Timing advance shift with P-mag

[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

[frankh]

What do you mean by "High compression"?

I run an IO360 on 92 octane autofuel (with ethanol) on a CR of 8.5:1 on the lower B curve (I think).

No issues.

The truth is nobody knows where the detonation limits are and its a function of temperature anyway..The hotter the engine gets and the more power its making the closer to the detonation point you will be.

Hence the max advance at high MP is 25 degrees with the Pmags.

You can add a negative advance shift until your comfortable if you like but in all honesty unless your running above 9.0:1 then I wouldn't worry about it.

Frank

[LouNathanson]

Frank,
The IO-390 compression ratio is right up there at 8.7, hence my concern.
-Lou

[flyeyes]

I've used a Pmag on an IO-360 with 8.7:1 (stock) pistons without any issues.

I'm not by any means an engine expert, but my understanding is as follows:

The somewhat arbitrary cutoff for "high-compression" pistons is anything over 8.5:1

8.7:1 is "high compression, but just barely, certainly not like the much higher compression pistons in the helicopter engine variants or "hot rod" pistons.

My engine specifies 20-degree timing, and I use the lower "b" curve advance. I've spoken at length with Tom and Brad about timing (I was a very early adopter) and they are certainly aware of this.

From the reading I've done, the detonation margins are pretty high on these normally aspirated engines as long as you stay away from the barely ROP "max power" settings.

The highest advance on the Pmag is at relatively low power cruise, where I typically operate LOP, with huge margins. At WOT takeoff power, the timing is no different than a mag.

[LouNathanson]

James,
My recollection from speaking with Brad is that the A-curve is what you get with the jumper installed, and is the less aggressive curve. Is that your understanding, and are you running with or without the jumper?

[SMO]

From the manual:

Variable Timing Limit:
The ignition will use engine rpm and manifold pressure to calculate firing angle over a range of operating conditions. There are two preprogrammed curves that can be called up by the operator by using a jumper between thermals #2 and #3. The ?B? curve will allow timing to advance as much as 39 degrees. The ?A? curve will allow timing to advance as much as 34 degrees. We recommend starting with A curve.
? A Curve advances (up to 34 degrees) - WITH jumper.
? B Curve advance (up to 39 degrees) - NO jumper.

[flyeyes]

Originally Posted by LouNathanson

James,
My recollection from speaking with Brad is that the A-curve is what you get with the jumper installed, and is the less aggressive curve. Is that your understanding, and are you running with or without the jumper?

You're right, I should know better than to rely on my memory.

I definitely have the jumper installed, and I'm using the less aggressive advance curve.

[nucleus]

I have a Superior with 9:1 compression ratio and dual pmags. Cut the jumper after break in. I saw no changes in cylinder head temps. EGT's went down some LOP, and it ran LOP smoother and could take it further LOP than on the A curve.

Hans

[elippse]

The manual on the O-235 F or G, with 9.7:1, called for 25 deg., but Klaus has all who have more than 8.5:1 to set their timing to 20 deg.; this is usually done by offsetting the trigger magnet(s) by 5 deg. retard. I have an O-235L2C with 9.7:1 and set my timing to 22 deg.! Keep in mind this is the timing you will have at full throttle.

[nomocom]

Originally Posted by LouNathanson

Am I worrying way too much about this???


Thanks,
-Lou

No, concerned and inquisitive, though what have you got for tracking CHT's? As Frank said, it's all about the temps. Would you know if you had detonation on any of your cylinders (multipoint monitoring)?