Timing advance shift with P-mag

[slyfox]

Originally Posted by LouNathanson

I'm in the middle of installing a P-model E-mag (P-mag) on the non-impulse side of my IO-390. After talking to the vendor, I'm getting a little nervous because he seemed surprised that my dataplate calls for a magneto timing of 20 degrees. He seemed to think that all Lycomings were timed at 25-28 degrees.

After speaking with the Lycoming rep, it appears that Lycoming changed the timing spec on essentially all their high-compression engines to 20 degrees about 15 years ago, ostensibly for easier starting and "...greater detonation margins at low ambient temperatures." I'm not concerned with the starting issue, since the electronic ignition will retard itself for starting. However, I am concerned with the detonation margin.

I certainly do want to take advantage of greater spark advance at lower MP and higher RPM, but I definitely don't want to have less than Lycoming's recommended 20 degrees under worst-case conditions of high MP and lower RPM.

With the E-mag/P-mag, you can't mess with the shape of the advance curve, but you can shift it up or down by a fixed amount across the whole curve. I'm trying to decide where to set it, my options being the "A-curve", the "B-curve" (4.2 more advanced than the A-curve), or hook up a laptop and shift it plus or minus from the A-curve.

The problem is that I can't seem to get the vendor to tell me what timing the A-curve will produce under the worst case detonation conditions -- the "floor" of the curve, if you will. I'd like to know this so that I can compare it to the Lycoming 20-degree spec). Perhaps they consider it proprietary, which is understandable. However, the fact that they were surprised that my stock timing was 20 degrees makes me concerned that I'm installing something that was not designed with Lycoming's current high-compression engines in mind, and I'm doing it in the blind because I don't know where the advance curve starts.

So, my questions for the group are:

Has anyone installed an E-mag/P-mag on a high-compression Lycoming that has a dataplate timing spec of 20 degrees?

If so, what advance curve or advance shift setting did you use?

How long have you been operating with the E-mag/P-mag?

Have you had any problems?

Am I worrying way too much about this???


Thanks,
-Lou

I read in my manual for installing the plasma3 to retard the timing for the higher compression engines by offsetting the TDC for no1 by 5 degrees. Seems you can do the same for the pmag. Put the flywheel to TDC and retard 5 degrees than do the rest of the setup after that, result, 20degrees base timing. Of course I would run this by emag first to make sure it's ok.