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Old 12-12-2020, 09:43 AM
Ron Springer Ron Springer is offline
Join Date: Apr 2012
Location: Rockville, MD
Posts: 12
Default Pmag Advance

I just installed a Pmag on my plane which already has an Electroair. I was curious how the timing advance for the Pmag compared to the Electroair so I did a test last night at three points using the monitor program. Shift was set to zero so this is the Pmag A curve. I am not sure if the results are normal for a Pmag or if there is a problem? The Electroair will always have a lot of advance at low MAP regardless of the RPM.

1) Idle, RPM = 1040, MAP = 12"
Pmag Advance = 2 deg
Electroair Advance = 13 deg
Below ~16", the Electroair vacuum advance is maxed out

2) Runup, RPM = 1700, MAP = 17"
Pmag Advance = 6 deg
Electroair Advance = 12 deg
I did a mag check here and got no drop when shutting off the Pmag and a 70-80 drop in RPM when shutting off the Electroair. That is to be expected based on the timing differences.

3) Full Power, RPM = 2400, MAP = 28.5"
Pmag Advance = 3 deg
Electroair Advance = 1 deg
The Electroair vacuum advance is zero above 24" and it was installed to be at approximately base timing during takeoff

Does everything look right with the Pmag values? Does it intentionally not advance at idle? I have not flown with the Pmag yet and have no idea how they would compare at cruise at 8000 ft (high RPM and low MAP).
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Old 12-12-2020, 04:53 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is online now
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,936

The short answer is “yes”.

Some thoughts:
- 2400 RPM is not full power (at least as far as the pMag is concerned), so the pMag may provide a little advance over the base 25 degrees.
- pMag advance is a based on both RPM and MP - this is reflected in your observations of timing when below typical flying RPM. As you point out, this is also why the ElectoAir had less drop when doing the Mag check.

I’d be interested in the readings you get at say 8,000’ to 12,000’, 2500 RPM and WOT.

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Old 12-12-2020, 05:51 PM
Ron Springer Ron Springer is offline
Join Date: Apr 2012
Location: Rockville, MD
Posts: 12

Originally Posted by Carl Froehlich View Post
The short answer is “yes”.
Thanks Carl!

I will have to collect some data at altitude. I hadn't planned on flying with my laptop. My plane is kind of tight for that. But, I do have my Electroair advance displayed on my GRT EFIS and by switching ignitions at altitude and monitoring EGT, I can tell if the Pmag is more or less advanced than the Electroair. I will probably do a qualitative check like that first.
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