P-Mag Timing Divergence at High Power - SOLVED

John Collier · #1 08-25-2012, 06:26 PM

I have been running P-Mags since my aircraft was completed about 1.5 years ago. I have an Electronic Ignition Commander (EIC) that monitors timing divergence and I've always received a timing divergence warning on Takeoff...around 7-8 degrees. The timing almost always went to less than 6 degrees post takeoff as the power was reduced (either by me or by altitude)...thereby eliminating the warning.

At cruise everything was fine...CHTs, EGTs, engine always ran great all the way around. I've sent the EIC back...I've sent a P-mag back. I quadruple checked the P-Mag timing index. All was normal...very frustrating!

Finally, thanks to VAF I narrowed the problem down to this: No filter/orifice in my Manifold Pressure line. The P-Mags were seeing a surge in Manifold Pressure from the engine and varying timing between the two P-Mags. My Skyview MP was always solid...maybe the skyview software smooths out the varying MP...not sure. My filter is a small foam earplug inserted into the T-fitting going to the P-Mags.

So for all you guys running P-Mags without a monitor...make sure you have installed a filter or orifice in the Manifold Pressure line. I can tell a difference in smoothness on takeoff.

Hope this helps someone out there running P-Mags.

Omega232Devils · #2 08-25-2012, 08:50 PM

John,
Thanks for the post. I have been running dual P-Mags for about 6 months and never heard of this before. Are others doing this, putting a filter in the MP line?

If so, what material for the filter?

Pictures are appreciated too.

Dan

John Collier · #3 08-25-2012, 09:16 PM

I stumbled across the filter/orifice issue by searching the forums regarding Manifold Pressure lines. Several threads mentioned erratic MP on Dynon displays without a filter/orifice on the MP line. I'm not sure why my Skyview MP indications were OK...maybe the Skyview software smooths out the pulses from a straight MP line?

I can give you a pic, but all I did was take a small earplug from my Halo headset...I rolled it up...stuck it into the MP line by my Skyview / PMag T fitting.

FWIW...I never would have known the variations in MP was affecting the timing on my PMags by feel or CHT/EGT indications. Only because I had the EIC monitor did I see the issue.

http://www.vansairforce.com/communit...essure+orifice

erich weaver · #4 08-26-2012, 11:14 AM

How do you stick a foam earplug in the line without completely blocking it? Are you sure you have not blocked it?

Erich

John Collier · #5 08-26-2012, 11:24 AM

I used a plug for my headset...so it has a small tube in the middle. Plus, it was the smaller sized plug. I've read some use cigarette filters...but one had it sucked into the engine. The plug isn't in the line that goes straight to the engine on my setup...plus it is on a clear tube...so I can see the plug.

I'm 99.9% sure it is working because during the trouble shooting process I completely disconnected the MP...ran like a Mag...and my divergence disappeared. When I reconnected the MP, with the plug, my CHTs were on the 400mark on climb out ...just like I always see with the PMags on a nice 110F AZ day. Lastly, I got 174TAS LOP with 6GPH at 11,500....I don't think that's possible without the ignition advancing...and the engine running smooth.

-John

Walt · #6 08-26-2012, 02:18 PM

Van's manifold pressure gauge kit has a nice little restrictor fitting in it, smoothing out the pulsations on the MP line is not a new problem.
Takes it from a -4 AN to a 1/8 ID line.
Part Number = IE VMP INSTALL KIT

N941WR · #7 08-26-2012, 06:06 PM

The other things that we have seen cause a Timing Divergence Alarm are:
(In no particular order.)

? Different timing maps in the two P-mags
? Zip-tied vacuum lines where one side was pinched and didn?t advance with the other side.
? A warn out timing gear
? Bad P-mag wiring and/or wiring to the EICommander
? Mistimed P-mags (Even an ?expert? can do it wrong.)

It is good to hear you solved this issue John. We have another client whom I think has the exact same issue. My preference is to drill a very small hole through a AN470AD4 rivet (The length doesn?t matter) and put it in a fitting where the head won?t allow it to move. That rivet won?t deteriorate like filter material will. Another good solution is to put a small lawnmower filter in the line like this: