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Problem or Feature? PMAG setting fluctuates

JDA_BTR

Well Known Member
My question is: does this sound like normal operation......

I recently installed the EngineBridge wifi module to see what my ignition is doing. I'm able to see 1/sec what the timing is advanced to for both PMAGs together on my phone screen via wifi.

IO390, dual PMAG, operating 2400 RPM with about 24 MAP. Steady cruise for quite a while.

I see the setting vary up and down 1.4 degrees (1.4 is the step size I know) and not just sit on one setting. It is 24.... then 25.... then 24..... etc. It will be the right side mainly; then I raise MAP a hair, then the right stabilizes and the left changes. Sometimes they both come and go a bit.

My instruments aren't showing me altitude, MAP or RPM changes when this is going on. So I wonder why the PMAG is so sensitive that it wants to change the timing so often. Of course this is assuming the EngineBridge is accurate.

Also, my system is timed at TDC, with 20BTDC being the timing mark of the IO390. I have the PMAG set for -7 setpoint and 29.4 max advance. The PMAG system starts at +4 on a -25 assumption for a TDC making it fire at 21 BTDC or +1 from what an IO390 wants. So when I see 25 degrees of advance in flight I assume it is really +7 ATDC -25 (assumed by PMAG) - 25 (the reported advance) =43 BTDC which would be 23 degrees of advance for my ignition at that point.

But why does the timing setpoint wobble around so much?
 
Check the fitting in your cylinder where you get the MAP and make sure it either has a very small hole you, as Bill R recommends, install a rivet with a small hole drilled in it to stabilize MAP fluctuations.
 
It does have a restrictor in it. The MAP is rock solid and not changing. It is the PMAG setting that moves all around.
 
1 degree is not very significant and there will always be some scatter with any EI which isn't crank triggered- gear lash, TV etc.
 
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Bit resolution

If the setting is only varying within the resolution, I would suspect it is not really changing. It may be that the recording software cant decide what it should be, so it vacillates between two readings. There is a hint this is happening because you said one becomes stable then the other starts acting up.

In general, in my opinion, it is vary hard to tell the true value of a parameter if the resolution is not fine enough.
 
I think I am with John S on this, specially that the serial link from PMAG is a rather low speed link. I am not an engine guru but I would think the gear lash would not apply since it is under the load and turning at a steady speed. Ironically, I don't see that in EICommander, and wondering if they are filtering it or rounding it off.
 
1 degree is very significant and there will always be some scatter with any EI which isn't crank triggered- gear lash, TV etc.

I assume you meant to say "not very significant"

+2 on instrument resolution. Most of the ignitions smooth between integer values, and the Engine Bridge probably rounds up or down, reporting only integers.
 
As someone that has spent a bunch of time with the PMag serial stream, what you are seeing is typical.
 
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