you view an analog gauge form an angle.  High accuracy analog gauges have a mirror on the face of the dial behind the needle so you can ensure your are correctly lined up to the face of the gauge.   Most likely the digital readout is more accurate than the mechanical gauge.

 

[xftrplt]

Quote: Piloto

 

You are right Dick, but I found that in some tachometers the error% maybe different at different RPM settings. It is best to calibrate at your cruise RPM setting.

José

 

[schule]

Thanks for the pointers.  I will definately give it a try.

 

-Mark

[Comatose]

I just had an EDM-730 installed and was a bit shocked that the factory gauge is reading 2770 on takeoff and 70 to 100 higher than the edm across the cruise range. I sure like the performance better at the factory settings... given that the engine monitor isn't a "primary" instrument, but is probably more accurate, what to go by?

[Jerry 5TJ]

Quote: Comatose

I just had an EDM-730 installed and was a bit shocked that the factory gauge is reading 2770 on takeoff and 70 to 100 higher than the edm across the cruise range. I sure like the performance better at the factory settings... given that the engine monitor isn't a "primary" instrument, but is probably more accurate, what to go by?

[jetdriven]

If I had a digital source I would all but ignore the mechanical tach.  They are only certified to be accurate within 25 RPM but ours, when we bought it, was reading 200 RPM fast.

[pjsny78]

my mechanical RPM Guage was bench tested recently to be within 25rpm.